What happened
On May 3, 2017, an Airbus A330-343, registration 9M-XXS, operated by AirAsia X Berhad, encountered moderate to severe turbulence while cruising at FL390. The aircraft was en-route from Taipei to Kuala Lumpur when it hit intense wind variations, causing the aircraft to roll rapidly to the right and pitch down significantly.
As the aircraft entered a period of extreme instability, the bank angle exceeded 90 degrees and the pitch angle reached -27 degrees. During the event, the autopilot and autothrust systems were manually disconnected, and the captain took manual control to manage the abrupt maneuvers. The turbulence was so intense that the crew declared a PAN call to Ho Chi Minh Air Traffic Control.
While the aircraft's flight control protections prevented a stall, the turbulence caused physical damage to some cabin ceiling panels. The incident resulted in 5 passengers injured, with one person requiring medical attention upon arrival in Kuala Lumpur.
The investigation
AAIB Malaysia examined flight data recorder (FDR) information, which revealed significant load factor excursions, including a vertical load of +2.094g. The investigation focused on the sequence of flight control movements and the technical response of the aircraft's systems.
Investigators analyzed the flight control computers, specifically noting that a PRIM 1 fault occurred during the upset. This fault was linked to a disagreement in elevator orders triggered by the extreme bank angle. The crew successfully reset the system, restoring normal flight control laws. Post-flight inspections of the airframe were conducted; while some fastener rotation was noted on the wing, it was determined to be unrelated to this specific event.
Findings
- The aircraft encountered severe turbulence characterized by significant wind variations across three axes.
- The flight crew's manual inputs, including dual sidestick inputs, may have contributed to the over-control of the aircraft during the recovery phase.
- The weather radar range selection may have been set too high, potentially preventing the crew from clearly identifying the adverse weather cells ahead on the navigation display.
- The aircraft's high angle-of-attack protections functioned correctly, preventing a stall despite the extreme maneuvers.
Safety action
Following the investigation, recommendations were made to AirAsia X regarding training and operational procedures, including:
- Reintroducing Upset Prevention and Recovery Training (UPRT) into the LOFT syllabus.
- Developing a dedicated Upset Prevention and Recovery Training Program.
- Emphasizing the importance of correct navigation display range selection during recurrent training to ensure better weather monitoring.