Reason Behind The crash of Palair Macedonian Airlines flight 301

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.

Arrangement of the fuel tanks inside the wing of a Fokker 100. (Dutch Safety Board)
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.

A technician examines an aircraft’s wing in search of ice. (NASA)
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. website