Seven Seconds to Survive: The crash of Independent Air flight 1851

Admiral Cloudberg
20 min readNov 7, 2020
The wreckage of Independent Air flight 1851 lies atop Pico Alto in the Azores. (Bureau of Aircraft Accidents Archives)

On the 8th of February 1989, an American Boeing 707 carrying Italian tourists departed Bergamo, Italy on a marathon trip to the Dominican Republic. But as the flight prepared to make a scheduled stopover on the Azorean island of Santa Maria, the plane crashed headlong into the summit of Pico Alto, instantly killing all 144 people on board. Investigators soon descended on the remote, windswept island, where the scattered debris of the 707 testified to the tragic end of a vacation that never had a chance to begin. The sequence of events that put the plane on a collision course with the mountain was long and complex, resulting from a slow accumulation of misunderstandings between the crew and the air traffic controller. Events crystallized out of a fog of confusion, tiny mistakes mounting one atop the next to create a perfect storm that led to Portugal’s worst air disaster. But the fate of the plane in fact might have been sealed years earlier and thousands of miles away — across the Atlantic in the United States, with the company policy of the scrappy charter airline that had been hired to operate the ill-fated flight.

N7231T, the Boeing 707 involved in the accident. (Martin Oertle)

In 1984, two business partners purchased an Atlanta flying club and turned it into a full-fledged airline. Their new airline, christened Independent Air, began operating charter flights using a pair of aging four-engine Boeing 707s, which it used to carry tourists on behalf of tour operators both in the US and abroad, and to run charters for the military.

Among Independent Air’s most lucrative markets was the Italian tour industry, where tour operators regularly hired the company to carry Italian tourists to holiday destinations in the Caribbean. Although Independent Air’s Boeing 707s couldn’t make the transatlantic trip in one leg, their low prices ensured that customers continued to choose them over other carriers despite the frequent need to stop for more fuel.

Location and close-up of Santa Maria Island. (Google)

The most frequent stopover point for Independent Air’s transatlantic charter flights was the tiny island of Santa Maria, the southeasternmost speck of land in the Azores, a Portuguese archipelago located 1,400 kilometers west of Lisbon. Despite having a population of less than 6,000 people, the island was a critical stopover point for commercial airliners in the decades before long-range jets entered service, and throughout the 1950s and 1960s its state-of-the-art airport saw dozens of planes come and go every day. But by the middle of the 1970s, traffic had plummeted as most airlines no longer needed to stop at Santa Maria to refuel their planes. By 1989, Santa Maria’s time in the limelight had long since ended, and the island had returned to its backwater status. Besides regular commercial flights from TAP Air Portugal and SATA, Independent Air was one of the only airlines which frequently visited the island. So few were the flights to Santa Maria that islanders who worked at the airport referred to them by name — there was “the TAP,” “the SATA,” and of course the Independent Air Boeing 707, which they called “the charter.” Some islanders even hitched rides on it when none of the scheduled flights were convenient.

Route of Independent Air flight 1851. (Google)

On the 8th of February 1989, one of Independent Air’s two Boeing 707s arrived in Bergamo, Italy to pick up a group of 137 Italian tourists bound for Santo Domingo, capital of the Dominican Republic. Because the 707 didn’t have the range to fly from Bergamo directly to Santo Domingo, the airline had scheduled a fuel stop at Santa Maria as usual. After a short delay due to weather conditions, the plane, on this trip designated flight 1851, departed Bergamo at 10:04 a.m.

In command of the flight were Captain Leon Daugherty, First Officer Sammy Adcock, and Flight Engineer Jorge Gonzalez. Captain Daugherty, an active member of the Tennessee National Guard, had a fair amount of experience but only 766 hours in the Boeing 707, low for a captain. Gonzalez had more, but Adcock had much less: he had just received his certificate to fly the 707 two weeks before the flight, and he had only 64 hours on the type, including training flights. He previously hadn’t flown anything bigger than a seven-seat Piper PA-31.

