# Aeroflot Flight F-637

The Aeroflot Flight F-637 was an aviation accident involving a Let L-410MA passenger aircraft operated by Aeroflot, which occurred on 4 December 1984 near Kostroma, resulting in the deaths of all 10 people on board.

## Aircraft

The Let L-410M Turbolet, registered CCCP-67225 (manufacturer number 770706, serial number 07-06), was manufactured on June 17, 1977 by the "Let" factory in Czechoslovakia. The aircraft was delivered to the Ministry of Civil Aviation of the USSR, which assigned it to the Kostroma (193rd) United Aviation Detachment of the Central Regions Civil Aviation Administration in August. The passenger cabin had a capacity of 8 seats. In August 1980, the aircraft was modified to the L-410MA model. At the time of the accident, it had completed 3,007 flight cycles and had accumulated 3,534 flight hours.

## Crew

* Captain: Viktor Igorevich Würtz.
* Co-pilot: Nikolay Petrovich Zvegintsev.

## Accident

The aircraft was operating a scheduled passenger flight F-637 from Kostroma to Ulyanovsk with an intermediate stop in Ivanovo. At 08:08, in the early morning twilight, flight 637 took off from Kostroma Airport with 8 passengers (including an unregistered passenger, Vladimir Lebedev, the head of the flight operations dispatch center at Kostroma Airport) and 2 crew members on board, heading on a magnetic course of 137°. The sky was covered with stratus clouds with a base at 220 meters, winds from 280° at 5 m/s, drizzle, haze, and visibility up to 4 kilometers. The crew reported takeoff, and the controller instructed them to climb to an altitude of 1,800 meters. Then, 35 seconds after the first takeoff report, the crew reported takeoff again. The controller asked if the pilots understood the instruction to climb to 1,800 meters, but there was no response. All attempts by the controller to contact flight 637 were unsuccessful.
According to data from the flight data recorder, 1 minute and 10 seconds after takeoff, the aircraft ascended to 200 meters and entered the clouds, prompting the crew to switch to instrument flight. For the next 70 seconds, the aircraft flew in the clouds without deviations and climbed to 530 meters, after which the crew made a right turn to a course of 150° towards Privolzhsk, reporting this to the ground. However, after completing the turn and leveling out from the right bank, the aircraft quickly entered a left bank, which reached 55° within 20 seconds. The pilots may have noticed this by the course change, as 2 minutes and 50 seconds after takeoff (30 seconds after the start of the turn), the bank angle stopped increasing and reduced to 20-25° over the next 6 seconds. However, the pilots seemed not to understand the aircraft's spatial position, as the left bank angle quickly increased again, reaching 73°. The aircraft began to descend with a vertical speed of up to 50 m/s, and the indicated airspeed exceeded the maximum permissible 410 km/h, reaching 485 km/h. The pilots attempted to stop the descent by deflecting the elevators, which, along with reducing the bank angle, led to the occurrence of vertical acceleration, but the descent rate only increased.
At an altitude of 200 meters, the aircraft emerged from the clouds, allowing the crew to regain orientation and level the wings. However, since the forward speed exceeded the maximum permissible, and the pilots were inexperienced in handling the aircraft at such high speeds and low altitude, they likely feared creating negative loads and thus did not deflect the elevators downward. Meanwhile, as the aircraft leveled off at an altitude of 60 meters, the wings reached a horizontal position, leading to a rapid increase in lift, causing the aircraft to soar upwards, climbing at a vertical speed of 20–25 m/s back into the clouds.
It is important to note a critical design feature of the L-410's artificial horizons AGB-2 — at an extreme bank angle of 80°, their indication systems shut off. As a result, during the second ascent, the artificial horizons were no longer providing accurate indications. Flying in the clouds without visible external references, the pilots lost spatial orientation. The aircraft climbed to an altitude of 520 meters, then began descending again. When it emerged from the clouds, the crew leveled the wings, but they were unable to prevent a rapid ascent. Descending to an altitude of 60 meters, the aircraft again ascended into the clouds. This time, it climbed only to 320 meters, before rapidly descending with a steep bank angle exceeding 80°, nearly vertical. The crew reduced engine power, keeping the indicated airspeed below 320 km/h.
When the aircraft emerged from the clouds at 200 meters for the third time, the crew corrected the bank angle. However, as the speed was now one and a half times lower and the engines were operating at low power, the descent rate only slowed from 40 to 10 m/s. The aircraft, flying on a magnetic course of 340°, struck a pine tree 24 meters high with its right wing, then another tree two meters later, causing the aircraft to yaw right by 10-15°. The right wing then struck two fir trees and detached. The aircraft began rapidly rotating right around its longitudinal axis, colliding with several trees and breaking apart. After rotating 330°, the aircraft crashed into the ground, slid along it, and then struck a chain-link fence on reinforced concrete posts, coming to a stop. The crash occurred at 08:13 near the village of Gridino, 11 kilometers southeast (azimuth 139°) of the departure airport. The entire flight lasted 4.5 minutes from takeoff. All 10 people on board were killed.

## Investigation

The investigation revealed that due to improper loading, the aircraft's center of gravity was shifted beyond the maximum allowable rear limit by 2.6% MAC. However, the aircraft was controllable, and the commission concluded that the shift in the center of gravity did not affect the outcome of the flight. The flight instruments, including the primary and backup artificial horizons, correction switches, and turn indicators, were operational until the aircraft broke apart. According to the medical examination, the pilots were in a state of stress (the so-called "shock kidney"), which may have influenced their actions during the developing emergency and when attempting to recover from the final descent that ended in the crash with trees.
Based on data from the flight data recorder, flight and simulator tests were conducted, revealing that until 530 meters, the crew followed correct instrument readings. However, after the aircraft was rolled out from a planned right turn and began entering an increasing left bank, reaching extreme values, the situation became typical of an instrument malfunction, known as a slow "failure" of the artificial horizon. However, since technical expertise showed that the artificial horizons were functional, and the gyroscopes were spinning, it was theorized that the pilot, during pre-flight operations at the airport, mistakenly turned off the artificial horizon while executing the checklist, and it was not noticed until the emergency began developing. Consequently, the pilots did not switch to backup instruments, and when the bank angle reached 80°, the artificial horizons turned off, leaving the crew without accurate information about the aircraft's attitude.

### Causes

Since small aircraft are not equipped with cockpit voice recorders, the investigators could not fully reconstruct the accident. Therefore, the exact cause of the crash was not determined. There is only the hypothesis that before takeoff, the artificial horizons were accidentally turned off, leading to incorrect bank indications, causing the crew to lose spatial orientation while flying in clouds. The design flaw of the artificial horizons AGB-2, which deactivate their indication systems at extreme bank angles, played a role in the crash, leaving the crew without accurate instruments to determine the aircraft's attitude.