Preceding Circumstances
The aircraft was operating flight F-77 from Moscow to Bugulma with an intermediate stop in Cheboksary. It was piloted by a crew from the 61st Flight Detachment, consisting of Captain V. A. Pastukhov, co-pilot A. S. Cheprasov, and flight engineer A. B. Shtein. Flight attendant N. A. Baskakova was working in the cabin. At 02:02 Moscow time, the An-24 took off from Cheboksary airport and, after climbing, leveled off at a cruising altitude of 4,500 meters. There were 34 passengers on board: 32 adults and 2 children. [1]
According to the weather forecast available to the crew, Bugulma was expected to have overcast conditions with a cloud base at 120 meters and an upper boundary at 3,000 meters, fresh southeast winds (160° 5 m/s), heavy snowfall, mist, and visibility of 1,500 meters. Occasionally, fog was expected, reducing horizontal visibility to 800 meters and vertical visibility to 80 meters. The actual weather in Bugulma almost matched the forecast, with visibility even reaching 4,000 meters — more than twice the expected. This weather was within the meteorological minimum for the captain. [1]
As the aircraft approached Bugulma, at 02:54 Moscow time (52 minutes into the flight), the crew, after receiving clearance from the dispatcher, disconnected the autopilot and began descending to the circuit altitude of 400 meters, which they reached 20 kilometers from Bugulma airport. Following the dispatcher's instructions, the approach was made with a right turn according to ILS with a landing course of 192°. At 16 kilometers from the runway threshold, the crew made the fourth turn and aligned with the final approach. Without deviation from the operating manual, the landing gear and flaps were deployed to 15°. The flight speed was 230 km/h, and the engine mode was initially set to 28-30° on the thrust lever position indicator. At 03:04 Moscow time (63 minutes into the flight), the crew extended the flaps to the landing position (38°) as per the manual. Due to the increased aerodynamic drag, the engine mode was increased to 40° on the thrust lever position indicator. [1]
Accident
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°. [1]
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°). [1]
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. [1]
Causes
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. [1]
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. [1]
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. [1]
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.
- The aircraft's transition to high sideslip angles and subsequent stall were caused by the following erroneous crew actions:
- 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;
- Uncoordinated countering of the yawing moment after engine failure (using only ailerons);
- Insufficient forward control yoke deflection to counteract the pitch-up moment from sideslip, resulting in a loss of speed.
- 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.
- 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: [1] 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.
An aileron is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll, which normally results in a change in flight path due to the tilting of the lift vector. Movement around this axis is called 'rolling' or 'banking'.
Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude.
A slip is an aerodynamic state where an aircraft is moving somewhat sideways as well as forward relative to the oncoming airflow or relative wind. In other words, for a conventional aircraft, the nose will be pointing in the opposite direction to the bank of the wing(s). The aircraft is not in coordinated flight and therefore is flying inefficiently.
Dutch roll is an aircraft motion consisting of an out-of-phase combination of "tail-wagging" (yaw) and rocking from side to side (roll). This yaw-roll coupling is one of the basic flight dynamic modes. This motion is normally well damped in most light aircraft, though some aircraft with well-damped Dutch roll modes can experience a degradation in damping as airspeed decreases and altitude increases. Dutch roll stability can be artificially increased by the installation of a yaw damper. Wings placed well above the center of gravity, swept wings, and dihedral wings tend to increase the roll restoring force, and therefore increase the Dutch roll tendencies; this is why high-winged aircraft often are slightly anhedral, and transport-category swept-wing aircraft are equipped with yaw dampers. A similar phenomenon can happen in a trailer pulled by a car.
Aircraft flight mechanics are relevant to fixed wing and rotary wing (helicopters) aircraft. An aeroplane, is defined in ICAO Document 9110 as, "a power-driven heavier than air aircraft, deriving its lift chiefly from aerodynamic reactions on surface which remain fixed under given conditions of flight".
A vertical stabilizer or tail fin is the static part of the vertical tail of an aircraft. The term is commonly applied to the assembly of both this fixed surface and one or more movable rudders hinged to it. Their role is to provide control, stability and trim in yaw. It is part of the aircraft empennage, specifically of its stabilizers.
The critical engine of a multi-engine fixed-wing aircraft is the engine that, in the event of failure, would most adversely affect the performance or handling abilities of an aircraft. On propeller aircraft, there is a difference in the remaining yawing moments after failure of the left or the right (outboard) engine when all propellers rotate in the same direction due to the P-factor. On turbojet and turbofan twin-engine aircraft, there usually is no difference between the yawing moments after failure of a left or right engine in no-wind condition.
Adverse yaw is the natural and undesirable tendency for an aircraft to yaw in the opposite direction of a roll. It is caused by the difference in lift and drag of each wing. The effect can be greatly minimized with ailerons deliberately designed to create drag when deflected upward and/or mechanisms which automatically apply some amount of coordinated rudder. As the major causes of adverse yaw vary with lift, any fixed-ratio mechanism will fail to fully solve the problem across all flight conditions and thus any manually operated aircraft will require some amount of rudder input from the pilot in order to maintain coordinated flight.
In aviation, coordinated flight of an aircraft is flight without sideslip.
P-factor, also known as asymmetric blade effect and asymmetric disc effect, is an aerodynamic phenomenon experienced by a moving propeller, wherein the propeller's center of thrust moves off-center when the aircraft is at a high angle of attack. This shift in the location of the center of thrust will exert a yawing moment on the aircraft, causing it to yaw slightly to one side. A rudder input is required to counteract the yawing tendency.
The yaw string, also known as a slip string, is a simple device for indicating a slip or skid in an aircraft in flight. It performs the same function as the slip-skid indicator ball, but is more sensitive, and does not require the pilot to look down at the instrument panel. Technically, it measures sideslip angle, not yaw angle, but this indicates how the aircraft must be yawed to return the sideslip angle to zero.
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A flight control mode or flight control law is a computer software algorithm that transforms the movement of the yoke or joystick, made by an aircraft pilot, into movements of the aircraft control surfaces. The control surface movements depend on which of several modes the flight computer is in. In aircraft in which the flight control system is fly-by-wire, the movements the pilot makes to the yoke or joystick in the cockpit, to control the flight, are converted to electronic signals, which are transmitted to the flight control computers that determine how to move each control surface to provide the aircraft movement the pilot ordered.
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