A couple hours later, flight 1851 entered the Oceanic Control Sector, a vast section of airspace controlled by the Oceanic Center on Santa Maria. But the crew had difficulty communicating with the controller, a problem which would continue throughout the rest of the flight. Over a period of less than 25 minutes, First Officer Adcock had to ask for clarification on six different items of information transmitted by the Oceanic controller because he couldn’t understand what was being said, or because he wasn’t listening carefully.

How the choice of primary navigational aid affected the path of flight 1851. (Carlos Sousa)

Meanwhile, the crew plotted out their course for the approach to Santa Maria. The coordinates to which they were supposed to fly, as listed in the flight plan, didn’t correspond to any navigational aid installed at the airport. So they inferred from their chart that they were supposed to begin by flying to the Santa Maria VOR, or Very high frequency Omnidirectional Range, a type of radio beacon which incoming planes can track in order to find the airport. However, Santa Maria was special — instead of using the VOR as the primary navigational aid, it used a less reliable non-directional beacon, or NDB, instead. This was because planes arriving from the east typically did so through the same oceanic waypoint called ECHO. A direct flight between ECHO and the VOR would pass right over the top of Pico Alto, which at 1,925 feet (587 meters) was the highest point on Santa Maria; this tended to block the signal and made the VOR harder to track. The NDB was located off the north end of the airport several kilometers from the VOR, where a direct line to ECHO didn’t have terrain obstacles that would interfere with the signal. However, the charts used by the pilots didn’t mention that planes were supposed to first fly to the NDB, not the VOR.

Because of the ambiguity of the approach chart, many planes inbound to Santa Maria flew directly to the VOR instead of the NDB. This was not a problem because the minimum safe altitude (MSA) indicated on the chart was 3,000 feet, which kept planes clear of the mountain regardless of which route was used.

As flight 1851 began its descent toward the island on course to the VOR, the pilots invited a female tour guide up to the cockpit (although it is widely assumed that the woman was Captain Daugherty’s fiancée Yvette Murray, who was also on the plane that day, this was actually not the case). The cockpit microphone recorded snippets of a conversation: “Don’t laugh…” “Close that door, it’s a jungle out there…” “First time in the Azores…?” “We’re havin’ fun now, hey hey!”

Meanwhile at the sleepy Santa Maria Airport control center, the two controllers on duty — a supervisor and a trainee — were taking an unauthorized break, and no one was in the tower at all. They finally returned around 12:44 p.m. local time, at which point the trainee made contact with flight 1851 and provided weather information. At the time, conditions were mostly clear below an overcast layer at 3,000 feet, but Pico Alto stuck up into the higher oceanic air currents and was surrounded by a localized area of lower clouds, as usual. “One octa at one two zero zero feet,” the controller said, using a term for how many eighths of the sky are covered by cloud, “and six octa at three thousand feet. Temperature one seven, QNH one zero one niner,” he added, giving the local air pressure in millibars. The crew acknowledged the pressure reading and set their altimeters accordingly. However, one item didn’t come across correctly: the phrase “one octa at one two zero zero” sounded uncannily like “one octa two two zero zero” when heard over the cockpit speaker, leading the pilots to believe they would not encounter any clouds below 2200 feet, when the clouds around Pico Alto actually bottomed out at 1200 feet.

This was but the first of several misunderstandings between the crew and the controller. At 12:56, the controller said, “You’re cleared to three thousand feet on QNH one zero two seven and uh, runway will be one niner.” At this moment several things happened. First of all, the controller had given an incorrect pressure setting of 1027 millibars, a massive difference compared to the setting of 1019 millibars he had transmitted 12 minutes previously. Simultaneously, First Officer Adcock, misled by the controller’s strong local accent, thought he heard him say they were cleared to 2,000 feet instead of 3,000.

After giving the crew this information, the controller paused for a second, leading Adcock to believe that the transmission was over. He pressed his push to talk button and said, “We’re cleared to two thousand feet and ah… one zero two seven.”

