Invisible Peril: The crash of Palair Macedonian Airlines flight 301

The wreckage of Palair flight 301 lies in a field outside Skopje, Macedonia. (Rune Lind)

On the 5th of March 1993, a Fokker 100 jet flying for the national airline of the newly independent republic of Macedonia lost control during takeoff from Skopje, rolling wildly from side to side before cartwheeling into a field and breaking apart, killing 83 of the 97 people on board. Macedonian authorities wanted to close the case quickly, drafting a final report just two months after the accident. But a Dutch team sent to represent the aircraft’s owner and manufacturer refused to accept this bare-bones inquiry, and kept investigating for another year, revealing the science behind why the plane went down. The investigators uncovered disturbing evidence of how small amounts of ice on the wings, interacting with warm and cold fuel mixtures inside the fuel tanks, could lead to a complete loss of roll control during takeoff — a terrifying emergency that the pilots didn’t have enough time to understand. The crash led to changes in the way Fokker pilots learn about the dangers of icing, and to greater awareness of the vulnerability of the aircraft type to ice on the wings. But the changes, as important as they were, did not prevent the same type of accident from happening again — not once, but twice.

A Palair Macedonian Airlines crew and plane, undated photo. (History of Macedonia)

In 1991, as Yugoslavia began to crumble, the small, landlocked country of Macedonia declared its independence as a sovereign nation. Largely avoiding the bloody conflicts which defined the breakup of Yugoslavia, Macedonia (known since 2019 as North Macedonia) set about the slow process of gaining international recognition. By the spring of 1993 the process was almost complete, but the new country was still about a month away from acceding to the United Nations. Nevertheless, it had already taken one of the first steps toward presenting itself as a modern, independent nation: setting up a national airline. Palair Macedonian Airlines came into being in late 1991 with a single Tupolev Tu-154, but by 1993 it was looking to expand, and early that year it added a Fokker F28 Fellowship and a larger Fokker 100 leased from foreign companies. Specifically, Palair acquired the Fokker 100 from a Dutch company called Aircraft Financing and Trading, or AFT, which was jointly owned by Fokker Aircraft and engine manufacturer Rolls Royce and specialized in leasing aircraft and crews to passenger airlines.

PH-KXL, the aircraft involved in the accident. (Torsten Maiwald)

The Fokker 100 arrived in Macedonia in January 1993, along with a complement of pilots from AFT, who would conduct line training for Palair’s pilots until they had enough experience to operate the plane themselves. Flight crews were thus usually split between one Dutch pilot and one Macedonian pilot. Maintenance was contracted out to Swissair, which performed service every time the plane made its scheduled stop in Zürich, and also sent a Flying Station Engineer, or FSE, who rode with the plane to all stops other than Zürich in order to carry out maintenance and other operational tasks.

Late on the morning of the 5th of March, the Fokker 100 arrived in Skopje, the capital of Macedonia, after a routine flight from Frankfurt, Germany. The same crew was supposed to fly the next leg to Zürich, Switzerland, but the flight had been delayed and the crew informed the company that they wouldn’t be able to conduct the second flight within duty time limits. AFT and Palair quickly summoned a new crew to replace them, consisting of Captain Peter Bierdrager, who worked for AFT, and an unnamed Macedonian captain who was undergoing training to transfer from the Boeing 737 to the Fokker 100.

Route of Palair flight 301. (Google + own work)

When the crew arrived at the airplane around 11:30 a.m., the weather was poor with no signs of improvement. The temperature hovered around 0˚C and light snow was falling, which immediately melted upon touching the ground. The dew point — the temperature at which water vapor will condense out of the air — was -1˚C, very close to the actual temperature, creating conditions perfect for ice formation, especially on cold surfaces.

