Insidious Truths: The crashes of Birgenair flight 301 and Aeroperú flight 603
Speed is the foundation of flight, the invisible hand that holds an airplane in the air, without which it will plummet to its doom. To fly without knowing one’s speed is to drive an automobile without knowing where the edge of the road is. Even worse is to believe in a speed that is false, at which point exiting the great aerial highway becomes all but certain.
On the 6th of February 1996, the pilots of a Turkish Boeing 757 found themselves in just such a situation moments after takeoff from the Dominican Republic. Climbing through the midnight darkness, they became confused by a faulty airspeed reading and misleading warnings. Unsure what was true and what was false, they lost control of the airplane, which stalled, rolled inverted, and plunged into the Atlantic Ocean, killing all 189 passengers and crew.
Just eight months later, on the 2nd of October, an Aeroperú Boeing 757 departing from Lima again encountered false speed readings, but this time their altitude was faulty as well. Terrified and confused, unsure where they were going or how fast, the pilots pleaded for help, but there was little anyone could do. After nearly half an hour of chaos, the aircraft descended into the Pacific Ocean, bounced off, turned over, and plowed again into the pitch-black water. Out of 70 passengers and crew, none survived.
The back-to-back disasters opened the eyes of the aviation industry to the true dangers of what is known as unreliable airspeed. This type of emergency presents unique difficulties that challenge human instincts and corrupt the actions of automated systems. Complete elimination of the diverse causes of unreliable airspeed is all but impossible, as the culprits have included everything from ice to protective covers, insects to masking tape. But in each of the above cases, readily available information could have helped the pilots fly their airplanes safely, despite the loss of crucial speed data. This is the story of two tragedies that resulted from a failure to recognize that fact — and how those tragedies have informed current pilot training and aircraft design.
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Part 1: The Turkish Venture
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When people who don’t go on Caribbean vacations try to think of that region’s quintessential destination, they tend to picture Jamaica or the Bahamas, but the current most popular Caribbean island among global tourists is actually the Dominican Republic.* Occupying the eastern half of the island of Hispaniola, the Dominican Republic leads regional tourism in part due to its size, which has allowed the country to position its own beaches and mountains as a more affordable — but no less beautiful — alternative to its smaller and pricier neighbors. That approach began paying dividends in the early 1990s, and tourism to the country has been increasing steadily ever since.
Historically, Germans were the third most common nationality among tourists to the Dominican Republic, especially during the 1990s. The number of tourists from Germany peaked at over 435,000 in 1999 and has been falling ever since, but during the initial boom, demand for cheap flights between Germany and the Dominican Republic compelled a number of charter airlines to offer such services. One of the smallest and cheapest was the Turkish outfit Birgenair, named for its businessman founder, Çetin Birgen.
*I actually surveyed my friends for this, asking them to guess the top Caribbean island country or territory by total visitors per year, and included the two most common responses. Nobody guessed the DR.
The story of how a scrappy Turkish airline with less than half a dozen aircraft came to be involved in the long-haul charter business between Germany and the Caribbean is a strange one.
Since its founding in 1988, Birgenair was involved in the German charter business thanks to a personal relationship between Çetin Birgen and the German-Turkish businessman Vural Öger, owner of the travel agency Öger Tours. The pair started out operating charter flights between Germany and Turkey, but by 1993 their ambitions began to stretch beyond the Atlantic, to the white sand beaches of the Caribbean.
According to the Freedoms of the Air outlined in the Chicago Convention on International Civil Aviation, an airline may operate a flight from a second country to a third country only for the purpose of connecting to the airline’s home country. Therefore, a charter service ferrying tourists between Germany and the Dominican Republic must be operated by an airline from one of those countries, since the purpose of the flights is not to connect to a third destination. At that time, several German airlines offered services to the Dominican Republic, including Lufthansa subsidiary Condor, which carried numerous tourists on behalf of Öger Tours and countless other travel agencies. But presumably in an effort to undercut his competitors on price, Öger wanted the ability to book his customers on the much cheaper Birgenair instead. It has also been alleged that Öger owned a financial stake in Birgenair, but this is disputed.
Fortunately for Öger and Birgen, there is a well-known solution to this type of international business problem: establish a shell company.
In 1995, apparently at the behest of Öger Tours and Birgenair, Finnish businessman Matti Puhakka and six unnamed Dominican investors established an airline in the Dominican Republic named Alas Nacionales (“National Wings”), which was granted a Dominican Air Operator Certificate (AOC) despite having no aircraft. This airline then entered into a lease agreement with Birgenair, under which Birgenair would supply aircraft to fly between the Dominican Republic and Germany under Alas Nacionales’ brand and AOC. According to German newspaper Die Zeit, Puhakka and the other investors received a flat fee for each passenger carried under this arrangement.
In reality, of course, Alas Nacionales only existed on paper. Die Zeit described the airline as a “mailbox company” (Briefkastenfirma). After the accident, German reporters attempted to track down this airline, and found the company headquarters inside an unmarked, dilapidated building on the outskirts of Santo Domingo, staffed by an individual who was unable to produce a business card. It is quite apparent that Alas Nacionales was nothing more than the local face of Birgenair, a legal fiction that allowed Birgenair to operate out of the country without connecting to Turkey. However, this arrangement wasn’t illegal, and Birgenair was far from the first or last airline to do it.
After setting up Alas Nacionales, Birgenair leased a Boeing 767 to its Dominican partner, which was re-registered in the Dominican Republic as HI-660CA. The word “Birgenair” was hastily painted over with the name “Alas Nacionales,” but the livery otherwise remained unchanged. Flight crews were apparently hired by Alas Nacionales directly from Birgenair. Regular nonstop flights between the Dominican Republic and Germany started shortly after.
At the same time, beginning in November 1995, Birgenair took advantage of its position in the Dominican Republic to lease a second aircraft, a Boeing 757 with the registration TC-GEN, to Argentine charter company Servicios de Transportes Aéreos Fueguinos, known as STAF Airlines, for irregular flights between the Dominican Republic and Buenos Aires. Very little information about this operation is available. However, it is apparent that this was what’s known as a “wet lease.”
In aviation, a dry lease is a lease in which the lessor supplies an aircraft to the lessee, while the lessee provides their own crew. The 767 lease to Alas Nacionales was technically a dry lease, because the crewmembers worked for the lessee. By contrast, in a wet lease, the lessor supplies both the aircraft and crew, while the lessee sells the tickets and pays the operating expenses. Airlines typically use wet leases to cover short-term gaps in their fleets and schedules.
A dry lease and a wet lease are legally very distinct because in a wet lease, the lessor remains the legal operator of the aircraft. Therefore, the 757 remained registered in Turkey, remained painted in Birgenair colors, and remained on Birgenair’s AOC.
Very little is known about Birgenair’s wet lease to STAF Airlines. Wikipedia states that Birgenair only operated five flights on behalf of STAF between November 1995 and January 1996, but this number isn’t in the cited source. What is known is that after the last flight in January, the aircraft sat unused on the apron in the northern Dominican city of Puerto Plata for an uncertain period of time. Dominican investigators would later state that it remained parked for 20 days beginning on January 17th, while Birgenair stated that it was parked for 12 days beginning on January 25th.
It just so happens that Puerto Plata was one of the cities served by Birgenair through its front company Alas Nacionales. In fact, the city is one of the most popular tourist destinations in the Dominican Republic, with 162,000 residents and tens of thousands of hotel beds, including high-end resorts. Puerto Plata also features a cruise port and an international airport, located on the coast a few kilometers east of the city. Today, Puerto Plata’s Gregorio Luperón International Airport is the fourth busiest in the Dominican Republic with over 700,000 passengers arriving and departing from its single runway every year aboard airlines like WestJet, TUI Nordic, and Edelweiss.
It was here that a deadly sequence of events took place on the night of the 6th of February, 1996.
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On that evening, crews were preparing the Boeing 767 to operate Alas Nacionales flight 301 from Puerto Plata to Frankfurt via Berlin when they discovered that a hydraulic pump wasn’t working. As this was a no-go item and an immediate repair could not be conducted, Alas Nacionales contacted Birgenair to arrange a last minute switch to that airline’s idle 757 instead.
The Boeing 767 is a wide body, twin-aisle, twin engine aircraft with seating for between 214 and 290 passengers depending on the configuration. The 767–200ER, the variant operated by Birgenair and Alas Nacionales, was capable of flying nonstop from Puerto Plata to Berlin.
By contrast, the Boeing 757 is a twin-engine narrow body aircraft with a shorter range and a capacity of 200 to 239 passengers for the -200 variant operated by Birgenair, depending on the configuration. The 757–200 was not capable of flying nonstop to Berlin, but it was large enough to accommodate the 176 passengers scheduled to depart that night. As a result, Birgenair agreed to operate the flight using the 757 on behalf of Alas Nacionales, with a fuel stop in Gander, Newfoundland hastily added to the flight plan.
This replacement flight was operated on Birgenair’s AOC and had to use a Birgenair crew. Although the 757 and 767 have a high degree of system commonality, allowing pilots to fly both types at the same time, my assumption is that it was not possible to switch the original crew over to the new aircraft because the original crew officially worked for Alas Nacionales, not Birgenair. Furthermore, even if this was possible, the original crew might not have been able to fly until the next day, depending on how long it had been since they were called up for duty.
Fortunately, Birgenair had its own full 757 crew already in place in Puerto Plata who had not flown since January 27th. It’s not clear from official documents how they got to Puerto Plata, or why. One would assume that they had been flying the wet lease flights to Buenos Aires, but the last such flight occurred no later than January 25th. Some other flight must have taken place on the 27th but I can’t say where, or how. Regardless, the pilots and flight attendants had been stuck in the Dominican Republic for at least 10 days by this point, and they probably welcomed an opportunity to return home. What would have been less welcome was the short notice callup, which came in just after 21:00 local time. Without advance notice of the flight, they might not have spent time resting up for the late night departure, so I can’t imagine they were too thrilled by the idea of spending the rest of the night flying to Europe instead of sleeping.
For a flight of this length, an augmented crew was required, with a third pilot who would rotate in part way through the flight. In command of this three-man crew was 61-year-old Captain Ahmet Erdem, a highly experienced pilot with 24,750 flying hours over a lengthy career, including 1,875 hours on the 757.
His first officer for the first part of the flight would be 34-year-old Aykut Gergin, whose 3,500 hours made him no longer new to flying, but he was new to the 757, with only 71 hours on type.
Later on, the plan was for Erdem to go off duty, at which point he would be replaced by the third pilot, 51-year-old Relief Captain Muhlis Evrenesoglu, who had a substantial 15,000 flying hours, but once again, very few — 121 to be precise — on the 757.
Finally, at some point before the end of the flight, Captain Erdem would return to relieve First Officer Gergin, and the two captains would finish the journey together.
In addition to the three pilots, the crew included six flight attendants and four maintenance engineers, who had also been on standby for a considerable period of time before Birgenair’s last-minute callup. Together, they would oversee the boarding of the 176 passengers, who were mostly Germans on a package vacation sold by Öger Tours. Two passengers were also sitting members of the Polish legislature.
By approximately 23:15 local time, all 13 crewmembers had arrived at the aircraft and the passengers were ushered on board. In the meantime, the pilots completed the pre-flight walkaround and checks, which confirmed that the 757 was seemingly in good working order despite its lengthy downtime.
Little did they know that this conclusion was wrong.
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When an aircraft is parked for an extended period of time, several routine maintenance actions must take place to ensure that it remains airworthy. Specifically for the purposes of this story, it bears mentioning that all external sensors and the engine inlets should be covered to prevent environmental contamination.
A modern aircraft relies on an extensive network of sensors that monitor everything from air pressure to exhaust temperature to angle of attack. But for now I want to focus on the sensors that measure airspeed and air pressure.
The ambient or static air pressure is a parameter used by modern aircraft in numerous critical processes, including but not limited to calculating barometric altitude and airspeed. This pressure is measured at the static ports, a series of sensors installed flush with the fuselage on the 757’s forward underside. Redundant static ports independently supply pressure data to the captain’s and first officer’s instruments, as well as the set of emergency standby instruments on the center console.