At exactly the same moment the controller pressed his own push to talk button and said, “Expect ILS approach runway one niner and report reaching three thousand.” On this radio system, when the push to talk button is depressed, messages can be transmitted but not received. Thus the crew of flight 1851 didn’t hear the controller say “report reaching three thousand” and the controller only heard “one zero two seven,” without Adcock’s incorrect readback of the altitude clearance. In such an event the controller was supposed to ask Adcock to verify that he had understood the whole clearance, but he never did.

Meanwhile on board flight 1851, Captain Daugherty said, “Make it three,” which was possibly an attempt to correct First Officer Adcock’s incorrect readback of their altitude clearance. But Adcock either didn’t hear him or didn’t understand what he was talking about, because he set the altitude alert system — which produces a chime when approaching the selected altitude — to notify them upon nearing 2,000 feet. Had they looked at their approach charts, they would have seen that the minimum safe altitude in the entire sector was 3,000 feet, but the pilots had skipped the approach briefing, which would have covered this information.

They had now set themselves up to level off at 2,000 feet, which would lead to a near miss with Pico Alto if they passed over it at that altitude. But there was one last piece to the puzzle: the incorrect pressure reading transmitted by the controller. The new QNH of 1027 millibars struck First Officer Adcock as odd, so he asked, “Is that what he said? Ten twenty-seven on the millibars?”

“Yep,” said Captain Daugherty, confirming that he had heard the same thing. Neither pilot questioned how the pressure could change by 8 millibars in 12 minutes, a movement more characteristic of the approach of a category 5 hurricane than a normal day at the airport. Having received confirmation of the reading from his more experienced colleague, Adcock set their altimeters using a sea level pressure of 1027 millibars, causing the altimeters to show an altitude that was 240 feet too high.

How each of the altitude errors came together to just barely put the 707 on course to strike Pico Alto. (DGCA Portugal)

The stage was now set for disaster. Flight 1851 was on course to pass over Pico Alto at a height of 1,760 feet, below the height of the mountain, but only slightly. At the same time, their altimeters would indicate 2,000 feet, above the height of the mountain, and they believed that the clouds ended at 2200 feet, leaving them with the expectation that they would break out of the clouds at any moment.

“Starting to pass through layers here,” Adcock said, observing the gathering clouds.

“Can’t keep this son of a bitch thing straight up and down,” said Captain Daugherty, as the plane flew through heavy turbulence rolling off the back side of Pico Alto.

“[Should I] help you?” Adcock asked.

“Nah,” said Daugherty.

Seconds later, the plane’s ground proximity warning system (GPWS) detected rising terrain beneath the aircraft and began to call out, “WHOOP WHOOP, PULL UP! WHOOP WHOOP, PULL UP!”

Stylized art of flight 1851 flying toward Pico Alto, used as the cover picture of Francisco Cunha’s book on the crash. In reality, there were more clouds. (Francisco Cunha)

Strangely, none of the pilots said a word. For seven seconds, the alarm blared in the cockpit, until it was cut short by the terrible sound of the plane impacting the mountain. Flight 1851 struck a descending ridge of Pico Alto, slamming into a retaining wall alongside the summit road at 420 kilometers per hour. The massive impact tore the plane into thousands of pieces, catapulting shattered wreckage over the top of the ridge and down through the forest on the other side. The tail section fell back and tumbled down the east side, taking some of the engines with it, while the rest of the plane lay scattered across the road and the western slope beyond it. So terrible was the crash that all 144 people on board died instantly, without time even to scream.