At Palair it was common practice to carry more fuel than was strictly necessary in order to avoid higher fuel prices at certain European airports, and when the plane landed in Skopje it still had several thousand kilograms of fuel on board. According to company procedure, the outgoing crew had added still more fuel after landing until they reached the company standard of 6,800 kilograms. Then, upon learning of bad weather in Zürich, Captain Bierdrager added another 900 kilograms just to make sure they had enough for extended holding if conditions prevented them from landing.

During the flight from Frankfurt, the fuel in the tanks was exposed to very cold temperatures at high altitudes, resulting in what is known as “cold-soaking.” As the plane sat on the ground, the fuel in the wings remained much colder than the ambient air temperature, allowing the falling snow to freeze to the wing instead of melting.

When more fuel was added to the tank, this fuel came from a fuel truck where the temperature was somewhat higher than that of the outside air. This warmer fuel entered via the collector tanks, located near the wings roots. In addition to the collector tanks, each wing tank also consisted of three compartments, labeled from inboard to outboard, which were connected to each other and to the collector tank only by small flap doors and holes in stringers. Consequently, the warmer fuel mixed thoroughly into the collector tanks, but for the most part did not migrate to the other compartments, especially compartment three. Consequently, a temperature gradient developed in the wings, where the wing surfaces (heated by the new fuel) were warmer near the wing roots and colder near the wing tips.

Around ten or fifteen minutes after refueling, the Flying Station Engineer performed a walkaround check of the airplane to look for mechanical problems and ice. As was called for in standard operating procedures, he checked the top of the wings for ice by standing on a baggage cart and running his hands over the surface. He found only wet, melting snow without any sign of ice. Several ground handlers also checked and reported similar findings. After the check, one of the ground handlers asked the FSE whether they would need to de-ice the plane, grabbing a bunch of slush off one of the flaps to contextualize his question. But the FSE said that the snow wasn’t sticking to the wings and would slide off during the takeoff roll, so no de-icing would be necessary. He then presumably went to the cockpit to tell the pilots about his decision, which they apparently accepted uncritically.

The problem was that the FSE had checked for ice near the wing root, where the warmer fuel was causing the snow to melt, and not near the colder wing tips, where snow was sticking to the wing surface and forming ice. Unaware of the danger, the pilots completed the pre-flight checks, started the engines, and taxied to the runway without once mentioning the weather conditions.

With 92 passengers (including the FSE) and five crew on board, Palair flight 301 to Zürich received takeoff clearance at 12:11 p.m. By this time the light snowfall had become moderate to heavy, with visibility restricted to less than 1,000 meters. But still the pilots, apparently unperturbed, did not discuss the weather.

With the Macedonian trainee captain sitting in the left seat and handling the controls, flight 301 sped off down the runway and lifted off normally some 30 seconds later. In the tower, the controller watched the plane vanish into the snow.

As soon as the plane started to climb, an unusual vibration began. “Positi-i-i-ve,” Captain Bierdrager called out as they started to climb, his utterance tinged half way through with sudden uncertainty.

What neither pilot knew was that the ice on the wings was severely affecting the performance of their airplane. By interrupting smooth airflow over the wings, even a tiny layer of ice can result in a significant reduction in lift. The reduced lift and increased drag will also cause the plane to stall at a much lower angle of attack than normal. The angle of attack (the angle of the plane relative to the airflow) where the Fokker 100 will stall is normally around 16.5 degrees, but with ice on the wings, this was reduced to between 10 and 11 degrees, slightly below the angle of attack used during a routine takeoff. As a result, when the trainee captain pulled back on his controls to climb, the airplane started to stall, causing violent buffeting as the airflow separated from the wings.

How ice on the wings affects lift. (Dutch Safety Board)

But the stall was only half of the problem. Due to the way the fuel was distributed, the ice had become concentrated near the wing tips, with less ice or even no ice near the wing roots. This was significant because the ailerons, which control bank angle, were also located on the outboard part of each wing.