An airplane’s airspeed, its speed relative to the surrounding air, is crucial to stable flight. Except during landing, knowing one’s speed relative to the ground is of little use because the amount of lift generated by the wings depends in part on the speed of the airflow over them, which can be affected by motions of the air mass itself — that is to say, wind. Therefore, the actual airspeed is approximately equal to the groundspeed plus the headwind component, or minus the tailwind component.*
*This breaks down a bit at high altitudes but the flight in this story never got high enough for that to matter.
Airspeed is measured using three independent sets of pitot tubes. As air flows into the pitot tubes, it exerts pressure against a sensor, which is then compared against the static pressure to calculate the speed of the airplane relative to the surrounding air mass. The resulting value is then depicted on each pilots’ airspeed indicator, located just to the left of the artificial horizon on the 757.
The raw data from the 757’s pitot tubes and static ports is processed and integrated with other parameters by three redundant air data computers, or ADCs. If one of the three airspeed indicators displays an erroneous value, the pilots can change its data source to a different ADC using the corresponding data source selector.
There are a number of reasons why an airspeed indicator might display erroneous values, but typically this is because something has blocked one or more pitot tubes. For instance, ice accumulation inside the tube during flight can reduce the amount of air reaching the sensor, resulting in a lower flow pressure and an erroneously low indicated airspeed. Alternatively, if the flight takes off with an already existing pitot tube blockage, sea-level air can become trapped inside the tube. The measured pressure then becomes a constant, while the static pressure decreases, resulting in an airspeed indication that starts out at zero, but increases proportionally with altitude.
In order to reduce the probability of such a blockage, special pitot tube covers must be applied whenever an aircraft is expected to remain parked for a certain period of time. Later, the parties involved in this story failed to agree on exactly how long an aircraft had to be parked before covers were required by the maintenance manual. Birgenair, for its part, cited “irritating and even conflicting procedures” that seemingly gave permission to omit pitot tube covers for some period of time ranging from seven days to two months, depending on the interpretation. However, it is accepted that contamination of the pitot tubes can occur when an aircraft is parked for any length of time, a fact which Birgenair also acknowledged.
It is not known whether or for how long pitot tube covers were used during the period of time in which TC-GEN was parked at Puerto Plata. Dominican investigators would later write that there was no evidence that pitot tube covers were used at any point during that period. On the other hand, Birgenair stated that pitot tube covers were in fact used up until approximately the 4th of February, when a routine engine runup was conducted, presumably in order to maintain the airplane’s flight-ready status. It is however undisputed that from that point onward, no pitot tube covers were installed. Çetin Birgen, writing on behalf of his airline, stated that Birgenair’s mechanics in Puerto Plata decided not to reapply the covers because they expected the aircraft to be ferried back to Europe within three days, which they believed was permissible under the relevant provisions of the maintenance manual.
If Birgenair is to be believed, then the aircraft stood without pitot tube covers for approximately two days before it was requisitioned to complete the flight to Berlin. Unfortunately, history has shown that this is plenty of time for the pitot tubes to become blocked by a particularly sneaky little villain — the mud dauber wasp.
Mud daubers are a collection of wasp species from several taxonomic families that are subjectively united by their habit of building simple tubular nests out of mud. The species Sceliphron caementarium is native to North and Central America, and like many mud daubers, it prefers to build its nests inside existing crevices and holes, where it carefully constructs a separate mud tube for each individual egg. The inside of a pitot tube happens to be the perfect place for such a nest, at least from the wasp’s perspective. From our perspective, however, it’s less than ideal. In fact, several previous accidents and incidents have been tentatively or conclusively attributed to unreliable airspeed indications caused by mud dauber nests inside the pitot tubes, including the crash of Florida Commuter Airlines flight 65, a DC-3 that went down off the coast of Florida in 1980 with the loss of 34 passengers and crew.
Other incidents have shown that mud daubers can commandeer an aircraft’s pitot tubes within a very short period of time. In perhaps the most extraordinary example, in 2013 an Etihad Airways Airbus A330 made an emergency landing after takeoff from Brisbane, Australia due to unreliable airspeed indications caused by the presence of a mud dauber nest inside the captain’s pitot tube. Even though the aircraft was only on the ground in Brisbane for two hours before the incident flight, that was apparently enough time for a mud dauber to claim the pitot tube as its sovereign territory.
Given these precedents, and the lack of alternative explanations, is believed — but not proven or provable — that a mud dauber most likely built a nest inside the captain’s pitot tube while TC-GEN was parked at Puerto Plata without pitot tube covers. This conclusion remains in the realm of supposition rather than fact because no living soul ever again laid eyes on the pitot tubes after the aircraft departed, but I’ll provide some more information about why a mud dauber is the preferred theory at the end of Part 1.
In any case, because the effects of a blocked pitot tube only become apparent once the aircraft is travelling at speed, there was very little if any opportunity to detect the blockage prior to getting underway. Although the pre-flight inspection represented an opportunity to check for obvious problems, it would have been impossible to see a mud dauber nest or any other contaminants inside the pitot tubes, because the probes are high off the ground and have very small openings.
For this reason, Boeing’s maintenance procedures recommended a more specific verification of the pitot-static system prior to returning a parked aircraft to service. The accident report doesn’t say what equipment was required to perform such a verification on a Boeing 757, but Çetin Birgen contends that this technology was not available at the poorly equipped Gregorio Luperón International Airport.
Consequently, the pilots concluded that the aircraft was airworthy and accepted it for flight, unaware that the captain’s pitot tube was blocked.
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At 23:42 local time, Birgenair flight 301 lined up for takeoff on Puerto Plata’s runway 08, with Captain Erdem at the controls. The weather was unsettled but not adverse, with one fourth overcast at 1,800 feet, near full overcast at 7,000, and intermittent rain over the field. Thunderstorms churned in the distance, over the ocean.
The pilots advanced the thrust levers to takeoff power, and First Officer Gergin called “power’s set.” The aircraft began to accelerate down the still-wet runway, propelled forward by the 757’s famously overpowered engines.
Within seconds, the aircraft reached the 80-knot checkpoint, and First Officer Gergin called, “80 knots.” But because Captain Erdem’s pitot tube was blocked by the mud dauber nest, it did not measure the increasing airspeed, prompting Erdem to exclaim, “My airspeed indicator’s not working.”
This sudden problem at the utmost outset of the flight could have prompted a variety of responses, but there was and remains no single right answer. Below 80 to 100 knots — the exact value depends on the aircraft — pilots are taught to reject a takeoff for any failure or warning indication. There is no risk in rejecting a takeoff at these speeds due to the size of the remaining runway margin and the relatively minimal demand on the brakes.
But above this threshold lies the “high speed regime,” where a narrower set of conditions restricts the decision to reject. Between 80–100 knots and decision speed, or V1, pilots are generally taught to reject the takeoff only in the event of an engine failure, fire, objects on the runway, windshear warning, or — crucially — whenever the pilots judge that the aircraft is “unsafe to fly.” This last point brings in a level of subjectivity that can lead to disagreement over the best course of action if an unexpected scenario arises.
There was no specific policy requiring the crew to reject the takeoff due to the failure of one out of three airspeed indicators, which means that the decision was left to the pilots’ judgment. In this case, First Officer Gergin’s “80 knots” callout indicated that the aircraft was entering the high speed regime, which begins at 80 knots on the 757. It was only at this point that the airspeed issue was detected.
In the opinion of the eventual investigation, the ideal action upon discovering an airspeed indication problem at 80 knots would be to reject the takeoff immediately. But in order to reject the takeoff in the high speed regime, the pilots would need to determine that the failure threatens the safety of the aircraft. And if too much time is taken to evaluate the significance of the failure, the aircraft could surpass V1, at which point they would have no choice but to continue into the air, because the remaining runway would be insufficient to stop the airplane.
In fact, by the time Captain Erdem acknowledged that his airspeed indicator was not working, the aircraft was already traveling at almost 100 knots. First Officer Gergin then affirmed that Erdem’s airspeed indicator was indeed not working, after which he called out, “one twenty.”
“Is yours working?” Captain Erdem asked.
“Yes sir,” Gergin affirmed.
By this point the aircraft was traveling at 132 knots, and V1was rapidly approaching. Therefore, Erdem had only a couple of seconds to decide whether this failure rendered the aircraft “unsafe to fly.” But from his perspective, the answer was probably obvious: since First Officer Gergin’s airspeed indicator was working, then a valid indication existed, and safe flight was possible. He also would have needed to weigh the risk posed by the failure against the inherent risk of rejecting close to V1, especially on a wet runway with no overrun safety area, only a jumble of boulders sloping down into the Atlantic Ocean.
In the end, Captain Erdem did not decide to reject the takeoff. It was a decision that many have questioned, but under closer scrutiny it appears understandable. He certainly could not have anticipated the dire sequence of events that would follow. So he said to his first officer, “You tell me,” with the understanding that they would use the first officer’s airspeed readout until further notice.
At 23:42 and 35 seconds — nine seconds after Erdem called out “My airspeed indicator’s not working” — First Officer Gergin announced “V1,” and then “Rotate.” Captain Erdem rotated for takeoff, and flight 301 lifted off the runway in a normal manner.
Erdem called out, “Positive climb, gear up,” and Gergin confirmed they were climbing, then raised the landing gear.
“Gear is up,” he announced. He then asked whether Erdem would like the autopilot to be configured in LNAV mode, to which Erdem replied, “Yes please.”
At this point, the autopilot was not actually engaged, nor would it be engaged for another two minutes. However, setting the desired modes before engaging the autopilot ensures that it engages in the desired configuration. In this case, the crew wanted to use the LNAV mode in the autopilot’s lateral channel, which would cause the autopilot to steer the plane along the series of flight plan waypoints entered in the flight management computer (FMC). Therefore, First Officer Gergin pressed the LNAV button and called out the selection, which Erdem acknowledged.
Just one second later, Gergin announced, “It began to operate.” But what was he referring to?
This line, translated from Turkish, appears to have been widely misinterpreted and/or misattributed. The final report on Birgenair flight 301, which is supposed to be the definitive account of events, attributes this line to Captain Erdem, and every single retelling in the three decades since has repeated this. But in his comments on the report, Çetin Birgen points out that according to the official cockpit voice recorder transcript, this line was actually uttered by First Officer Gergin — and the transcript, which I have viewed, confirms that Birgen was correct and the Dominican investigators were wrong.
Having misattributed this line, the final report states that Erdem was referring to his airspeed indicator, which had indeed begun to operate, in a sense. Because the mud dauber nest had trapped a pocket of sea level air inside the captain’s pitot tube, and because ambient pressure decreases with altitude, the difference between the pressure inside the tube and the static pressure outside the airplane was now increasing as the aircraft climbed. This change manifested as an increasing airspeed indication, but the parameter actually being measured was effectively altitude, not airspeed. By the time the comment in question was made at around 500 feet above ground level, the captain’s airspeed indication had increased to 125 knots and was rising at a rate of about 4 knots per second. However, the actual airspeed was likely around 200 knots.*
*Note: the flight data recorder only recorded the captain’s airspeed indication, so no accurate record of airspeed was available from the flight. However, the recorded ground speed was 196 knots at this time, with a slight headwind.
On the other hand, the discovery that the First Officer Gergin actually uttered this crucial line complicates the narrative. It follows that either he was glancing over at the captain’s airspeed indicator, or he was referring to something else entirely — but what? In his response to the accident report, Birgen argued that Gergin meant that LNAV mode had successfully engaged, and that the comment had nothing to do with airspeed at all. However, in my opinion it would be pretty strange to say that LNAV “began to operate” when the autopilot was off, because LNAV mode wasn’t doing anything — it was just sitting there, waiting.
It’s also possible that the line was poorly translated from Turkish, and in fact Birgen did write that some lines were translated improperly, but he didn’t specify which lines he was talking about. The original Turkish language transcript is not available and I don’t speak a word of Turkish, but I do have academic knowledge of linguistics and translation more generally, and it’s not inconceivable that a phrase best translated as “it has engaged” could be misleadingly rendered as “it began to operate.” But if this line is accurate, then I would prefer the explanation that Gergin was keeping an eye on Erdem’s airspeed indicator and called out when he saw it start working.
In either case, Erdem didn’t acknowledge the first officer’s callout, and two seconds later he asked Gergin to turn off the windscreen wipers, proving that his mind was elsewhere. So far there was no evidence that Erdem had lent any credence to his airspeed indications, despite the investigators’ assertions to the contrary.