The crash site viewed from the west, opposite the point of impact. (Francisco Cunha)

In the nearby village of Santa Barbara, just east of Pico Alto, the sound of the plane passing overhead caught the attention of a local priest, who looked up to see the silver bottom of the 707 gliding through the shifting clouds. Its low height caught him by surprise, and with sudden horror, he realized it was going to strike Pico Alto. He counted seven seconds before a thunderous explosion echoed out from the fog-shrouded mountain, signaling that the plane had crashed. As Italian travel brochures and documents rained down around him, the priest jumped in his car and raced toward the site of the crash, hoping to find survivors. Instead, he found a scene of total desolation. The airplane had been completely destroyed, and its occupants along with it. Pieces of human bodies were strewn across the road and in the branches of trees. When rescuers from Santa Maria Airport arrived several minutes later, they found that they could do nothing to help; of the 137 passengers and 7 crew, it was obvious that there were no survivors. The firefighters on the sleepy island were totally unprepared for the catastrophe that confronted them. Upon reaching the site, one of the firemen radioed the control tower and said, with terror in his voice, “Tower, it’s a disaster up here! This is horrible, a horrible thing has happened here!”

Another aerial view showing the point where the 707 first struck the mountain. (Francisco Cunha)

When word of the crash reached Lisbon, Portugal’s Directorate General of Civil Aviation hastily put together a team of experienced investigators, who finally arrived at the crash site at around 3 o’clock in the morning. The main question that they would need to answer was why the plane struck a ridge at 1,760 feet in an area where the minimum safe altitude was 3,000 feet.

It took over a month for the plane’s black boxes to reveal the story of how flight 1851 ended up on a collision course with Pico Alto. First, the pilots failed to conduct an approach briefing, where they would have discussed the minimum safe altitude of 3,000 feet. Then the crew chose to fly directly to the VOR, taking them over the mountain, because their charts did not indicate that the NDB was the start of the approach. Next, First Officer Adcock misheard “cleared to three thousand” as “cleared to two thousand,” and the other pilots failed to correct him. The simultaneous transmission of the controller’s request to “report reaching three thousand” and Adcock’s incorrect readback caused the messages to cancel each other out, so neither the crew nor the controller picked up on the error. At this point flight 1851 was still on track to miss the mountain, but the controller also transmitted an incorrect pressure setting which the pilots did not question, causing an altimeter error which put the plane 240 below the indicated altitude. Without any of these factors the crash would not have happened. In fact, they almost missed the mountain anyway: had they been flying just 10.5 meters (35 feet) higher, they would have cleared the ridge.

Smoldering wreckage tumbled down through the forest on the west side of the ridge. (FotoPepe)

Another major question that needed answering was why the crew didn’t react to the ground proximity warning system. The alarm sounded seven seconds before impact, and it usually does not take a pilot more than about five seconds to respond. Given how close they already were to clearing the mountain, the remaining two seconds would have been sufficient to gain 35 feet and avoid the ridge. And yet no one made any move to prevent the accident. To understand why, investigators turned to the National Transportation Safety Board for help in examining Independent Air’s pilot training program.

The NTSB was disturbed to find that Independent Air was not teaching its pilots how to respond to GPWS alerts, even though this training was required by federal regulations. US investigators had previously recommended that the Federal Aviation Administration check whether operators were complying with this rule, but the FAA inspector assigned to Independent Air had not done so, and responses to GPWS were not covered in the airline’s training manual. But that wasn’t even the worst of it. Independent Air didn’t have its own flight simulators, so it sent pilots to train at simulators owned by another airline which had configured its 707s differently, in violation of regulations. When speeds and descent rates used at Independent Air were replicated in these differently configured simulators, the GPWS tended to go off during normal approaches. When this occurred, instructors either turned off the GPWS or outright told student pilots to ignore it! This had conditioned pilots to believe that GPWS alerts during an approach were usually not real, and it was no surprise that when the alert sounded on flight 1851, the pilots reacted exactly as trained — by doing nothing at all.