Aircraft wings are designed to ensure that as the plane approaches a stall, the airflow separates first near the wing roots before progressing outward toward the wing tips. Because the ailerons rely on smooth airflow over the wings in order to operate properly, this helps ensure that it is possible to control the plane’s roll angle during a stall. But the ice on the wing tips disrupted this sequence and caused the airflow separation during the stall to progress outboard to inboard, opposite to the normal direction. As a result, one of the first signs of the stall was a sudden and unexpected loss of roll control as the air stopped flowing smoothly over the ailerons.

Just seconds after liftoff, at the same time as Captain Bierdrager called out “positive,” this manifested in the form of an uncommanded 11-degree roll to the right, prompting the trainee captain to turn left using his control column. Strangely, he had to apply nearly full left aileron before the plane returned to wings level, allowing him to relax his inputs.

But three seconds after that, all hell broke loose: without any input from the pilots, the plane abruptly rolled fifty degrees to the left within the space of about one second. The trainee captain instinctively turned his control column all the way to the right, but his inputs seemed to have no effect.

“Ah shit!” Bierdrager exclaimed.

“What is it?” said the trainee captain.

Two seconds after rolling hard to the left, the plane suddenly rolled all the way through wings level and into a 63-degree bank to the right. The trainee captain slammed his controls back to the left again, but was unable to arrest the extreme roll.

Bierdrager was now just as confused as his copilot. The roll clearly wasn’t deliberate, so perhaps it was the autopilot? He then called out, “Oh, deselect!” before reaching out to disconnect the autopilot. But the autopilot had never been turned on in the first place.

FDR data from the accident flight shows how the plane swayed from side to side despite opposite inputs by the crew. Note that the data from the last two seconds was lost due to damage caused during the crash. (Dutch Safety Board)

The stick shaker stall warning momentarily activated, but it was programmed based on the stall characteristics of a clean wing, and only came to life well after the airplane had already stalled. While in the steep right bank, the trainee captain pitched the nose down, decreasing the angle of attack and recovering enough roll control to return to a shallower, 15-degree bank. But now they were descending at a rate of 2,000 feet per minute from a height of just 150 feet above the ground.

“Nose up!” shouted the engineer from the cockpit jump seat. In a desperate attempt to avoid hitting the ground, the trainee captain pulled up sharply, but this again caused the airflow to separate from the wing tips, and they once again lost control of the ailerons. The plane suddenly rolled 90 degrees to the right and plowed sideways into a snow-covered field 380 meters beyond the end of the runway. The right wing sliced a furrow across the ground and through the airport’s barbed wire perimeter fence before the fuselage crashed to earth and broke apart, splitting into several pieces as fire erupted from the riven fuel tanks. Within seconds, it was all over, and the smoking remains of the jet came to a halt beneath a curtain of falling snow.

The remains of the Fokker 100 were strewn for several hundred meters across a field. (Rune Lind)

No one at the airport witnessed the crash, but a United Nations helicopter pilot walking back to his office after parking on the ramp heard a loud boom coming from the end of the runway and rushed back to his helicopter to search for the crashed plane. Less than a minute later he arrived upon a scene of devastation. The plane had disintegrated, spewing seats and passengers all over the field, while only the tail section and the left side of the cockpit remained intact. Spot fires burned throughout the debris field. But in the back of the plane, some people had managed to survive, including one of the flight attendants. The UN pilot crammed seven survivors into his helicopter and rushed them to hospital, before turning around and flying right back to pick up more. Another helicopter also arrived within minutes, along with police and firefighters, and together the two helicopters ferried another eight survivors to nearby hospitals. But after that, rescuers could only find bodies. One of those taken to hospital also soon died, leaving 83 people dead with only 14 survivors.

The left side of the cockpit was found somewhat intact with part of the fuselage wall still attached to it. (FAA)

The Macedonian Civil Aviation Authority quickly set up an investigation commission, which was to be run by Yugoslav investigators based in Belgrade, as Macedonia had not yet set up its own investigative agency. They also invited investigators from the Dutch Safety Board, who were entitled to participate as the aircraft was manufactured and registered in the Netherlands.