Instead, flight 301 continued its climb without any signs of trouble. At 1,000 feet, Erdem called for the thrust to be reduced from takeoff to climb power, and Gergin acknowledged. He then called on Gergin to set the autopilot’s vertical mode to VNAV, which he did. Gergin then reported that they had reached flap retraction speed, which they had, and Erdem instructed him to set flaps 5, then flaps 1, then flaps up. At the same time, the Puerto Plata tower controller handed them over to Santo Domingo area control. Moments later, at 23:43 and 48 seconds, Gergin called the after takeoff checklist complete.
Next, Gergin called Santo Domingo to report their position. The controller replied with an instruction to climb to 28,000 feet and report passing waypoint Pokeg, which Gergin read back correctly.
Finally, at 23:44:07, while climbing through 3,500 feet over the ocean, Captain Erdem called for Gergin to engage the autopilot. Specifically, he asked for the center autopilot — the 757 has three; left, right, and center — but this would have no practical impact on the sequence of events.
On many modern aircraft, especially fly-by-wire aircraft, the extensive automation suite draws its flight data in aggregate form from several redundant air data computers that compare the raw data sources and reject any source that doesn’t agree with the others. But the Boeing 757–200, designed in the late 1970s and in service since 1982, did not have this capability. While it was among the first aircraft to come with a heavily computerized cockpit, these computers didn’t automatically compare inputs to reject faulty data sources, and the automation could only draw data from one set of sensors at a time, rather than aggregating data from two or three sources. In fact, by default, the left and center autopilots used the №1 air data computer as their data source, which was supplied by the captain’s static ports and pitot tubes.
As you may have already guessed, this meant that the autopilot had no source of airspeed data other than the captain’s blocked pitot tube. Furthermore, the readings could not be rejected as invalid because the analog sensor was measuring the actual local pressure inside the tube, which was then reported faithfully to the air data computer. Consequently, the autopilot immediately began to control the airplane based on the captain’s incorrect airspeed indications.
The significance of this fact is sometimes lost in retellings of the Birgenair story. It was crucial to the sequence of events, but the final report didn’t quite convey its importance, despite acknowledging its role. What in fact took place here was a failure of technical knowledge. The captain — knowing full well that his airspeed indicator was not working — lacked sufficient understanding of his highly computerized aircraft to predict that this false information would be fed to the center autopilot, and that the autopilot would react to that information as though it were true. To a modern pilot trained from the outset on modern aircraft, this fact would be self-evident, but to Captain Erdem, it was not.
As soon as the autopilot engaged, it began to navigate laterally according to the programmed flight plan (LNAV). But in order to navigate vertically (VNAV), it needed to configure the aircraft to achieve the programmed altitude, and to do that, it activated a secondary mode called flight level change, or FLCH. This is a mode common to Boeing aircraft in which the autothrottle commands continuous thrust (climb thrust, in this case) while the autopilot adjusts pitch to maintain a constant airspeed, resulting in the optimal rate of climb up to the selected flight level. Pitching up slows the aircraft down; pitching down speeds it up.
In this case, the actual selected speed isn’t that important — it was presumably a normal cruise speed, somewhere between 220 and 250 knots. But as the airplane climbed, the captain’s indicated airspeed kept increasing proportionally with altitude until it exceeded the target airspeed. And with the autopilot’s vertical channel in flight level change mode and the thrust levers locked at climb power, the autopilot responded to the increasing airspeed by pitching up in order to slow down. This pitch up was slow at first, but pitching up also caused the plane to climb faster, which caused the difference between the trapped pitot air and the static pressure to increase faster, which caused the indicated airspeed to rise faster, which caused the autopilot to pitch up more… and you can see where this is going.
As the airspeed readings increased, but before the pilots had a chance to realize what was occurring, two mysterious caution messages appeared on the screen of the Engine Indicating and Crew Alerting System, or EICAS. These messages read “rudder ratio” and “mach/speed trim.”
The first message, rudder ratio, was produced by a system that adjusts the ratio of rudder pedal deflection to actual rudder deflection with respect to airspeed. The purpose of this system is to ensure a consistent aircraft response to a given pedal deflection at all speeds, even though the effectiveness of the control surfaces actually increases proportionally with airspeed.
The second message, “mach/speed trim,” came from a system that subtly adjusts the position of the horizontal stabilizer to compensate for the tendency of the aircraft’s center of lift to move aft at high Mach numbers (that is, closer to the speed of sound).
As far as I can tell, both of these systems took into account airspeed and Mach number information from either all three pitot-static systems, or from a pitot-static system other than №1. This matter is purely academic but if anyone happens to know which is the case, please let me know. Regardless, these systems continued to function normally with respect to the aircraft’s actual airspeed, but because the captain’s indicated airspeed was very high, caution messages were generated to warn the flight crew that there was a discrepancy between the indicated airspeed and the settings of the rudder ratio and Mach trim systems. The designer’s intention was that these messages would indicate a failure of the aforementioned systems, but in this case it actually meant that the messages were generated by a computer that was getting the wrong airspeed information.
None of the pilots aboard Birgenair flight 301 had been taught that these caution messages could be the result of an erroneous airspeed indication. In fact, they probably had very little understanding of what the messages referred to at all. Clearly confused, Captain Erdem simply read out, “Rudder ratio? Mach airspeed trim?”
“Trim, yes,” said First Officer Gergin.
“There is something wrong, there are some problems,” Erdem cryptically declared.
For 15 seconds, Gergin clarified a clearance with air traffic control. But as soon as that conversation ended, Erdem repeated, “Okay, there is something crazy, do you see it?”
Presumably he pointed at his airspeed indicator, because Gergin replied, “There is something crazy there, at this moment two hundred only is mine and decreasing sir.” Indeed, as the autopilot steadily pitched up to counter the ever increasing indicated airspeed, the actual airspeed — which was still displayed correctly to the first officer — began to decrease below the target value.
The correct action at this point would have been to disconnect the autopilot, establish a stabilized flight path, then evaluate which airspeed was correct by comparing to the standby airspeed indicator on the center console. But even this may have been unnecessary if the pilots had simply trusted their previous determination that the captain’s indicator was faulty.
If the crew had previously noticed the captain’s airspeed indicator coming to life, then this could have complicated their assessment. For a brief period after Gergin stated “it has begun to operate,” the captain’s airspeed indicator happened to display an airspeed that was quite close to the correct value. The final report endorses the theory that this false sense of normalcy caused the pilots to believe that the problem had corrected itself. However, I’m skeptical of that assumption because of Captain Erdem’s bombshell reply: “Both of them are wrong. What can we do?”
In my opinion, if Erdem believed his own airspeed indicator was correct, then he wouldn’t have announced that both indicators were wrong. Rather, I think he knew all along that his airspeed was incorrect, but falsely believed that the autopilot was now maintaining the selected speed, due to an incomplete understanding of the 757’s air data distribution architecture. So when Gergin announced that his airspeed indicator showed 200 knots — well below the target speed and decreasing — Erdem’s first assumption was that something was now wrong with Gergin’s airspeed indicator too.
It may seem obvious, from the vantage point of 2025, or whatever year it is when you’re reading this, that a faulty airspeed indication is probably caused by bad data, and that this bad data will also be fed to other aircraft systems. But in 1996, the transition to highly computerized aircraft was still underway in most of the world, and unreliable airspeed emergencies on these aircraft were not widely understood. The vast majority of Captain Erdem’s 24,000 flying hours were spent on a variety of 1950s-era and 60s-era aircraft, including the Vickers Viscount, Douglas DC-8, Douglas DC-9, Boeing 707, and Boeing 727. Pilots transferring from these aircraft to “glass cockpit,” computer-heavy aircraft like the Boeing 757 at that time were not necessarily taught to understand the complex interplay between the sensors and computers that controlled the aircraft’s automation technology and digital flight displays. As I previously discussed in my article on the infamous Gimli Glider, much of this interplay was not considered “need to know” information. As a result, I think it’s possible that Erdem’s instinct upon seeing an erroneous instrument indication was to assume that the instrument itself was faulty, without considering the implications of a fault with the data source. But that’s simply my untested opinion.
Without this understanding, Captain Erdem found himself facing a confusing barrage of indications that seemed to lack any obvious common origin. He almost certainly didn’t understand why the rudder ratio and mach/speed trim caution messages appeared, nor for that matter would most 757 pilots at the time. And he had lost faith in his instruments too, because neither pilot’s airspeed indicator matched his conception of what the plane should be doing.
Erdem had been flying for long enough to know that there was no checklist for this situation. So what was he to do? In the event, his fallback response was to declare, “Let’s check the circuit breakers.”
Unfortunately, this was not an appropriate response to the indications he was receiving. The circuit breakers might be relevant if a digital instrument has gone completely blank, but not when the instrument is displaying wrong information. It should also go without saying that messing with circuit breakers in flight, without following an emergency procedure, is not a course of action envisioned by the manufacturer and should be attempted only as an absolute last resort, if at all.
In response to Erdem’s call to check the circuit breakers, Gergin said, “Yes,” but before he could actually do so, Erdem announced, “The alternate is correct.”
In his own submission, Çetin Birgen wrote that this line was actually spoken by the relief captain Evrenesoglu, but the cockpit voice recorder transcript doesn’t confirm this. In the absence of any primary evidence indicating otherwise, I’ve attributed this line to Captain Erdem, as does the final report. But regardless of who said it, there are two possible interpretations of this line that lead to very different conclusions about what may have happened next.
According to the final report, Erdem was most likely referring to the standby or alternate airspeed indicator on the center console. This instrument was indeed showing the correct airspeed value, which was identical to the first officer’s value. However, it’s apparent that Erdem did not use the standby airspeed indicator as a reference, nor did he cross-check it against either of the other airspeed indicators, as would be expected in the event of an instrumentation problem.
In his submission, Birgen argued that according to the standard phraseology used by Birgenair pilots, the word “standby” would be used to describe the standby instruments, while the word “alternate” would refer to the alternate data source. Therefore, he proposed that the person who spoke this line was not referring to the standby airspeed indicator, but was actually suggesting that the problem might be solved by switching one of the data source selector switches to “alternate.”
During my research, I found it difficult to evaluate which of these interpretations was more likely to be correct, and there are arguments against both. For instance, if Erdem had determined that the standby airspeed indicator was correct, then why didn’t he use it as his airspeed reference? Without getting too far ahead of myself, it’s apparent that at no point after this statement did Erdem accurately determine their airspeed. But if he meant to suggest the use of an alternate data source, then why didn’t he move his data source selector switch to “alternate,” which would have caused his instruments to draw correct data from the №2 ADC?
Birgen argued in his submission that at time 23:44:16, about 40 seconds before someone said “alternate is correct,” captain Erdem briefly switched his air data source selector to “alternate,” then moved it back again because the indication still didn’t match his mental model of what the aircraft was doing. However, I concluded that this was not true. The raw flight data from the FDR did show that the captain’s air data selector switch was positioned at “alternate” for 1 second at 23:44:16, but the investigation determined that the data from that particular second was corrupted and could not be used. In fact, during that exact second a large number of parameters recorded unreasonable or obviously false values, including a roll angle of 179 degrees, a 50% drop in engine fan rotation speed, a 50% decrease in airspeed and altitude, a ground speed of 500 knots, and an angle of attack of 45 degrees. All of these parameters returned to normal values by 23:44:17, as did the air data switch position. It can therefore be stated with a high degree of certainty that Captain Erdem did not touch his air data switch at that point, nor did he do so at any other time during the flight, according to the FDR.
However, Birgen also argues that when Evrenesoglu — according to the transcript, actually Erdem — announced “alternate is correct,” First Officer Gergin mistakenly reacted by moving his air data switch to “alternate.” This would have caused his instruments to draw incorrect airspeed data from ADC №1. And while he should have known in theory that this would happen, it’s already clear that the pilots of flight 301 did not understand the 757’s air data architecture, and he might not have realized that moving the air data switch to “alternate” causes his instruments to draw data from the other pilot’s ADC, rather than from the third, standby ADC.
Unfortunately, this hypothesis is impossible to prove beyond doubt because the flight data recorder did not record the position of the first officer’s air data switch, only the captain’s. Nevertheless, as we will soon see, it’s a possibility worth considering.