Rescue workers line the road to the crash site. (FotoPepe)

The behavior of the air traffic controllers also came under scrutiny. The controller on duty, who was working both the approach and tower positions, was a young trainee who had been on the job for only five months. An experienced supervisor was supposed to be monitoring him, but she failed to notice when he made numerous mistakes, such as giving out the wrong QNH and not requesting a full readback of the descent clearance. (The airport did not have radar, so they could not have otherwise detected that the plane was too low.) The ATC tapes showed that the supervisor was distracted answering phone calls during much of the time the trainee was talking to flight 1851. Furthermore, both controllers were on an unauthorized break between 12:00 and 12:44, leaving the control tower empty. All of these facts suggested that the Santa Maria control tower was understaffed and had developed several lax operating habits as a result of its light traffic load. As for why the controller gave out the wrong QNH, investigators couldn’t say for sure, but it was possible he simply had a momentary mental lapse and mixed together the QNH (1019 millibars) with the wind direction (270 degrees) to come up with a reading of 1027. It was also noted that these transmissions occurred right in the last moments of the controllers’ shift and they might have been hurrying to end their working day, increasing the likelihood of errors.

Wreckage of the 707 lies next to the road atop Pico Alto. (Bureau of Aircraft Accidents Archives)

All of that said, the conduct of the flight crew also left much to be desired. Throughout the flight, it was apparent that the pilots were not properly communicating with each other. Not only did they skip the approach briefing, they also failed to correct First Officer Adcock when he made numerous mistakes, such as the incorrect clearance readback and incorrect altitude alert setting. Captain Daugherty was supposed to repeat the clearance out loud to ensure everyone heard it, but he never did so. The crew also should have known that an 8-millibar increase in airfield pressure was impossible under the circumstances, but Daugherty did not question the figure, suggesting that he hadn’t been paying attention last time the QNH was transmitted. First Officer Adcock also did not push Daugherty on the point even though he clearly knew something was off about the QNH, probably because he was new to the airplane and lacked confidence in his own ability. On top of all this, the pilots engaged in an off-topic conversation with the tour guide during the descent, demonstrating poor judgment. The sheer number of mistakes led investigators to wonder if the pilots might have been fatigued. Although they had 48 hours rest time before the flight, the investigators openly speculated that the crew might not have used it “in the best way,” implying that they were perhaps out on the town the previous night rather than sleeping. However, no specific evidence for this theory was provided.

The 707’s tail, possibly after being transported to a different site temporarily. (Bureau of Aircraft Accidents Archives)

There were several factors which complicated the task facing the pilots. Among these was the fact that their approach chart did not indicate that the primary navigational aid was the NDB, not the VOR; had this distinction been made clear, the crash would not have occurred as the plane would not have flown over Pico Alto in the first place. It didn’t help that the official published information about Santa Maria Airport hadn’t been updated since 1962 and was covered with 27 years of confusing handwritten changes. In the absence of information suggesting otherwise, it made sense that the crew would have chosen to fly directly to the VOR instead of the NDB — not only would this have been this the correct choice 99.9% of the time, International Civil Aviation Organization regulations actually required that the primary navigational aid be a VOR, and Santa Maria Airport was not in compliance.

Wreckage lined the road after being pushed aside to clear a path for emergency vehicles. (Bureau of Aircraft Accidents Archives)

Investigators also found that the pilots were poorly trained for international operations. None of the pilots had received special training on how to understand regional accents outside the United States, which led to confusion during communications with air traffic control. This may have been the reason why Adcock heard “two thousand” when the controller actually said “three thousand.” This could have been exacerbated by the fact that this was the first international flight of Adcock’s aviation career. The NTSB had previously recommended that the FAA verify that airlines operating international flights had adequately trained their pilots to handle the unique challenges of flying abroad, but this had not occurred at Independent Air, and FAA inspectors didn’t have any guidance on how to evaluate airlines in this area anyway.

Another factor was Adcock’s inexperience. Investigators found that Adcock actually had not received the minimum training necessary to be a first officer on the Boeing 707; however, Independent Air had received permission to break this rule under an FAA waiver system. After finishing ground school, Adcock spent just five hours training in a simulator and six hours in the real aircraft before being released for regular duty only two weeks before the accident. Although his shocking lack of training wasn’t illegal, Portuguese investigators felt that it was clearly insufficient and the rules ought to be changed.