The Dutch team soon took over much of the technical side of the inquiry, which involved complex tests carried out on special simulators at Fokker Aircraft. But in May 1993, well before any of the tests had concluded, the Yugoslav and Macedonian investigators wrote up a final report which they then proceeded to sit on for several months until newspapers began reporting that the investigation had been completed. The Dutch Safety Board was certainly not in agreement that the investigation was over, but in a meeting in September 1993 the Macedonians told the Dutch representatives that they should either agree with the findings or add their comments that very day. The Dutch Safety Board rejected this outright, because in their view the report was woefully incomplete, as it made no mention of the numerous time-consuming experiments they had been conducting with regards to fuel temperatures, ice distribution, and loss of roll control, and it contained no discussion of the human factors involved in the decision not to de-ice. Furthermore, international rules gave the accredited representatives 60 days, not one, to make comments on the report. Fortunately, after the presumably explosive meeting, Macedonia agreed to give the Dutch investigators the full 60 days, after which they presented a list of comments longer than the final report itself.

The tail section was where all the survivors were seated. However, the left engine penetrated the cabin during impact sequence and triggered a fire which burned over most of what was left of this area after they escaped. (Rune Lind)

Macedonia seemed to take offense at the implication that incorporating the Dutch comments would involve an almost total rewriting of the report, and Dutch investigators described apparent pressure on the Macedonians, possibly political in origin, to get the investigation over with and publish the findings. After more negotiations, Macedonia agreed to publish a mutually acceptable probable cause while the commission tried to resolve the disagreements over the details. The Dutch Safety Board ultimately ended up writing its own final report based around the skeleton of the report produced in May 1993 and submitted this to Macedonia in 1994 — only for them to sit on it again until 1996, when they informed the Dutch Safety Board that they would be submitting the original 1993 report to the International Civil Aviation Organization, and that if the Netherlands wanted to publish their own report they should do so separately. No obvious reason was given for the two-year delay, which the Dutch Safety Board said could have been detrimental to aviation safety.

Another view of the wreckage shows how half the cockpit was torn away. (Rune Lind)

Needless to say, the much more thorough Dutch report has become the work of reference concerning the crash of Palair flight 301, especially since the Macedonian report was never publicly released. Its findings pointed to more critical vulnerabilities in an aircraft type that was already known to be at elevated risk of ice-related accidents.

The basic sequence of events began when an uneven temperature distribution within the fuel in the wing tanks allowed ice to build up on the outboard parts of the wings but not on the inboard parts. The Flying Station Engineer examined the inboard parts of the wings, but he didn’t have a good view of the wing tips and couldn’t have seen the ice which was accumulating there. He probably checked the wing roots out of force of habit: up until the week of the accident, he had been handling Swissair Fokker 100s, which had special ice detectors located in this area. Palair’s Fokker 100 didn’t have any ice detectors, so there was no reason to check any particular part of the wing vis-à-vis another, but the engineer simply did what he had always done and examined the wing root.

United Nations personnel examine the wreckage. (Rune Lind)

Apparently convinced that the snow was melting and not sticking to the wings, the FSE told the ground handlers that they wouldn’t need to de-ice the plane, and presumably said the same thing to the pilots. However, the pilots hadn’t followed a company procedure which called for the cockpit voice recorder to be turned on before starting any checklists, so any possible record of this conversation was lost when the pilots and the FSE died in the crash.