Immediately after Erdem called out “alternate is correct,” First Officer Gergin affirmed, “The alternate one is correct.” Birgen’s submission claims this line was spoken by Captain Erdem, but the transcript contradicts this.
Erdem then said, “As [the] aircraft was not flying and on [the] ground, something happening is usual. Such as an elevator asymmetry and other things.” Indeed, it is common for issues to arise while an aircraft is parked for a long time.
He then declared, “We don’t believe them.” Most likely he was referring to the caution messages on the EICAS display, but he might have been referring to the airspeed indicators as well; it’s hard to say.
Meanwhile, Relief Captain Evrenesoglu asked, “Shall I reset its circuit breaker?”
“Yes, reset it,” Erdem agreed.
“To understand the reason,” Evrenesoglu continued.
“Yeah,” said Erdem.
Seconds later, at 23:45:28, the overspeed warning suddenly burst to life, filling the cockpit with an ominous rapid-fire clacking sound. The warning was based on the captain’s airspeed indication, which had now risen to 353 knots, above the maximum operating speed of the aircraft. Meanwhile, the actual airspeed was below 200 knots and falling.
Correctly recognizing that the warning was based on a false indication, Captain Erdem stated, “Okay, it’s no matter. Pull the airspeed, we will see.” One of the other pilots responded by pulling the circuit breaker to cancel the overspeed warning.
One second later, First Officer Gerin said, “Now it is three hundred and fifty yes?”
Once again, this line has several interpretations. For instance, the final report states that he most likely glanced over at Captain Erdem’s airspeed indicator and read off the erroneous value. It is possible to read the other pilot’s airspeed indication, although on the 757–200 it would have taken some effort due to the size and position of the displays. If one subscribes to the interpretation that Gergin was looking at Erdem’s display when he announced “it began to operate” earlier in the flight, then this would be consistent with the assumption that he was still monitoring the captain’s airspeed indicator. However, if one subscribes to Çetin Birgen’s argument that Gergin improperly switched his air data source to “alternate,” then he could have been reading his own airspeed indication, which would have been the same erroneous value displayed to Captain Erdem. And on top of these two possibilities, I would add a third — that he was simply reciting, from memory, the maximum operating speed of the aircraft, as an explanation for the overspeed warning.
In the seconds before, after, and during this callout, events built to a startling crescendo. As you hopefully recall, the autopilot had been slowly increasing the pitch in an attempt to reduce the erroneously high airspeed. Eventually the pitch attitude reached a maximum value of 15.1˚, where it remained for a considerable period of time. It’s not entirely clear if this was the maximum pitch attitude that could be commanded in VNAV/FLCH, but that would be my assumption. But this pitch attitude was too high for the thrust setting and configuration, which is why their actual airspeed had been slowly decreasing. And as their airspeed decreased, their rate of climb also decreased, even though the pitch attitude wasn’t changing.
At 23:45 and 47 seconds, Captain Erdem declared, “Let’s take that like this,” and then a confusing series of events happened within a very short time. First, he apparently switched the vertical autopilot mode from VNAV/FLCH to vertical speed (V/S), in which the autopilot pitches up or down to maintain a climb or descent rate selected by the crew. Switching to V/S mode also causes the autothrottle to enter speed mode (SPD), in which it modifies engine thrust to achieve a target airspeed. I’m not an authority on Boeing 757 autoflight systems, but as far as I can tell, switching to V/S and SPD mode from VNAV after retracting the flaps during climb will result in a default target airspeed of 250 knots until the pilots select something else. And because the №1 indicated airspeed was at that moment close to 350 knots, the autothrottle apparently decreased thrust in both engines to idle in order to lose what it believed was an excess of 100 knots’ airspeed.
At the same time, the flight data shows that after V/S mode was selected, the rate of climb increased from 1,344 feet per minute at 23:45:46 to 1,600 feet per minute at 23:45:52, while the pitch increased from 15 degrees to 18 degrees nose up. In my opinion, this suggests that the target vertical speed was above the actual vertical speed, which caused the autopilot to pitch the nose up even further in an attempt to climb faster. The exact vertical speed that was selected is not stated in the report or included in the flight data, nor do I know whether that target value was selected by the pilot or entered by default from the flight management computer.
The final report states that during these crucial six seconds, “the flight crew reduced power and increased elevator deflection.” However, in my opinion, the flight data clearly shows that this was not the case. Up until 23:45:52, the autopilot and autothrottle were both engaged, and the increase in elevator deflection (thus pitch angle) and decrease in engine power appear to be consistent with the expected autoflight system behavior given a switch from VNAV to V/S mode at this phase of flight. Therefore, I don’t think the evidence supports a conclusion that these inputs were made by the flight crew.
Another question not answered by the final report is why Captain Erdem selected V/S mode in the first place. None of his comments on the CVR provide a clear explanation. However, the simplest assumption is that he saw that their rate of climb was decreasing below the value he would expect at this phase of flight, so he selected a mode in which he would have more control over their vertical speed. It has also been suggested that he wanted to increase the vertical speed in order to reduce airspeed in response to the overspeed warning, but in my opinion the evidence indicates that he recognized the overspeed warning as erroneous.
In any case, the sudden pitch up and decrease in thrust at a moment when the real airspeed was already below normal for this phase of flight resulted in an immediate and dramatic exit from the normal flight envelope. At 23:45:52, the stick shaker activated, vibrating the pilots’ control columns to warn of an impending stall. The aircraft was on an upward trajectory that was completely unsustainable with the current airspeed and thrust, and unless the pilots increased airspeed right away, they were going to fall from the sky.
In 1996, pilots were taught react to the stick shaker by following “approach to stall” procedures. The idea was that a stall could be avoided by increasing engine power and maintaining a reasonable pitch angle, provided that these steps were accomplished sufficiently far in advance of the stall actually taking place. “Approach to stall” training tended to focus heavily on thrust and emphasized minimizing altitude loss. But these procedures were not adequately aggressive to handle the situation that flight 301 had encountered.
Captain Erdem was probably caught off guard by the airplane’s reaction to the mode change, but the flight data shows that he began to increase engine power within one second of the stick shaker activation, exactly as he had been trained. This action also disconnected the autothrottle, but the autopilot remained engaged in vertical speed mode, where it kept trying to pitch up to achieve the selected climb rate.
The final report provides almost no useful information about the behavior of the autopilot during the period between the stall warning and its disconnection, which occurred 6 seconds later at 23:45:58. To make matters worse, the only publicly available FDR readout is missing the crucial page containing the pitch angle, elevator deflection, and stabilizer deflection data between 23:45:45 and 23:46:23. However, it is possible to say that the stabilizer setting was at 6 units at 23:45:45, just before V/S mode was engaged, and by 23:46:23 it had increased to 10 units.
The purpose of the horizontal stabilizer is to adjust the aircraft’s natural or stable pitch-speed combination. A higher setting will cause the aircraft to stabilize at a higher pitch angle and lower airspeed, while a lower setting will stabilize the aircraft at a lower pitch angle and higher airspeed. So we simply cannot ignore the fact that the stabilizer setting increased from 6 units to 10 during the period of the stall, precisely when the pilots needed to reduce pitch and increase airspeed. However, the accident report perplexingly makes no mention of this information at all.
When the autopilot is engaged, the stabilizer position is controlled automatically in order to stabilize the aircraft in the pitch-speed combination required to achieve the target aircraft state. Therefore, during the 13 seconds that V/S mode was active, it should have continuously increased the stabilizer position to try to stabilize the aircraft at a lower airspeed, because the indicated airspeed was very high. However, my understanding is that the rate of automatic stabilizer motion is quite slow, so it’s doubtful that 13 seconds is long enough to explain a deflection of 4 units. Çetin Birgen explained this discrepancy by suggesting that the autopilot had not properly disengaged due to a system fault, but there is no evidence of this, and the flight data clearly shows that the autopilot disconnected at 23:45:58 and was never engaged again. So, without knowing the exact timeframe during which the stabilizer movement occurred, or what the automatic stabilizer deflection rate is, it remains possible that some or even most of the change from 6 units to 10 units was the result of pilot action during the ensuing chaos.
Right at the moment that the stick shaker activated, flight 301’s altitude peaked at 7,264 feet before entering a shallow but steepening descent, wallowing along with its nose high in the air. The pitch attitude peaked at 21 degrees nose up right before the autopilot disconnected. The reason why the autopilot disconnected is not stated in the final report, but it could have been done manually by one of the pilots, or automatically in response to some kind of limitation exceedance.
In the cockpit, the captain and first officer both repeated the word “God” several times, while multiple alert tones chimed against the clack-clack-clack of the stick shaker. Both pilots seemed utterly paralyzed, overwhelmed by the confusing sequence of indications.
Despite the evident failure of his thrust increase to silence the stick shaker, Captain Erdem took no further action to prevent a stall. It’s quite possible that he believed the stick shaker was erroneous, much like the overspeed warning — but it wasn’t. Although stalls are often discussed with regard to the airspeed at which they occur in a given configuration, the only parameter that determines the point at which an aircraft will stall is its angle of attack, the angle of its lifting surfaces relative to the oncoming airflow. If this angle is too steep, the air will cease to flow smoothly over the wings and the aircraft will stop flying, no matter its speed. For this reason, stall warnings are generated by the angle of attack sensors, with no input from the airspeed sensors at all. That means that even in the absence of any valid airspeed indications, the stick shaker remains trustworthy. It’s not clear that the pilots of flight 301 understood this.
In the absence of any effort to avert it, the aircraft stalled. Now the only way out was to pitch steeply nose down, putting the airplane into a dive in order to restore airflow over the wings. The pilots would have known in theory that this was the way to recover, but putting it into practice was another matter entirely, especially when they were unsure of the stick shaker’s provenance.
From the jumpseat, Relief Captain Evrenesoglu possibly realized that the aircraft was in a stall and called out, “ADI” — attitude direction indicator — referring to the pilots’ pitch and roll displays. If this was a call for the pilots to pitch down, it was not straightforward enough to convey the message, and Erdem did not react. First Officer Gergin did shout, “Nose down!” then recited the Islamic phrase bi-smi llahi r-rahmani r-rahim — “In the name of God, the Most Gracious, the Most Merciful.” No sooner had those words left his lips than he again exclaimed, “Thrust!”
“Disconnect the autopilot, is the autopilot disconnected?” Captain Erdem asked. It’s possible that he was having difficulty controlling the plane in a stalled condition and had begun to wonder whether he was fighting the autopilot. Alternatively, he might have been trying to push down, only to encounter resistance due to the nose up stabilizer setting.
“Already disconnected, disconnected sir,” Gergin replied.
“ADI!” Evrenesoglu called out again.
At around this time, someone pulled back the thrust levers to idle, making the problem even worse. It is unknown who did this or why.
Over the radio, the Santo Domingo controller asked them to set a new transponder code, but Gergin simply told him to standby. This would be the last anyone heard from flight 301.
“Not climb? What am I to do?” Erdem exclaimed.
“You may level off altitude okay, I am selecting the altitude hold sir,” Gergin replied. He then pushed the “altitude hold” button on the autoflight panel. With the autopilot disengaged, the only effect of this action was to instruct the flight director, an overlay on the pilots’ primary displays, to indicate the flight control inputs required to hold their current altitude. This was not in any way useful because the airplane was in a stalled state from which recovery was only possible by trading altitude for speed.
“Okay, five thousand feet,” Gergin called out, watching their altitude tick steadily downward.
“Thrust levers, thrust, thrust, thrust, thrust!” Erdem shouted.
“Retard?” Gergin asked.
Reducing (“retarding”) engine thrust was not what Erdem wanted. “Thrust, don’t pull back, don’t pull back, don’t pull back, don’t pull back!” he yelled.
“Okay, open, open,” Gergin said, referring to “opening” the throttles.
“Don’t pull back, please don’t pull back!” Erdem repeated.
“Open sir, open!” Gergin said. “Bi-smi llahi r-rahmani r-rahim!”
Gergin pushed the thrust levers forward, but by this the angle of attack was so high that the airflow into the engine inlets had become highly oblique. In fact, the airflow into the left engine was insufficient to achieve the requested thrust, resulting in a compressor surge as compressed air inside the engine burst forward violently. This caused the left engine’s power output to drop dramatically, while the right engine spooled up as commanded.
