Aeroflot Flight F-77

However, a second after increasing the mode, at a speed of 225 km/h, the left engine's automatic feathering system spontaneously activated, feathering the left propeller. This caused asymmetrical thrust, resulting in a right yawing moment, and the aircraft began to bank to the left, reaching a 20° bank angle within 5 seconds, and deviated to the left. The crew noticed the failure of the left power unit almost immediately and attempted to counter the left bank by deflecting the ailerons to 19° for a right bank and pressing the right rudder pedal forcefully to turn the rudder right. However, by pressing the right pedal, the pilots only neutralized the rudder, as the aircraft began slipping to the left. The forces applied to the pedal (15 kg) merely held the rudder in a neutral position, failing to counteract the yawing moment. However, through aileron deflection, the crew managed to reduce the left bank to 9°.^[2]

Due to the high sideslip angle, speed began to decrease, prompting the pilots to push the control yokes forward, attempting to increase speed by pointing the nose down. However, this measure was ineffective, so the crew moved the remaining operational right engine to takeoff mode, forgetting that, according to the manual, they should first level the aircraft out of the left bank and into a right one. As a result, the left bank increased, exceeding 50°, and the sideslip and pitch angles also increased. Aerodynamic drag increased by 1.5 times, causing speed to drop. The crew attempted to correct the bank with full aileron and rudder deflection, but these measures were too late. By this time, the airliner was flying at a speed of 155 km/h with a sideslip angle of 18-21° and had deviated 50° from the landing course (to 142°).^[2]

At a speed of 140 km/h, the An-24 stalled, and its bank angle rapidly reached 110°. Twenty-five seconds after the left engine shutdown, the aircraft, with a 40° nose-down angle and a 3° left bank, flying at a heading of 15°, hit the ground at a forward speed of 320 km/h and a vertical speed of 40 m/s, 8 kilometers from the runway threshold on an azimuth of 15° (500 meters from the runway centerline). The airliner was completely destroyed on impact, and the debris scattered over an area of 136 by 40 meters, but no fire ensued. All 38 people on board perished.^[2]

According to data from the flight recorder, when the crew increased the engine mode after extending the flaps at 03:04, the left engine's feathering pump activated, leading to the feathering of the left power unit. Thus, the engine shutdown and propeller feathering occurred not due to engine failure but because of an electrical signal, with no reverse thrust applied during the flight.^[2]

The commission determined that this electrical signal was caused by a malfunction in the left engine's automatic feathering sensor DAF-24, as the micro switch KV-9-1's contacts closed due to wear on its stop and contact spring. The KV-9-1 micro switch in actual operational conditions within DAF-24 was not reliable against vibration loads, and from 1981 to 1985, there had been 22 cases of such failures. On the crashed An-24 CCCP-46423, there were also two previous cases of automatic feathering of the propeller on the left engine: on January 28, 1985, in level flight at an altitude of 6,000 meters and on February 21, 1986 (nine days before the crash) on the ground during takeoff preparation. The cause in the latter case was not identified and rectified. During periodic inspections of the DAF-24, conducted every 300±30 hours, detecting all instances of KV-9-1 micro switch wear was impossible, and the failures were not eliminated even after the industry implemented special measures.^[2]

Regarding the crew's actions, simulation results indicated that if the crew had intervened in the yaw control within the first eight seconds of the emergency situation (engine shutdown) and countered the yawing moment by deflecting the rudder to 10°, while half-deflecting the ailerons, the aircraft would have banked right and maintained straight flight on the set descent trajectory. The recommended actions in the manual for the crew during engine failure on final approach were correct.

Based on the investigation results, the following conclusions were made:

1. The spontaneous shutdown of the left engine and feathering of the propeller blades occurred due to the failure of the DAF-24 automatic feathering sensor because of wear on the KV-9-1 micro switch components. The defect was structural.
2. The aircraft's transition to high sideslip angles and subsequent stall were caused by the following erroneous crew actions:
   1. Not deflecting the rudder to counter yaw after engine failure and insufficient rudder deflection after increasing the right engine to takeoff mode without first creating a bank towards the operational engine;
   2. Uncoordinated countering of the yawing moment after engine failure (using only ailerons);
   3. Insufficient forward control yoke deflection to counteract the pitch-up moment from sideslip, resulting in a loss of speed.
3. The crew had the opportunity to timely deflect the rudder (both in terms of effort and time) to counter the yaw after engine failure and to recover the aircraft from the bank and sideslip, restoring the original speed and flight direction.
4. The aircraft's stability and controllability characteristics after engine failure allowed for recovery from the bank and sideslip, and for restoring the original flight speed.

Conclusion (translated): "At night, in clouds, on the final approach with fully extended flaps and landing gear, spontaneous feathering of the propeller and shutdown of the left power unit occurred. In this situation, the crew made piloting errors, leading to a loss of speed and a stall, followed by the aircraft's collision with the ground."