The National Transportation Safety Committee (NTSC) of Indonesia has released their final report on the December 2014 crash of Indonesia AirAsia flight QZ8501, which plunged into the Java Sea while enroute from Surabaya to Singapore-Changi International. As with any accident, this crash occurred after a series of cascading events took place, ultimately resulting in the deaths of all 162 passengers and crew onboard.

Airliner crash investigations are painstakingly exhaustive, and rarely the result of any single incident (the recent Metrojet Airbus crash in Egypt could be one of those rare instances) but more often the chain of cascading events that lead to an unrecoverable situation. Indonesia’s NTSC identified several contributing factors in their final report, so let’s take a look at what happened.

Three days prior to the crash of QZ8501, the Airbus A320 registered PK-AXC was operating a flight from Surabaya to Kuala Lumpur. The same captain (with over 20,000 total hours, over 4600 in type) made a call to maintenance with a failure of the rudder travel limit system. After resetting the flight augmentation computer (FAC) circuit breakers, the problem went away, only to resurface shortly. Upon replacing the #2 FAC, the aircraft departed for Kuala Lumpur without incident.

Fast forward to December 28th, and the same captain was flying the same aircraft out of Surabaya with the first officer acting as pilot flying. Once enroute, the Flight Data Recorder (FDR) showed the master caution activating four times with the rudder travel limiter failure. After the first three master cautions, the crew complied with the Electronic Centralized Aircraft Monitoring (ECAM) procedure. Yet on the fourth, the FDR recorded a different response, this time akin to the action several days prior of resetting the FAC circuit breakers. This action produced a less than desirable (but not yet dangerous) result when the master caution activated again as a result of the FAC 1 & 2 faults. Consequently, the autopilot and auto-throttle systems failed and the aircraft’s flight control logic switched from Normal Law to Alternate Law.