With the aircraft already in a stalled state — essentially falling toward the ground — the sudden application of differential thrust caused it to enter a spin about its vertical axis. The airplane pitched over to a maximum nose down attitude of -53 degrees while simultaneously performing a full 360-degree left roll. From that point onward, recovery was impossible, and the fate of flight 301 was sealed.
In the cockpit, confusion surrendered to fear.
“Pull up!” Evrenesoglu shouted.
“What’s happening?” Erdem exclaimed.
“Oh, what’s happening?” Gergin repeated.
The ground proximity warning system roared to life, calling out, “SINK RATE! WHOOP WHOOP, PULL UP!”
The pilots attempted to pull out of the spin, raising the nose as high as -9 degrees, but the airplane was completely out of control. The Atlantic was rising up beneath them; they were out of time. Still locked in a hopeless spiral, the mighty 757 breathed its last.
In the cockpit, First Officer Gergin calmly stated, “Let’s do like this” — the final words on the cockpit voice recorder. Four seconds later, pitched 34 degrees nose down and banked 35 degrees to the left, Birgenair flight 301 slammed into the dark waters of the Atlantic Ocean, instantly killing all 189 people on board.
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After flight 301 disappeared from radar in Santo Domingo, a search and rescue effort was initiated. Within hours, search vessels located floating debris and bodies in the ocean approximately 23 kilometers northeast of Gregorio Luperón International Airport, strewn across the sea in a grim testament to the violence of the impact. Few of the bodies were intact, and the rescuers described finding coffee cans from the galley that had been crushed into disks by the immense forces.
Immediately after the accident, an investigation was organized under the leadership of the Dominican Republic’s Junta Investigadora de Accidentes Aéreos (Air Accident Investigation Board), or JIAA. In accordance with international law, representatives of the United States National Transportation Safety Board were invited on behalf of the state of manufacture of the aircraft, while the Turkish Directorate General of Civil Aviation was invited on behalf of the aircraft’s state of registry. The US and Turkish investigators were accompanied by representatives of Boeing and Birgenair, respectively.
A specialized salvage ship was commissioned from the United States to locate and recover the flight recorders. Arriving on the scene early in the morning on the 28th of February, the vessel’s sonar equipment detected the battery-powered pingers from the flight recorders within two hours of searching, and both boxes were successfully recovered that same day from a depth of 7,200 feet (2,190 m).
After the contents were downloaded, it quickly became clear that the flight data recorder had captured erroneous airspeed data throughout the flight. The 757’s FDR draws its airspeed data from the captain’s pitot tube only. This data showed the airspeed increasing to a peak of 353 knots shortly before the aircraft reached its maximum altitude, despite a pitch angle and thrust setting that would not have permitted such a high airspeed. In fact, these data were consistent with a pitot tube that was completely blocked, causing it to measure an airspeed proportional to altitude.
Very little wreckage was retrieved from the seabed, and the pitot tubes were neither observed nor recovered, so there was no way to conclusively determine the nature of the hypothetical obstruction. However, ice could be ruled out due to the balmy temperatures in Puerto Plata. A pitot tube cover left in place could affect airspeed readings, but maintenance crews testified that covers were not used, and a cover would have been obvious during the pre-flight inspection. In light of the fact that covers were not installed for at least two days prior to the flight, and the fact that Sceliphron caementarium is native to the area and is known to build nests inside pitot tubes, investigators concluded that the most likely explanation was that wasps had stuffed the tube full of mud.
Çetin Birgen disputed this explanation. In his own submission, he argued that there had been “numerous cases where rainwater had entered the pitot tube and had affected the operation of the connected systems.” He did not cite any examples, nor did he explain how rainwater, which should be flushed out as soon as the aircraft accelerates, could trap sea level air inside the pitot tube in such a way as to produce the recorded data.
How exactly this bad data led to the loss of 189 lives is also a matter of some dispute. In fact, throughout this article, I’ve presented competing interpretations of the pilots’ statements and actions, so you should be already familiar with the substance of most of the arguments. But it’s also worth examining why there’s so little consensus about the exact sequence of events.
Unfortunately, the final report produced by the Dominican JIAA is among the worst I’ve ever read. It is unacceptably short, lacks crucial details, and contains numerous factual errors, ranging from the age of the captain to the exact timing of the autopilot disconnection. Its analysis of the events is sometimes unsupported by the raw data, and some parts are based on apparently misattributed statements from the cockpit voice recorder. It does not appear that the investigation conducted any tests to better understand the autopilot behavior or to place the pilots’ statements into the context of what they were seeing.
The NTSB, the Turkish DGCA, Boeing, and Birgenair all wrote responses to the accident report. The NTSB and Boeing responses simply clarified a few points and requested some minor changes, while the Turkish DGCA questioned just a couple of the core findings. By contrast, Çetin Birgen’s bombastic response on behalf of his airline questioned practically everything that the JIAA wrote, including the very basis of the agency’s jurisdiction over the case. In fact, he argued that the aircraft had crashed in international waters and that in the absence of a valid “state of occurrence,” the investigation should have been led by Turkey as the “state of registration,” effectively accusing the JIAA of seizing control of the investigation without regard for international law. However, his position seems to be based on a fundamentally flawed and carelessly researched application of the definition of “international waters.” Under international law, “international waters” begin 12 nautical miles offshore, and indeed flight 301 crashed about 12.5 nautical miles away from the airport, but the crash site is actually only about 8 nautical miles from the nearest point on land, placing it firmly inside the territorial waters of the Dominican Republic.
Not all of Birgen’s arguments were as bad as this one, and some of them I actually agree with — but only when verified by the raw flight data and cockpit voice recorder transcript. Fortunately, those documents are publicly available and I relied heavily on them to tell this story. What follows now is my evaluation of the arguments made by all parties about the fundamental causes of the accident, and especially those made by the JIAA and Çetin Birgen, in light of the available hard evidence and developments in aviation safety over the last 30 years.
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In its own analysis, the JIAA argued that the pilots initially identified the faulty airspeed indicator, but failed to appropriately reject the takeoff. They were subsequently reassured that all was normal when the captain’s indicated airspeed started increasing through a range of reasonable values. The pilots then attempted to continue a normal climb, without recognizing that the autopilot was steadily pitching up. When additional abnormal indications started to appear, including the “rudder ratio” and “mach/speed trim” EICAS messages, the pilots became confused about the actual nature of the problem. The first officer accurately determined that their airspeed was decreasing, and the captain accurately observed that the standby airspeed indicator was correct, but nobody attempted to unite these observations into a plan of action. They should have carefully determined which airspeed indicators were trustworthy, handed control to the first officer, and assumed a safe pitch attitude and thrust setting, but they didn’t. Instead, they tried to troubleshoot, searching for the cause of the disparate indications before trapping their consequences. Having failed to accomplish this, they were unprepared for the false overspeed warning. Despite telling the first officer to ignore the warning, Captain Erdem hesitated to practice what he preached, and began to decrease engine thrust and pitch up, which sent the aircraft into a stall. The crew failed to recognize that the stick shaker was providing an accurate warning of the impending stall, and they did not execute the stall recovery procedures. Disjointed and irrelevant inputs made the loss of control worse, until recovery became impossible.
As reasons for this chaotic behavior, the JIAA cited a lack of training. Birgenair’s flight crews did not receive crew resource management (CRM) training, which was not required in Turkey at the time. This training would have helped them take advantage of all three pilots’ extensive experience to determine the safest course of action. But perhaps even more significantly, they had not been trained to react to an unreliable airspeed indication. Boeing’s flight crew training manual at the time assumed that pilots could handle such a situation with “little difficulty” by comparing the three airspeed indications and discarding the odd one out. But pilots at most airlines around the world were not actually trained to do this, nor were they given even basic written guidance. In 1995, Birgenair did introduce some simple unreliable airspeed guidance to its flight crew operations manual, which instructed pilots to adopt a known pitch angle and thrust setting to ensure a safe flight path before troubleshooting, but this guidance was not used in training and there is no evidence that the pilots of flight 301 knew of its existence.
As a result, the pilots did not properly integrate their observations related to the airspeed indications and did not know which warnings were likely to be false and which were likely to be true. Unsure what to do, they became paralyzed by their own confusion and did not react to save the plane.
This is the story of the accident that is most often told, but as you’ve already gathered by now, I don’t think it accurately reflects the true causes of the crash. The points about training are spot on, but not for the reasons that the JIAA claimed. I’ll go into that in more detail shortly. But first I want to similarly summarize Çetin Birgen’s argument.
Birgen’s submission was at times conspiratorial and generally worked hard to deflect responsibility from Birgenair. He called the final report “flawed and misleading in many material respects,” which it was, and wrote that it demonstrated an “obvious lack of care in dealing with facts.” But Birgen was throwing rocks in a glass house, because many of his claims were equally dubious, especially his argument that Turkey should have led the investigation, and his argument that Captain Erdem selected his alternate air data source then switched it back, which was based on obviously corrupted data. So, discounting the clearly false parts of his account, what follows is a distilled version of Birgen’s overall argument.
In his view, the pilots recognized that the captain’s airspeed indicator was faulty from the very beginning and maintained this understanding throughout the flight. They chose not to reject the takeoff because the aircraft had already entered the high speed regime, and a fault with one airspeed indicator did not seem to outweigh the risks of a rejected takeoff. Subsequently, when the first officer said “it has begun to operate,” he meant that LNAV mode was armed. The pilots didn’t understand that the autopilot was using the captain’s false airspeed data because the air data architecture was not sufficiently covered during the pilots’ recurrent training in the United States. The next sign of a problem was the appearance of the “rudder ratio” and “mach/speed trim” EICAS messages, which were unhelpful because the procedures associated with these messages made no mention of the possibility that they could be triggered by a false airspeed reading. But the pilots didn’t even consult those procedures because the messages were marked as advisory only.
When the overspeed warning sounded, Captain Erdem correctly recognized that the warning must be false because even the overpowered Boeing 757 can’t achieve such a high speed with the engines in climb power and a pitch of 15 degrees nose up. However, at the same time, First Officer Gergin possibly reacted to the call that “alternate is correct” by switching his air data source to alternate, causing him to receive the same false airspeed data as the captain. This error was caused by a lack of understanding of the air data architecture, the details of which were not provided to him during training or in the manual. Without a valid airspeed reading on either display, the flight crew became confused about their actual airspeed. The autopilot kept pitching up until the stall warning sounded. According to the Boeing 757 training manual, the pilot should react to an approach-to-stall with the autopilot engaged by increasing engine power to return the aircraft to a normal airspeed, and if this doesn’t work, disengage the autopilot. Captain Erdem immediately performed these exact actions.
After the stall began, Birgen argued that the autopilot actually re-engaged due to a “known fault” with the 757’s autopilot system, which could cause the autopilot to re-engage following an automatic disconnection if the pilot didn’t follow up by pressing the autopilot disconnect button. He claimed this fault was known to Boeing and had been observed in the past but provided no examples nor any credible information related to this alleged issue. In his opinion, this could have caused the autopilot to re-engage in altitude hold mode, resulting in pitch-up inputs on the elevator and stabilizer that made it harder for Erdem to recover from the stall. However, the FDR data shows that the autopilot didn’t re-engage at any point, and in the absence of any evidence whatsoever, this argument can easily be dismissed.
In Birgen’s opinion, the cause of the accident was Boeing’s failure to provide flight crews with the technical information and procedures required to recognize that an unreliable airspeed event was occurring, predict its consequences, and respond in an appropriate manner.
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After weeks spent researching this accident, I came to believe that neither the JIAA nor Çetin Birgen articulated a sequence of events and causal factors that adequately reflects the evidence. I’ve already shared my opinions at various points throughout this article so far, but I will summarize them again here. But once again I must highlight that what follows is fact-based, but is not fact; it is not the product of a professional investigation. It is my personal opinion based on examination of the raw evidence.
My belief is that Captain Erdem made a judgment call that it was safer to take off than to abort, because the first officer had a valid airspeed reading. A pilot today might not necessarily make the same decision, but at that time a single inoperative airspeed indicator was thought to present “little difficulty” to the flight crew.