The tail section where it initially came to rest, in the forest below the crash site. (Bureau of Aircraft Accidents Archives)

Looking back, the problems at Independent Air and at the Santa Maria control tower suggested that neither party was running a particularly airtight operation. In comments on the accident, well-known human factors expert Malcolm Brenner noted that in 1989, the more “remote corners” of the aviation industry tended to have a certain sloppiness about them, which would have manifested both in the controllers on a quiet island airport and among the crewmembers and management of a second rate air charter company like Independent Air. He added that the number of violations and the way in which they were handled suggested that a similarly casual attitude toward the rules was pervasive at the airline and would have been considered normal.

These assumptions were further corroborated when former Independent Air Chief Pilot Cecil Mullins released his memoir in 2012. As Chief Pilot, he trained Leon Daugherty; however, he wrote that in hindsight, he shouldn’t have allowed Daugherty to be promoted to captain. According to Mullins, Daugherty frequently made errors like misreading approach charts and missing assigned altitudes, and that it took considerable effort to get him into a state where he could pass a check ride. “In retrospect,” he wrote, “it was probably a mistake giving him the reins on the big bird for international flying.” He also documented several scary incidents at the airline, such as a time when he and his crew were approaching an airport when they discovered that all the approach charts had been removed from the plane, and they needed to radio another flight crew for help. On another occasion the first officer messed with the navigation system for fun during an Atlantic crossing and sent them considerably off course. At one point, Mullins also discovered that on flights to the Azores, people were using Independent Air planes to smuggle all kinds of items ranging from canned food to car parts, none of which was declared on the load manifests or the weight and balance sheets. However, these were Mullins’ personal recollections, not the results of a formal inquiry.

Wreckage lined the road after being pushed aside to clear a path for emergency vehicles. (Bureau of Aircraft Accidents Archives)

In its final report on the crash of flight 1851, Portugal’s DGCA recommended that the status of the Santa Maria VOR be clarified, and that the minimum safe altitude over Santa Maria be raised to 3,100 feet, both of which were implemented. The NTSB also issued several recommendations, including that the FAA ensure its inspectors have all the materials necessary to evaluate the airlines under their purview; that the FAA more carefully review air carrier training programs to ensure compliance with regulations, especially those related to GPWS response; and that a minimum experience level for international flights be established. In addition to these, Santa Maria Airport later installed radar.

At the memorial cross atop Pico Alto, a small piece of the plane was placed as a memento. (Fabio Tavares)

After the DGAC released its official report, court proceedings opened in Portugal in order to determine whether anyone was legally liable for the accident. The inquiry determined that the pilots were mainly at fault for descending below the minimum safe altitude, and since the pilots were dead, the court declined to charge anyone. Independent Air later admitted to negligence and settled with the victims’ families for $34 million before the compensation case ever went to trial. As a result of fallout from the accident, which wiped out half its fleet, Independent Air went out of business in 1990.

The crash of Independent Air flight 1851 remains memorable today not just as the worst air disaster in the history of Portugal, but also as a sobering example of just how much has to go wrong to cause a plane crash. The sequence of events that put the 707 on a collision course with Pico Alto was long and complex, and even after all that, had the plane been just a few meters higher, the crash wouldn’t have happened. In fact, if the wind had been blowing at 250 degrees instead of 270, leading the controller to accidentally transmit a QNH of 1025 instead of 1027, 144 people would still be alive. For the families of the victims, this fact will haunt them for the rest of their lives. At the memorial cross that now stands at the site of the accident, open air extends to the horizon in both directions — so much space for the plane to climb away, and yet it never did. Visitors yearn to reach out, to push the plane up and out of danger, but that final chance to prevent the crash has long since faded into the mists of time.

Special thanks to Francisco Cunha for his book IDN 1851: The Santa Maria Air Disaster, without which I could not have written this article.


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Admiral Cloudberg

Kyra Dempsey, analyzer of plane crashes. @Admiral_Cloudberg on Reddit, @KyraCloudy on Twitter and Bluesky. Email inquires ->