The incorrect crew response to a faulty rudder control system caused the AirAsia Airbus A320 passenger jet to stall and eventually cras...
The incorrect crew response to a faulty rudder control system caused the AirAsia Airbus A320 passenger jet to stall and eventually crash into Java Sea on 28 December 2014, investigation by the Indonesian National Transport Safety Committee revealed in its final report released today.
The Airbus A320-216 aircraft registered as PK-AXC was cruising at 32,000 feet on a flight from Juanda Airport, Surabaya, Indonesia to Changi Airport, Singapore with total occupants of 162 persons onboard.
The Flight Data Recorder (FDR) recorded that 4 master cautions activated following the failure of the Rudder Travel Limiter Unit (RTLU) in flight beginning at 2301:10 UTC, which triggered Electronic Centralized Aircraft Monitoring (ECAM) message of AUTO FLT RUD TRV LIM SYS.
The crew performed the ECAM procedure on the first three master caution activations. After the 4th master caution, the FDR recorded different pilot action and the parameters showed similar signature to Flight Augmentation Computer (FAC) Circuit Breaker's being reset resulting in electrical interruption to the FAC.
This pilot action resulted on the 5th and 6th master caution activations which correspond respectively to ECAM message of AUTO FLT FAC 1 FAULT and AUTO FLT FAC 1+2 FAULT.
Following the two FAC fault, the autopilot and auto-thrust disengaged and the flight control reverted to Alternate Law which means the aircraft lost several flight envelope protections available in Normal Law.
After electrical interruption the autopilot disengaged and the rudder deflected at 2°, causing the aircraft to roll to left without pilot input at a rate of 6° per second. This rate of roll was two times faster than normal roll rate operation.
The SIC who acted as Pilot Flying responded 9 seconds after the autopilot off when the roll angle had reached 54°. Normally a pilot will respond immediately to level the wings when an aircraft is rolling without input by the pilot or normal system.
Subsequent flight crew action leading to inability to control the aircraft in the Alternate Law resulted in the aircraft departing from the normal flight envelope and entering prolonged stall condition that was beyond the capability of the flight crew to recover.
The experience of the PIC witnessing problem solving by resetting the FAC CBs on 25 December 2014 when the aircraft had a RTLU problem on the ground, might have influenced the PIC to adopt the same procedure when confronted with the same problem.
Resetting the FAC CB on the ground and in flight has different consequences. The FAC CBs were not included in the list of the CB allowed by Airbus to be reset in flight. The consequences of resetting FAC CBs in flight are not described in Airbus documents. It requires good understanding of the aircraft system to be aware of the consequences.
The investigation concluded that contributing factors to this accident were: The cracking of a solder joint of both channel A and B resulted in loss of electrical continuity and led to RTLU failure. Failure of both RTLUs will stop the rudder limiter at the last position, while the operation of the rudder will not be affected.
The failure does not affect the continuity of the safe flight as the autopilot, auto-thrust and other systems controlled by the FAC are still available. The maintenance records showed that there were 23 Rudder Travel Limiter problems starting from January 2014 to 27 December 2014.
At 2317:41 UTC the aircraft reached the highest altitude of 38,500 feet (ISIS) and largest roll angle at 104° to the left. The aircraft then descended with a rate up to 20,000 feet per minute momentarily afterward the rate of descent was recorded at average of 12,000 feet per minute until the end of recording.
The last data recorded by the FDR were at 2320:35 UTC with the airspeed of 83 kts, pitch 20° up, AOA 50°, roll 8° to left, with the rate of descend of 8,400 ft/minute at a radio altitude of 187 feet.
The Indonesian Pilot in Command (PIC) acted as Pilot Monitoring (PM) and the French Second in Command (SIC) acted as Pilot Flying (PF).