I think Erdem was aware throughout the flight that his own airspeed reading was incorrect, and I think the first officer’s call that “it has begun to operate” either was not referring to the airspeed indicator at all, or did not cause Erdem to believe his airspeed indicator. His actions throughout the flight are inconsistent with someone who believed the airplane was flying at an unreasonably high speed. However, due to an inadequate understanding of the air data architecture and autoflight systems, he did not recognize that the center autopilot would fly the airplane using air data from the left pitot-static system. The pilots did not adequately monitor their pitch attitude, probably because they didn’t expect the autopilot to behave in a deviant manner. The “rudder ratio” and “mach/speed trim” EICAS messages were confusing and might have sowed a lack of trust in the aircraft’s warning systems. This was compounded when First Officer Gergin reported that his airspeed indication was falling below 200 knots, which did not correlate with Erdem’s belief that the autopilot was flying the plane normally. He then concluded that Gergin’s airspeed display must also be incorrect. By this point the situation would have been quite concerning, but with no training on what to do next, Erdem continued to trust that the autopilot would fly the plane in a safe manner while he tried some basic troubleshooting. His call to disregard the overspeed warning was consistent with his understanding that his very high airspeed indication was erroneous.
Eventually, the increasing pitch angle caused the airplane to bleed off too much speed to maintain the desired climb rate, which became the first indication to Erdem that the autopilot was not behaving normally. Unsure why VNAV was not producing the expected climb rate, he decided to take more direct control of their flight path by engaging V/S mode with a higher climb rate selected. This caused the autopilot and autothrottle to switch from a continuous thrust, speed-on-pitch philosophy to a speed-on-thrust, vertical speed-on-pitch philosophy. Because the indicated airspeed was 350 knots while the probable target airspeed was 250 knots, the autothrottle reduced thrust to idle in order to slow down, while the autopilot increased pitch to its command limit in an attempt to make the aircraft climb faster. This combination of inputs caused the angle of attack to exceed the stick shaker threshold within six seconds.
After the stick shaker activated, Captain Erdem responded according to his training by increasing engine power. However, he didn’t apply maximum power, he didn’t pitch down, and he didn’t disengage the autopilot. I think at this point he still believed that the autopilot would react appropriately with his help. Six seconds later, the autopilot did disconnect, possibly because Erdem had realized that it was actively trying to stall the airplane. But by then it was too late; the stall had already taken place. Once in a fully developed stall, the only way to escape was to trade altitude for airspeed by pitching down well below the horizon — but pilots in 1996 had no opportunity to practice this. Simulator training focused on preventing stalls by increasing engine power and maintaining a slight nose-up attitude, which is what he did. When this failed to silence the stall warning, he might have concluded that the warning was false, without realizing that the stick shaker operates independent of airspeed.
After this point, the actions of the crew became chaotic and difficult to explain. I was not able to determine beyond reasonable doubt why the stabilizer setting increased to 10 units, but it probably resulted in control forces that captain Erdem would have found confusing. Someone also decreased thrust to idle for a considerable period of time, which I think might have been done by First Officer Gergin, given that Captain Erdem seemed quite emphatic about his desire that thrust should be increased. None of the pilots recognized that they were in a fully developed stall. Eventually, the left engine suffered a compressor surge, and differential thrust sent the aircraft into a spin from which recovery was impossible.
In my opinion, the causes of this accident were a lack of systems knowledge, which led to incorrect decision-making, especially with regard to the autoflight systems; and a lack of training and procedures for an unreliable airspeed emergency. The nature of stall training at the time, which didn’t allow pilots to practice recovering from a fully developed stall, also may have contributed.
The JIAA, Birgen, and I all agree that the failure to cover the pitot tubes while the aircraft was parked contributed to the accident, although Birgen argued that this wasn’t technically required. The JIAA also argued that Captain Erdem’s decision not to abort the takeoff was a contributing factor, but Birgen and I both agree that his decision would have seemed reasonable in the moment. The JIAA and I also agree that the pilots might have been fatigued.
Overall, I think the case of Birgenair flight 301 vividly illustrates how an unreliable airspeed emergency can quickly overwhelm even a relatively competent flight crew in the absence of clear guidance on what to do. In this case, the pilots had two valid airspeed readings available to them, and if Captain Erdem had simply disengaged the autopilot and handed over control to First Officer Gergin, the rest of the flight probably would have been uneventful. Instead, a single wasp’s nest in a single pitot tube led to a complete loss of control because the pilots didn’t understand that bad data on their instruments could mean the autoflight systems are getting bad data too. As I said earlier in this article, that fact seems self-evident now, but that’s in part because today’s pilots already know stories like this one. And they know these stories because unreliable airspeed events are rigorously drilled in initial and recurrent training — but that wasn’t the case in 1996. The crash of Birgenair flight 301 intensified calls for such training, but it would take another accident for those voices to reach their crescendo.
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Part 2: Panic in Peru
Founded in 1973, Aeroperú was the flag carrier of its namesake nation for 26 years until its bankruptcy in 1999. The airline was initially state-owned before undergoing gradual privatization starting in 1981, which culminated in the purchase of a controlling stake by Mexican flag carrier Aeroméxico in 1993. By 1996, its fleet included McDonnell Douglas DC-10s, Fokker F-28s, Boeing 727s, and of course several Boeing 757–200s, the same model involved in the crash of Birgenair flight 301.
The Boeing 757 at the center of this story bore the United States registration N52AW, although it was owned and operated by Aeroperú. I wasn’t able to determine why the aircraft was registered in the US, but everything else about its operation appears to have been normal, unlike the sketchy origins of Birgenair.
Although Aeroperú outsourced heavy maintenance of its 757s to parent company Aeroméxico, regular line maintenance and repairs were accomplished in-house at Jorge Chávez International Airport in Lima. That was where N52AW found itself on the 1st of October 1996 after suffering a bird strike into its right engine.
Although the damage from this incident was relatively minor, it did require technicians to replace two fan blades and repair the engine hydraulic pump. But that’s not key to this story — what is key is that the work also involved polishing the underside of the forward fuselage. The final report doesn’t say whether this was to clear dust and debris left by the maintenance work, or to clean off bird viscera, but my imagination tends to pull me toward the latter.
The underside of the forward fuselage happens to contain the static ports for all three pitot-static systems, which could be contaminated if left open during cleaning. For this reason, some airlines have special covers that can be placed over the static ports, but Aeroperú lacked anything of the sort. Instead, Aeroperú technicians decided to cover the static ports with silver aviation-grade masking tape. As far as I’m aware, this isn’t inherently unsafe, so long it’s removed again before flight. But as it turns out, that’s a huge caveat that greatly affects the overall wisdom of the decision.
The problem with this tape was that it was almost the same color as the lower fuselage itself, which was silvery-gray in Aeroperú’s livery. Due to this color similarity, the tape was difficult to spot during a cursory inspection unless the technicians explicitly divulged that they had put it there. But in this case, a breakdown of communication occurred. Details are scarce, but the final report states that the supervisor in charge of the work was replaced at some point due to illness, and the new supervisor then appointed a new mechanic to “attend to the aircraft on the apron.” Neither of these individuals noticed the tape during final inspection of the work. However, the exact roles and expectations of these people are not explained in the report and it remains unclear from that document who was responsible for taking the tape off. One technician was later convicted of negligent homicide as a result of the events, but he was widely considered a scapegoat.
As you’ve probably guessed, the tape was inadvertently left in place. The final report doesn’t explicitly state that the tape was left on all three sets of static ports, and in fact only the captain’s and first officer’s static ports were found, so the status of the standby system is technically unknown. Updates from the first few months of the official investigation suggest that investigators did not yet know whether all the static ports were covered, but the cockpit voice recorder transcript doesn’t support a conclusion that they had accurate pitot-static information. In fact, the transcript clearly suggests the opposite. This isn’t particularly surprising, because if mechanics had removed the tape from one set of static ports, it’s difficult to understand why they would not remove the tape from the rest of the ports too. So my assumption going into this story is that all the static ports were taped over.
Later that night, the aircraft was released from maintenance to perform the scheduled flight 603 from Lima to Santiago, Chile. This red-eye flight was lightly booked, with just 61 passengers scattered throughout the 757. Nine crewmembers were also rostered, including two flight crew.
The captain was 58-year-old Eric Schreiber Ladrón de Guevara, a highly experienced pilot with nearly 22,000 flying hours, including over 1,500 on the Boeing 757. The first officer was 42-year-old David Fernández Revoredo, who was also fairly experienced, with almost 8,000 total hours, including 719 hours on the 757. The final report doesn’t include detailed information about their career backgrounds or type rating histories.
After arriving at the aircraft sometime around midnight, Captain Schreiber conducted a routine walkaround inspection, searching for anything out of the ordinary. But on the poorly lit apron, the silver tape against the silver fuselage was very difficult to see, positioned as it was above Schreiber’s head height. That being said, the thoroughness of his walkaround inspection is unknown; all that can be said with confidence is that he didn’t see the tape.
The effect of tape over the static ports is not the same as the effect of a blocked pitot tube, nor is it particularly intuitive. Because tape is somewhat porous, a small amount of air will leak through the tape, but at a very reduced rate. Before takeoff, the pocket of air trapped inside the port would be pressurized to sea level, but as the aircraft climbs, this pocket will slowly depressurize until it eventually equalizes with the lower ambient pressure at altitude. Then, if the aircraft descends, the air will take an equal amount of time to leak back through the other way. The effect is to create a lag of several minutes in the measured static pressure.
This “lag” will result in an altitude reading that is too low when climbing, becomes accurate after a period of level flight, then becomes too high when descending. And because airspeed is calculated based on the difference between pitot pressure and static pressure, the erroneously high static pressure during climb will cause the airspeed to read too low, and the erroneously low static pressure during descent will cause the airspeed to read too high. This makes the false indications significantly more complicated than in the case of a single blocked pitot tube, where airspeed increases proportionally with altitude. This is especially true if all the static ports are blocked, as they most likely were in this case, not only because all three altitude and airspeed readouts will be wrong, but also because they might be wrong in different ways, due to differences in the speed of air leakage through each strip of tape.
In this case, the usual tactic of comparing the three indications and eliminating the odd one out will be ineffective. Instead, the solution is to evaluate the reasonableness of each indication with respect to the pitch attitude and thrust setting. If all three airspeed indications are unreasonable, then the pilot can still approximate their speed by referring to the ground speed indicators, which are independent of the pitot-static system but don’t account for wind. Height above the ground can also be determined by using the radio altimeter, which on the 757 is functional below 2,500 feet and is independent of the pitot-static system. Above that height, it may not be possible to reliably determine the aircraft’s altitude.
Everything I just said is included in basic unreliable airspeed training today. However, as I said in Part 1, that training did not yet exist in 1996 outside of a few airlines at the forefront of safety. Neither pilot on Aeroperú flight 603 had received such training, nor had they received any guidance on what a blocked pitot tube or static port might look like. No unreliable airspeed procedure existed in the quick reference handbook.
Both before and after the Birgenair accident, several US airlines began developing FAA-approved emergency procedures for unreliable airspeed events, but these procedures were not added to Boeing’s official 757 manuals, nor were they shared with other airlines around the world. The Birgenair crash prompted the US NTSB to write a letter urging the FAA and Boeing to make several changes, including updating the 757 flight manual to indicate that the appearance of the “rudder ratio” and “mach/speed trim” messages could indicate unreliable airspeed; developing a computerized cross-check that can warn the pilots if the airspeed is unreliable; developing and distributing an official emergency procedure for an unreliable airspeed event; and requiring unreliable airspeed scenarios during training. However, by October 1996 none of these measures had been implemented, nor was anyone at Aeroperú aware that the recommendations had even been issued.
This type of communication failure has become much less common with the advent of the internet, which has provided a platform for companies and pilots to actively seek out official information about contemporary accidents. By contrast, in 1996 most pilots still relied on their employers to distribute this information through print publications. Therefore, because Aeroperú had not distributed the interim recommendations, it’s unlikely that the pilots of flight 603 knew much about the still-ongoing investigation into Birgenair flight 301 beyond whatever was reported in the Peruvian media.
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At 00:41 local time, now on the 2nd of October, Aeroperú flight 603 lined up on Lima’s runway 15 and received clearance for takeoff. The weather conditions were ideal, with a single cloud layer between 900 and 3,100 feet, and only 3 knots of wind. With First Officer Fernández at the controls and Captain Schreiber monitoring the instruments, the pilots set takeoff thrust, and the 757 sped away down the runway. Because the static pressure was correctly reading sea level, everything appeared normal, and the airspeed indicators dutifully reported their actual airspeed. Captain Schreiber called out 80 knots, then V1, and finally “rotate,” prompting Fernández to lift the aircraft off the runway. Schreiber then announced “positive rate,” and Férnandez ordered “gear up.”