A major question that the investigators needed to answer was why the pilots didn’t decide to de-ice anyway, given that conditions were perfect for ice formation. The temperature was around freezing with a similar dew point amid falling snow; it would be hard to ask for weather more conducive to ice. But the pilots may not have had good situational awareness with regards to the weather. It didn’t start snowing until shortly before they arrived by car at the airport, and even then there were only a few flakes which melted immediately. After that, they went directly inside to prepare the plane for departure, a task which kept their focus inside the cockpit rather than outside on the weather. When the FSE reported that there was no need to de-ice, this could have reinforced their outdated notion that the weather conditions were nothing to worry about. They would have had little reason to question the judgment of the FSE, who was highly regarded by all who worked with him, and represented Swissair, a respected carrier. At the time there were also no specific weather criteria which would obligate a pilot to de-ice the plane, so the judgment of the qualified engineer was considered good enough. Following this, the pilots could not have failed to notice the increasing snowfall as they taxied to the runway, but since the snow still appeared to be melting when it touched the ground, it might not have occurred to them that it could nevertheless freeze onto the wings.

Even with ice on the wings, the plane was not doomed to crash on takeoff. But as long as the pilots remained unaware of the ice and its potential effects, it might as well have been. The detrimental effects of the ice caused the stall angle of attack to reduce below the angle of attack used during takeoff; as a result, the plane started to stall almost as soon as the trainee captain pulled up to climb. The stall came unexpectedly because the stall warnings were programmed to activate at a point calculated based on the plane’s configuration and altitude, and could not account for the fact that ice was significantly altering the jet’s aerodynamic characteristics.

The concentration of ice on the wingtips allowed the stall to propagate from the wingtips toward the wing roots, causing airflow to separate from the ailerons early in the process, a scenario which the design of the wings would normally prevent. This resulted in a loss of roll control beginning four seconds after takeoff, starting with the 11-degree roll to the right, then the 50-degree roll to the left. The pilots had no idea that a loss of roll control could be symptomatic of an ice-induced stall, as this phenomenon had never been observed on a Fokker 100 before. Captain Bierdrager’s attempt to disengage the autopilot (which was not actually engaged) also suggests that he might have been searching for a mechanical or computer-related cause for the loss of control.

Side view of the cockpit, where the impact with the ground stripped away everything to the right of the captain’s seat. (Rune Lind)

In fact, unbeknownst to the pilots, the sudden rolling motions were directly correlated with the plane’s angle of attack. Whenever the angle of attack decreased below 10 degrees, the trainee captain was able to recover to wings level. (In fact, had he known what the problem was, he could have just kept the pitch below this value and climbed away without any trouble.) But each time he regained control, he proceeded to follow his flight director, an overlay on his attitude indicator, which was showing him the nominal climb angle that he should be maintaining. He didn’t realize that the ice on the wings prevented him from actually maintaining this angle without stalling the airplane. Whenever he pitched up to reach this nominal climb angle, the angle of attack went above ten degrees, the plane started to stall again, and the uncommanded rolling would return. The terrified pilots did not have enough time to spot the connection. Only 18 seconds passed between liftoff and impact, during which time the plane rolled slightly right, steeply left, sharply right again, returned to slightly right, then flipped completely on its side. The pilots understandably focused on trying to level the wings, unaware that the root cause was something totally different.

Another view of the cockpit. The tail section can be seen in the background. (Rune Lind)

Because the physical evidence was ephemeral, the investigators couldn’t say exactly how much ice was on the wings at the time of the accident. But the extreme effects of relatively small amounts of ice on the Fokker 100 and its smaller sister craft, the Fokker F28 Fellowship, were already well known in the industry. The basic problem was that heavily swept wings without leading edge slats tended to lose more lift due to smaller amounts of ice than other wing shapes. The Fokker F28 and Fokker 100 were among the most widely used airliners with this relatively uncommon design feature, and their vulnerability had already led to a number of accidents. The first fatal icing accident involving the F28 occurred in Turkey in 1974, but several others had taken place since then. In 1989, Air Ontario flight 1363, an F28, crashed on takeoff from Dryden, Ontario after operational concerns compelled the crew not to de-ice, killing 24 of the 69 people on board. In 1992, USAir flight 405, another Fokker F28, crashed on takeoff from New York’s LaGuardia airport under very similar circumstances, killing 27 of the 51 passengers and crew. In both cases it was found that tiny amounts of ice invisible to the pilots had prevented the planes from becoming airborne. The two accidents led to an increasing push in the industry to ensure that pilots or other qualified personnel physically touch the tops of the wings when deciding whether or not ice is present. The crashes also led to increased use of anti-icing fluids in addition to existing de-icing mixtures, and to greater emphasis on the dangers of ice in Fokker’s documentation. But these measures failed to prevent the crash of Palair flight 301.

