[Avionics Today 12-3-2015] The crash of AirAsia flight QZ-8501 late last year was caused by series of aircraft computer technical failures and the way pilots responded to them resulted, according to an accident investigation report released Tuesday, Dec. 1, by Indonesia's National Transportation Safety Committee (NTSC). In the report, investigators concluded that the failure of the AirAsia Airbus
A320's Rudder Travel Limiter Units (RTLUs) and the pilots’ response to the malfunction ultimately lead to the crash.
AirAsia Airbus A320-216. Photo: Flickr User Aero Icarus.
departed from Surabaya, Indonesia en route to Singapore on Dec. 28, 2014, with an Airbus A320-216 that climbed at an abnormally fast rate prior to disappearing from Air Traffic Control (ATC) radar screens and crashing. All 162 passengers and flight crew onboard perished in the incident. The fatal AirAsia flight was one of several major accidents that occurred on flights operated by Asia Pacific region carriers in 2014, most notably MH370
, which lead to an international aviation industry effort to improve global flight tracking
Indonesian air safety officials report that defective equipment caused the A320's airplane rudder movement regulating system to repeatedly malfunction during the flight. More specifically, the report states that the rudder movement malfunctioning was the result of a cracked solder joint.
"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 report said.
NTSC's report states that the problems began when the flight reached a cruising altitude of 32,000 feet (FL 320). At that point the aircraft's Flight Data Recorder (FDR) showed there were four consecutive master caution alerts indicating malfunctioning RTLUs. Following the first three master caution activation, the pilots performed Electronic Centralized Aircraft Monitor (ECAM) actions that restored the normal functionality of the RTLUs.
However, according to the report, after the fourth master caution alert, the FDR did not record any ECAM actions. At that point, the FDR indicates that a Jakarta Radar air traffic controller cleared the Pilot in Command (PIC) for a requested climb to FL 340, however there was no response from the cockpit. Another two master caution alerts indicated malfunctioning with the aircraft's flight augmentation computer, which manages RTLU functions. At this point, the A320's autopilot and auto thrust systems became disengaged, causing the fly-by-wire system to exit the aircraft's normal flight envelope.
"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 two FAC fault, the autopilot and auto-thrust disengaged and the flight control reverted to Alternate Law, which means the aircraft lost several protections available in Normal Law. The aircraft entered an upset condition and the stall warning activated until the end of recording," the report said.
A review of the A320's aircraft maintenance records showed that aircraft's rudder movement malfunctioned 23 times during the year before the crash. The intervals between those incidents were also reduced during the three months immediately prior to the crash date.
As a result of the investigation, the NTSC has issued recommendations to AirAsia to "re-emphasize the importance of the standard call-outs in all phases of flight." The safety board has also recommended that AirAsia "re-emphasize the taking over of the control procedure in various critical situations of flight."
Additionally, the safety board recommends that Indonesia's Directorate General Civil Aviation require more upset recovery training of air operators under CASR 121. Airbus has also received recommendations from NTSC to consider "developing a means for flight crews to effectively manage multiple and repetitive Master Caution alarms to reduce distraction." The French airframe manufacturer, Airbus, has also received recommendations to consider and review the Flight Control Training Manual (FCTM) concerning the standard call-outs in all phases of flight.