A few seconds later, immediately after raising the gear, Schreiber noticed that something was amiss. “The altimeters are stuck,” he said, noting that his altimeter was reading near zero, and presumably both others were, too.
Two seconds later, an electronic voice called out, “WINDSHEAR! WINDSHEAR!”
This warning was generated by the reactive windshear warning system, which monitors aircraft performance in order to detect sudden changes in wind speed and direction (windshear) that may be affecting the flight path. In this case, because the vertical speed was zero with the aircraft at full takeoff thrust and a nose-up pitch, the windshear computer interpreted this set of readings as evidence that the aircraft was encountering a severe downdraft.
Neither pilot commented on the warnings, which were clearly false given that the tower was reporting negligible wind, nor was there any real weather activity in the airport vicinity at all. Instead, Captain Schreiber again called out, “Hey, altimeters have stuck!”
“Yeah,” said Férnandez, glancing around the cockpit. “All of them.” At an altitude of just 300 feet, he leveled off momentarily, unsure how to proceed with no altitude readouts.
“This is really new,” Schreiber drily commented. “Keep V2 plus ten, V2 plus ten.” Keeping a consistent known airspeed would help ensure that the aircraft remained on a safe trajectory as it moved out over the Pacific Ocean. Fernández acknowledged, and the aircraft began to climb again, vanishing from sight into the bottom of the cloud layer. Neither pilot realized that as they climbed higher, their airspeed would read further and further below the actual value.
For some time, the pilots expressed confusion about their airspeed readings. From the contents of their conversations, it’s possible that Fernández was unable to keep the indicated airspeed above V2 plus ten knots without pitching forward, causing Captain Schreiber to call for 10 degrees pitch. This was a reasonable course of action consistent with guidance for an unreliable airspeed event.
Into the middle of this confusion, the “rudder ratio” and “mach/speed trim” messages appeared as a result of the increasing divergence between the various airspeed readings, exactly like on Birgenair flight 301. Captain Schreiber read these messages aloud but didn’t appear to understand what they meant. Férnandez then attempted to engage the center autopilot, but it wouldn’t connect.
While Schreiber tried to figure out the meaning of the “rudder ratio” messages, Fernández attempted to switch his air data source to “alternate,” but this made no difference because all data sources were erroneous. “Let’s go back to basic instruments, everything has gone shit,” he said. He then keyed his microphone and reported, “Tower, Aeroperú 603, we are in an emergency… we have no basic instruments, no altimeter, no speedometer, we declare an emergency!”
By now, the altitude readout had started to creep upward as pressurized air in the static ports leaked through the tape — but each altimeter was showing something different. Amid the resulting confusion, both pilots cursed the mechanics who had worked on the plane — “What shit have they done!?” Schreiber exclaimed.
At 00:45, Fernández began requesting vectors for an approach back to Jorge Chávez, but Schreiber told him “Not yet, not yet, let’s stabilize.” He began to read from the quick reference handbook section on the “rudder ratio” caution, while Fernández acknowledged the controller’s instructions to turn right onto heading 330 and maintain present altitude. But what altitude was that? Fernández’s altimeter was showing 4,000 feet, but it didn’t stop increasing when he tried to level off. “What level do we have, do we have 4,000 feet?” he transmitted, hoping that the Lima departure controller could verify their altitude using radar.
Unfortunately, what seemed like a good idea on paper was actually a major mistake based on an incorrect understanding of both the 757’s air data architecture and the capabilities of ATC radar. In fact, controller’s secondary radar doesn’t independently determine an aircraft’s altitude; instead, it simply displays the barometric altitude transmitted by the plane’s own transponder, which comes from the pitot-static system. So as it turns out, the controller was seeing the same wrong altitude readout that the pilots were — and by verifying that the readouts were the same, he inadvertently encouraged the pilots to trust an instrument that was providing false information.
For several minutes, the pilots continued their futile struggle to understand what the aircraft was doing, repeatedly expressing their frustration without zeroing in on a course of action. They were unable to determine why the rudder ratio caution had appeared, which disproportionately occupied their attention. Throughout this time, they continued to fly to the southwest over the ocean instead of turning north toward the heading provided by ATC, apparently because they wanted to get as far away from terrain as possible.
With the altimeters now indicating nearly 10,000 feet, they debated what altitude to level off at, without being entirely sure what altitude they were really at. They also realized that they had forgotten to retract the flaps after takeoff, which they now did. For some time they argued about whether the autopilot was on or off. Eventually they appeared to level off at 12,000 feet, but their actual altitude is uncertain and was probably fluctuating, with an estimated peak around 13,000 feet. The pilots continued to observe that their airspeed was unreasonable for the current thrust and pitch, prompting them to switch the air data source again, but this made no difference.
Finally, the crew decided to fly the instructed heading 330 in an effort to intercept the landing course. This heading took them outbound parallel to the extended centerline of runway 15. ATC later modified their suggested course to 360, or due north. After confirming that the crew were able to read the Lima VOR radio beacon, the controller then instructed them to cross radial 315 of the VOR, then turn to intercept the landing course and descend to 4,000 feet, in the event of a loss of communication.
In the cockpit, the pilots struggled to understand their airspeed readouts amid multiple changes in engine power. Eventually, Fernández again gave up and asked air traffic control to help read out their speed and altitude, which was moderately helpful because the controller did have access to their ground speed — but of course, so did the pilots.
In the meantime, Fernández attempted to initiate a descent, but even with the engines at idle, their airspeed continued to increase. (Remember what a blocked static port does to the airspeed during descent?) “We have all engines cut and it’s accelerating… accelerating!” Fernández exclaimed. He extended the speed brakes in a further attempt to slow down, but this seemingly had no effect.
Seconds later, the overspeed alarm sounded, triggered by the erroneously high airspeed readings. “What would be the real speed?” Schreiber asked.
“This one is okay,” said Fernández. “They are okay, the speed… airspeed…”
“But with all power cut down, it can’t be, with all power cut down… there’s a problem with the source instrument,” Schreiber reasoned. The pilots again messed with the air data source selector, to no avail. Schreiber then began trying to work out what inputs would get them where they needed to go with no airspeed readout: “Let’s see, how many miles… at 30 miles from Lima, we start descending with spoilers and flight level change…”
“You are crossing the 260 [radial] of Lima, at 31 miles west,” ATC reported. “Flight level is 10,700, and approximate speed is 280 [knots] over the ground.” Once again, the altitude was false, but the ground speed was correct.
“Yeah but we have an indication of 350 knots here,” Fernández replied, recognizing that their airspeed and ground speed were much too far apart to be realistic.
Moments later, the overspeed alarm sounded again. “Fucking shit! I have speed brakes!” Fernández shouted in exasperation. “Everything has gone, all instruments went to shit, everything has gone, all of them!”
As if to punctuate his confusion, the stall warning suddenly activated, and the heavy clack of the stick shaker broke through the high-pitched rattle of the overspeed warning. Incredibly, the airplane was telling them that they were flying too fast and too slow at the same time. The confusion this must have caused is difficult to fathom. But once again, the overspeed warning was false, while the stall warning — derived from the angle of attack sensors, not the pitot-static system — was very much real. By reducing engine power and extending the speed brakes, Fernández had caused the airplane to bleed away speed extremely quickly, causing the angle of attack to increase in order to maintain lift, and now the angle of attack was approaching the critical point. Any higher and the wings would cease to generate lift, and the airplane would fall from the sky.
“We are going down!” Fernández exclaimed. “I don’t think so… it can’t be overspeed.” He managed to arrest the loss of speed just before the aircraft actually stalled, preventing a catastrophic situation from developing, but the stick shaker continued to activate intermittently.
At this point, Fernández had a novel idea: what if they flew in formation with another aircraft, allowing them to match its speed and altitude, like following a pace car in motorsport? Calling ATC, he asked, “Is there any aircraft that can take off to rescue us?”
Schreiber was apparently taken aback by the idea. “Er… wait, no, no no, no,” he exclaimed.
“Any plane that can guide us, an Aeroperú that may be around? Somebody?” Fernández continued.
“Don’t tell him anything about that!” Schreiber admonished.
“Yes, because right now we are stalling,” said Fernández.
“Attention, we have a 707 that will depart to Pudahuel [Santiago Airport], we are telling him,” said the controller.
“We are not stalling! It’s fictitious, it’s fictitious!” Schreiber insisted. Much like the Birgenair pilots before him, he didn’t understand that the stall warning remains valid even if all pitot-static data is lost.
“No! If we have stick shaker how would it be not?” Fernández argued.
“Shaker… but it is… but even with speed brakes on we are maintaining 9,500,” Schreiber said, sounding confused. Indeed, their altimeters were still reading 9,500 feet, despite the fact that the aircraft was descending. “Why do we read the same? I don’t understand… power. What power do we have?” he asked.
Fernández read a value of 395 knots off his airspeed indicator; the overspeed warning was still rattling away in the background.
“Aeroperú 603, you have turned slightly to the left, now you are heading 320 and your level is 100 [10,000 ft], approximate speed of 220 knots and a distance of 32 miles northwest of Lima,” the controller transmitted.
The pilots reacted again with confusion. They had been trying to descend for approach, why were they still at 10,000 feet? That didn’t seem possible. Fernández started frantically searching for a switch to silence the overspeed warning, although the only way to do this is by pulling the circuit breaker.
“Nine thousand feet if it indicates you… it’s fictitious, everything is fictitious, all the pitot has gone, the air data has gone to shit,” Schreiber exclaimed.
By now, Aeroperú maintenance was in contact with ATC, having been made aware of the ongoing situation. On behalf of maintenance, the controller asked flight 603 if their flight computers were working, and Fernández simply replied that all their instruments were wrong.
At 01:02–20 minutes after takeoff — ATC reported, “The 707 will be ready in some 15 minutes to fly west and help you.” But at this rate, staying airborne for 15 more minutes was a tall order.
Suddenly, the ground proximity warning system roared to life, calling out “TOO LOW, TERRAIN! TOO LOW, TERRAIN!” The warning sounded continuously for the next 45 seconds, repeating no less than 22 times.
“What’s happening?” Schreiber asked.
“Too low, terrain,” Fernández repeated.
“Let’s go left,” Schreiber declared. With the altimeters still showing nearly 10,000 feet, he apparently concluded that they had strayed over land and were approaching Peru’s coastal mountains, which exceed 10,000 feet within 35 nautical miles of the coast. Therefore, his suggestion was to turn left, pointing the plane out to sea and away from the high terrain. But in fact, they were already over the water, and the GPWS sounded because they were descending into the ocean — Schreiber just couldn’t tell, because it was a cloudy night, and the ground was invisible. It must also be noted that the old style GPWS installed on flight 603 was independent of the pitot-static system; it operated using the radio altimeter, and its warnings were valid.
As if to make matters even more confusing, after a few seconds the GPWS alarm was joined by three “WINDSHEAR!” callouts. This indicates that their actual height above the water was less than 1,500 feet, because the reactive windshear warning is inhibited above that altitude.
“We have a terrain alarm, we have a terrain alarm!” Fernández reported to the controller.
“Roger, according to monitor… it indicates flight level one zero zero [10,000 ft] over the sea, heading a northwest course of 300,” the controller replied.
“We have a terrain alarm and we’re supposed to be at 10,000 feet!?” Fernández said.
“According to monitor you have 105 [10,500],” the controller repeated. This report reassured the pilots that the terrain alarms were false — but they weren’t.
“We have all computers crazy here,” Fernández reported.
“Shit, what the hell have these assholes done,” Schreiber cursed.
After a back-and-forth exchange with ATC about why they were flying west, they again compared their speed readouts: 370 knots airspeed on the aircraft, 220 knots ground speed on radar.
“Shit! We will stall now,” Schreiber exclaimed. 220 knots in their configuration was too slow.