Little remained of the cabin near the front of the plane. (Rune Lind)

As a result of the accident in Macedonia, Dutch authorities issued an airworthiness directive requiring all Fokker F28 and Fokker 100 operators to incorporate language in their operations manuals warning of the danger of various icing patterns caused by fuel distribution and the potential loss of roll control. The updated documentation included a technique for taking off with a lower climb angle when the absence of ice cannot be assured — a method which would have prevented the crash of flight 301. Manufacturers, including Fokker, also started including specific weather criteria that would obligate a pilot to de-ice their plane, regardless of whether ice was noticed during the pre-flight inspection.

The cockpit is taken away from the scene on a UN flatbed truck. (Rune Lind)

Unfortunately, Palair flight 301 was not the last Fokker 100 to crash due to ice on the wings. On the 25th of January 2007, Air France flight 7775, a Fokker 100, prepared to take off from the city of Pau under conditions very similar to those in Skopje 14 years earlier. The temperature was once again 0˚C with a dew point of -1˚C in light falling snow. A crewmember did a pre-flight walkaround check, but he couldn’t find a step ladder which would allow him to access the tops of the wings, so he only checked the leading and trailing edges, where he found no ice. The pilots were unaware that ice had in fact formed on top of the wings.

During takeoff the plane started to stall and lost roll control; the jet banked 35 degrees to the left, then 69 degrees to the right, then 59 degrees to the left. The pilots pitched down in an attempt to land back on the runway, in the process averting the stall and regaining roll control. The plane touched down shortly before the end of the runway, then skidded off the end and across a road before coming to rest in a field. All 54 passengers and crew survived, but a truck driver was killed when the plane’s wing struck his vehicle as it was crossing the road. Apart from the outcome, the incident was very similar to Palair flight 301, except that the crew violated all the procedures put in place after that crash to prevent a recurrence. In the wake of the accident in Pau, the European Aviation Safety Agency mandated on-ground wing leading edge heaters on all Fokker 100s, although these still can’t remove ice from the entire wing.

The aftermath of the 2007 crash of Air France flight 7775. (BEA)

And yet, after all of that, this type of crash still kept happening. On the 27th of December 2019, Bek Air flight 2100, another Fokker 100, crashed on takeoff from Almaty, Kazakhstan after the captain decided not to de-ice the plane despite the presence of freezing fog. The plane failed to get more than 20 feet off the ground before slewing off the side of the runway and hitting a building, killing 12 of the 98 people on board. Although the investigation into the crash is still ongoing, preliminary findings showed that Bek Air, despite operating in a cold environment with a plane known for ice-related crashes, had not given its crews any winter operations training or provided any training related to the dangers of ice. The victims of all the previous crashes must have been rolling in their graves. Fortunately, Kazakh authorities permanently grounded Bek Air due to these violations (and others, including illegally removing data plates from key components, possibly to sell them on the black market, a finding which raised questions about whether the entire airline was some kind of organized crime front from the very beginning).

The aftermath of the 2019 crash of Bek Air flight 2100. (AP)

In any case, with fewer and fewer Fokker 100s in service with each passing year, it is probable that this will be the last ice-related crash involving this aircraft type. But then again, many experts probably would have said the same thing before the Bek Air crash too. If there’s one thing every pilot should take away from this sordid story, it’s that ice on the wings is not a joke.

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Analyzer of plane crashes and author of upcoming book (soon™). Contact me via @Admiral_Cloudberg on Reddit or by email at kylanddempsey@gmail.com.

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