An-24 crash near Bugulma

Causes

According to data from the flight recorder, when at 03:04, after the flaps were extended further, the crew increased engine power, the feathering pump of the left engine switched on, which led to the feathering of the left powerplant. Thus, the engine shutdown and propeller feathering occurred not because of an engine failure, but because of an electrical signal being sent, and there was no reverse thrust in flight.

According to the commission, this electrical signal arose as a result of a malfunction of the DAF-24 automatic feathering sensor of the left engine, since the electrical contacts in the KV-9-1 microswitch closed due to wear of its stop and contact spring. The KV-9-1 microswitch, under real operating conditions as part of the DAF-24, is insufficiently reliable against vibration loads, and previously, in the period from 1981 to 1985, as many as 22 such failures had occurred. On the crashed An-24 CCCP-46423 itself there had also previously been two cases of automatic propeller feathering, both on the left engine: on 28 January 1985 in level flight at a height of 6000 metres and on 21 February 1986 (9 days before the crash) on the ground during preparation for takeoff. In the latter case specifically, the cause was not identified or eliminated. During periodic inspection of the condition of the DAF-24, carried out every 300±30 hours, it is simply impossible to detect all cases of wear of the KV-9-1 microswitch, and the failures were not completely eliminated even after special measures were introduced by the industry.

As for the crew's actions, the results of simulating the situation showed that if, within the first eight seconds from the onset of the emergency (engine stoppage), the directional channel had been controlled and the yawing moment countered by deflecting the rudder by 10°, with the ailerons deflected to half of their full travel, the aircraft would enter a right bank and maintain straight flight on the assigned descent path. At the same time, the recommendations given in the flight manual for crew actions in the event of an engine failure during the pre-landing glide were correct.

Thus, on the basis of the results of the investigation, the following conclusions were drawn:

The spontaneous shutdown of the left engine with the propeller blades moving into the feathered position occurred due to the failure of the DAF-24 automatic feathering sensor as a result of wear of parts of the KV-9-1 microswitch. The defect is a design one.

The aircraft reaching large sideslip angles and stalling was caused by the following erroneous actions of the crew:

failure to deflect the rudder to counter the yaw after the engine failure, and insufficient deflection of the rudder after setting the right engine to takeoff power without first establishing a bank towards the operating engine;

uncoordinated countering of the yawing moment after the engine failure (with the ailerons only);

insufficient forward deflection of the control wheel to counter the nose-up pitching moment from the sideslip, which led to loss of speed.

The crew had the opportunity to deflect the rudder in time (both in terms of forces and in terms of time) to counter the turn after the engine failure and to ensure recovery of the aircraft from the bank and sideslip and restoration of the initial speed and direction of flight.

The stability and controllability characteristics of the aircraft after the engine failure ensured recovery of the aircraft from the bank and sideslip and restoration of the initial flight speed.

Conclusion: at night, in cloud, on the pre-landing descent path with the flaps and landing gear fully extended, spontaneous feathering of the propeller and shutdown of the engine of the left powerplant occurred. In this situation the crew made errors in piloting technique, which led to loss of speed and a stall of the aircraft with its subsequent collision with the ground surface.