The July 2014 accident involving an MD-83 in the Mali desert was caused by engine icing and the crew’s lack of appropriate repsonse to a stall, according to a report released by the investigation committee from Mali.
The airplane was operated by Spanish airline Swiftair on behalf of Air Algérie as flight AH5017 from Ouagadougou, Burkina Faso to Algiers, Algeria.
Flight AH5017 took off at night. At an altitude of about 10,500 ft, autopilot 1 was engaged, the autothrottle having been active since takeoff. At 01:28, the airplane was transferred to the Ouagadougou CCR and the crew said that they were turning to the left on heading 356° to avoid an bad weather in the area.
During the climb towards FL310, the crew made three heading alterations to the left (of 28°, of 4° then of 8°), then an alteration to the right of 36° to return to heading 019°, close to the initial heading.
Between 01:41:38 and 01:44:29, the Niamey CCR and flight AH5017 tried to get in contact, but did not manage to do so. Flight RAM543K offered to act as the intermediary. The crew of flight AH5017 said, at 01:44:29, that they were at FL310 on a weather avoidance manoeuvre. The Niamey CCR heard this radio exchange and asked them to call back passing GAO and to transmit estimates for MOKAT point. No answer, nor any other messages from flight AH5017, reached the Niamey CCR.
At 01:44, EPR and N1 fluctuations on both engines appeared for about 45 seconds. Subsequently the autothrottle disengaged.
At 01:45, the calibrated airspeed was 203 kt, the Mach 0.561 and the airplane started to descend. Pitch increased until it reached 10°, then decreased slightly while the deflection of the elevators and the position of the trimmable horizontal stabiliser continued pitching up. The EPR and the engines’ RPM started to decrease towards values corresponding to idle. The roll oscillations continued and the speed continued to decrease.
Then the autopilot disengaged. The altitude decreased by about 1,150 ft in relation to the flight level, the calibrated airspeed was 162 kt, the Mach 0.439 and both engines were almost at idle. The airplane’s pitch began to decrease and the bank was increasing to the left. The aircraft had entered a stall and continued to descend until it impacted the ground.
Causes (translated from French):
The speed of the aircraft, controlled by the auto-throttle, decreased due to clogging of the pressure sensors located on the nose cone of the engines, presumably by ice crystals. The autopilot then gradually increased the attitude of the aircraft to maintain altitude until the aircraft stalled. The stall of the aircraft was not recovered. The plane descended in a nose down attitude and a left bank while the rudders remain largely steered in the direction of a bank to the right. The plane hit the ground at high speed.
The accident resulted from the combination of the following:
– Non-activation of engine anti-icing systems;
– Clogging of pressure sensors Pt2, presumably by ice crystals, resulting in erroneous EPR values which caused the auto-throttle to limit the thrust delivered by the engines to a level lower than the thrust needed to maintain FL310;
– Late reaction from the crew at the decrease in speed and erroneous EPR values, possibly related to the crew workload associated with the avoidance of convective zone and communication difficulties with air traffic control;
– The lack of reaction from the crew to the appearance of the buffet, the stick shaker and the stall warning;
– The absence of appropriate action on the flight controls to exit a stall situation.
These events can find their explanations in the combination of the following factors:
– FCOM procedure for activating anti-icing systems was not adapted to the situation of blockage of Pt2 sensors by ice crystals;
– Insufficient information for operators on the consequences of blocking of the Pt2 sensor by an icing phenomenon;
– The trigger logic of the stick shaker and stall alarm that led these devices to trip late to the stall of the aircraft in cruise;
– The logic of the autopilot allowing it to continue to give orders to pitch beyond the impact of stall aggravating the situation leaving and increasing the difficulty of recovery by the crew.
Lack of usable CVR recording limited the possibilities to analyse the crew behavior during the flight.
In particular it was not possible to study aspects CRM or to assess the potential contribution of the employment framework and experience of the crew.