Four times, the GPWS called out “SINK RATE!” as the aircraft suddenly lost altitude. “Let’s go up, let’s see, let’s go up here,” Fernández decided, increasing thrust and pitching up to put the airplane into a climb. This action thankfully saved the plane both from a possible stall and possible ground impact. The overspeed warning continued to sound, while the altimeter continued to indicate above 9,000 feet, leading to another long and confused exchange between the pilots, but eventually they leveled off at a real altitude thought to have been about 4,000 feet.
With the aircraft now in apparently stable, level flight, despite the overspeed alarm, Schreiber decided that they were now in a position to maneuver for approach. The pilots made a rudimentary attempt to determine which instruments were trustworthy, and concluded that the only thing they could trust were the artificial horizons. Fernández then began another descent in order to intercept the instrument landing system for runway 15, from a reported altitude of 9,700 feet — but they were actually less than half that high.
At 01:08, ATC reported that the 707 was taking off to assist them, then instructed them to fly heading 070 to intercept the ILS. By this point they were 50 nautical miles west of Lima with a ground speed of 270 knots, which was within the safe range. Still, the pilots seemed confused by the overspeed alarm as Schreiber again said, “How can it be flying at this speed if we are going down with all the power cut off?” But the airplane was under control, and as he continued their descent, Fernández commented that it was “flying well.”
Suddenly, at 01:10 and 17 seconds, the GPWS again started calling out, “TOO LOW, TERRAIN!” But at the exact same moment, the controller affirmed that their altitude was still showing 9,700 feet. “What is the indicated altitude on board? Have you any visual reference?” he asked.
“9,700 but it indicates too low terrain,” Fernández replied. “Are you sure that you have us on the radar 50 miles?”
As the GPWS continued to blare, Schreiber said, “Hey look, with 370 we have…”
“Have what? 370 of what?” Fernández asked. “Do we lower the gear?”
“But what do we do with the gear? Don’t know… that…” Schreiber started to say. Apparently he was worried about extending the landing gear while above the gear’s maximum rated speed. Their airspeed indicators showed way above the limit, so he might have been worried that the gear wouldn’t extend, or perhaps he was simply uncomfortable extending the landing gear without knowing their real speed. Regardless, the debate was purely academic, because they were still 50 miles from the airport.
At that moment, 40 seconds after the GPWS first sounded, traveling at 250 knots with a 5 degree bank to the left, the 757’s trailing left wingtip struck the surface of the Pacific Ocean, sending a massive shudder through the aircraft.
“We are impacting water!” Fernández shouted over the radio. “Pull it up!”
“Go up, go up if it indicates pull up!” the controller urged.
“I have it, I have it!” Schreiber shouted. Pulling hard on the controls, he managed to drag the wingtip out of the water, but it was too late. The impact severely damaged the left wing, leading to an imbalance of lift that was impossible to overcome. The aircraft climbed to a height of just 300 feet, then began to fall, rolling inexorably left despite the pilots’ desperate efforts to level the wings.
“We are going to invert!” Schreiber yelled.
“WHOOP WHOOP, PULL…!” blared the GPWS. But the alarm was cut off mid-annunciation by a tremendous crash as the 757, banked 70 degrees to the left, plowed headlong into the Pacific. The last sound on the cockpit voice recording was the dreadful roar of the impact. The time was 01:11 a.m. and 16 seconds.
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As soon as the aircraft disappeared from radar and radio contact, emergency services were notified, and vessels were dispatched to the flight’s last known location. Due to poor organization, aerial assets were not deployed until later that morning, but by then a Peruvian navy ship had already found floating wreckage and bodies on the ocean 48 miles northwest of Lima. Rescuers determined that none of the 70 passengers and crew had survived.
Later, the US Navy recovered a small amount of wreckage from the sea floor on Peru’s behalf. Among the recovered items was a mangled section of the lower forward fuselage containing the captain’s static ports. In a remarkable feat of adhesive technology, the strip of silver masking tape was still attached.
The investigation by Peru’s Accident Investigation Board (AIB) found that the accident was precipitated by maintenance missteps, as technicians covered the static ports with tape that was insufficiently conspicuous. Multiple layers of personnel, including the flight crew, subsequently failed to notice and remove the tape. In a letter written after the accident, the US NTSB noted that Airbus and McDonnell Douglas provided brightly colored static port covers for their aircraft, and at least one US airline covered their static ports using tape with brightly colored streamers attached. Neither of these products was available at Aeroperú’s maintenance facility in Lima. As a result, the NTSB and the AIB both recommended requiring the use of brightly colored flags or covers when protecting the static ports.
The loss of all static pressure data created a situation that was extremely difficult for the flight crew to understand. Broadly speaking, the airspeed data started off correct, while the altitude read too low. The airspeed then slowly started reading too low as well, until the indicated altitude nearly caught up with the actual altitude. After the plane started descending from 13,000 feet, the airspeed and altitude both started reading too high, prompting the first officer to pull back power and deploy the speed brakes. This slowed the aircraft too much, leading to a near stall. The airplane descended to less than 1,500 feet above the ground, then climbed back to 4,000 feet, during which time the altitude continued to read around 10,000 feet. After receiving valid ground speed data from ATC, the pilots more or less stabilized the flight path, then initiated a descent toward the airport. This descent continued until the aircraft struck the water.
Unlike Birgenair flight 301, Aeroperú flight 603 was under control when it impacted the ocean. The aircraft collided with the sea primarily because the flight crew trusted their erroneous barometric altimeter readings, leading them to believe that they were much higher than they actually were. This misplaced trust developed because the controller kept verifying their false altitude readout using his secondary radar display. Neither the pilots nor the controller understood that ATC radar gets its altitude data from the aircraft’s own pitot-static system, which the pilots already knew was untrustworthy.
If the pilots had realized this, they could have descended until the radio altimeter came alive at 2,500 feet, then leveled off. The aircraft could then have been flown safely back to the airport at this altitude. The pilots also had ground speed indicators available in the cockpit that could have been used at an early stage to determine the reasonableness of the airspeed indications. Unfortunately, neither of these alternate systems were used.
It must be noted that it’s very easy to say, from the comfort of an armchair 30 years down the road, that they should have just used their radio altimeter and ground speed indicators. In reality, these kinds of outside-the-box ideas need to be nurtured in a stable environment. In this case the pilots never felt that the aircraft was sufficiently stabilized to engage in systematic troubleshooting, so alternative means of determining their speed and altitude were never properly considered. This is why the very first action in response to unreliable airspeed or altitude indications must be to assume a known pitch and thrust combination that will keep the aircraft in a safe, shallow climb — because there’s no time to think holistically when the airplane is threatening to stall or strike the ground.
Overall, the pilots’ actions were characterized by chaos and confusion, punctuated by moments of clarity that sadly didn’t translate into a successful outcome. Like the Birgenair flight crew, they were faced with a situation that was recoverable but ended in tragedy because of a lack of knowledge — a lack of knowledge of how the aircraft uses air data; a lack of knowledge of the symptoms of a pitot-static failure; and a lack of knowledge of strategies to ensure continued safe flight with unreliable airspeed and altitude indications. And like the Birgenair pilots, this lack of knowledge stemmed from a lack of relevant training.
While the Aeroperú and Birgenair accidents were different in some key ways, they are mentioned in the same breath not only because they involved the same type of plane and happened in the same year, but because they both could have been prevented by the implementation of unreliable airspeed training. And that brings me to the final part of this story — a survey of the extensive measures that have been implemented to prevent this sort of tragedy over the last three decades.
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Part 3: The Legacy Left Behind
Mere days after the crash of Birgenair flight 301, German authorities banned the airline from flying in Germany, which caused Birgenair to immediately cease operations, followed by bankruptcy later that year. Aeroperú, being a larger flag carrier, held on longer, but it too declared bankruptcy and ceased operations in 1999 following a series of costly settlements on behalf of the victims of flight 603. But the legacy of each disaster also profoundly advanced industry knowledge of the unreliable airspeed problem, ultimately kicking off a series of safety improvements that have completely transformed the way pilots handle this type of emergency.
This series of changes began within months of the two crashes, as Boeing drafted an unreliable airspeed emergency checklist for the 757 and introduced new EICAS messages that would alert the pilots if their altitude or airspeed indications disagreed. The checklist eventually spread to all Boeing models.
Soon, training scenarios based on unreliable airspeed events began to proliferate. Instead of relying solely on a checklist, manufacturers and airlines began to introduce “memory items” — actions that the pilots should take immediately from memory. On Airbus aircraft, for instance, these memory items called for the pilots to apply takeoff/go-around (TOGA) thrust and adopt a safe pitch angle, either 15 degrees below 1,500 feet; 10 degrees between 1,500 and 10,000 feet; or 5 degrees above 10,000 feet. Only then would the pilots begin troubleshooting using the checklist. All major manufacturers also introduced new quick reference handbook (QRH) pages that contain tables of thrust and pitch settings that will result in a safe flight path at a variety of aircraft weights and in different configurations, which allow pilots to quickly stabilize the situation in any phase of flight. Airlines in most countries are also now required to repeatedly expose pilots to these scenarios during recurrent training.
Later, Airbus introduced a backup speed scale (known as the BUSS). If, after following the checklist, the pilots determine that all air data units are unreliable, the checklist instructs them to turn the air data units off, which will cause the BUSS to appear. The BUSS replaces the normal airspeed scale with an angle of attack scale, depicting the boundaries of safe flight in terms of AOA instead of airspeed. In 2016, this system was further updated on some Airbus models to allow the pilots to engage the BUSS with the push of a button.
But even this tool is now obsolete. Some of the newest Airbuses now come with a system that automatically detects an air data unit that is providing bad data and generates a warning message indicating which unit is faulty. Additionally, this new modification includes a digital backup speed scale rendered in knots, computed algebraically by solving for airspeed in the lift equation with the angle of attack, weight, and load factor as known inputs.
If a failure results in erroneous altitude information, the newest Airbus systems can detect this, and will replace the affected altimeters with an alternate, GPS-derived altitude indication. This feature also generates a warning message prompting the crew to turn off the altitude broadcast from the transponder in order to prevent ATC from receiving erroneous altitude information.
On the Boeing 787, the most advanced Boeing model, bad air data can also be detected automatically by a feature that constantly compares the indicated airspeed to the algebraically derived airspeed. If a discrepancy is detected, a warning message is displayed to the crew, along with an automatic reversion to GPS altitude and/or algebraically derived airspeed if necessary.
As advanced as these systems are, not all aircraft have them, and the process of improvement continues. For instance, a 2020 study pointed out that many pilots still do not, and cannot, fully understand the air data architectures of state-of-the-art aircraft. The authors proposed to mitigate this problem with the use of newly designed synoptic pages that show how air data is flowing between different aircraft systems in real time. “This technology will create procedural changes in the way pilots handle system failures, but the benefit is the significantly reduced time to complete complex checklists and a greater pilot understanding of what needs to be done and why,” the study’s authors wrote.
This is far from an exhaustive list of the features incorporated into modern airliners as a result of accidents like Birgenair 301 and Aeroperú 603. It’s also far from a complete summary of the extent and nature of the training that pilots now receive. It cannot be overstated how far the industry has come since those accidents, when the pilots were given no tools whatsoever to combat one of the most insidious failures. Those men had everything stacked against them, and they lost.
Today’s pilots are drilled from an early stage to remember two items: pitch and thrust. Pitch and thrust are a lifeline. If you can maintain both, the airplane will stay on the great highway in the sky, even if you’re blindfolded. Only then can you have a nice conversation about the problem over a proverbial cup of tea.
Despite these measures, unreliable airspeed remains one of the trickier scenarios that a modern airline pilot might face. Very occasionally, unusual edge cases still arise that genuinely endanger the safety of flight. In those cases, good systems knowledge can easily mean the difference between life and death. In fact, the pilots in this story weren’t just flying blind because of bad data, they were flying blind because they didn’t understand their own aircraft, and because the required level of understanding wasn’t seen as important. As long as the industry doesn’t lose sight of that fact, then the 259 people who lost their lives in 1996, who were dashed against two oceans aboard two airliners that could have been saved, will not have perished in vain.
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Don’t forget to listen to Controlled Pod Into Terrain, my podcast (with slides!), where I discuss aerospace disasters with my cohosts Ariadne and J! Check out our channel here, and listen to our latest episode about a titanic battle between a BAC 1–11 and some wind. Alternatively, download audio-only versions via RSS.com, or look us up on Spotify!
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