What happened
On November 6, 2018, an Airbus A340-642 was cruising at flight level 380 within Swiss airspace when it encountered a rapid change in wind direction and speed. This atmospheric disturbance caused the aircraft to exceed its maximum operating Mach number.
In response to the overspeed condition, the pilot in command disconnected the autopilot and transitioned to manual flight, initiating a climb. As the aircraft reached flight level 400, the airspeed continued to decay, causing the aircraft to reach its maximum angle of attack multiple times and triggering the stall warning. The pilot eventually initiated a descent, managing to stabilize the flight path at flight level 340. There were no injuries and no damage to the aircraft or third parties.
The investigation
The BFU investigation examined the flight data, cockpit voice recordings, and the aircraft's technical systems. The investigation focused on the meteorological conditions, specifically the presence of mountain waves, and the crew's application of established procedures. The investigators also reviewed the airline's training programs, specifically regarding the application of Operations Engineering Bulletin (OEB) No. 49 and upset recovery techniques. The investigation also noted that while the aircraft's V-alpha protection system had been modified to comply with EASA directives, the crew's handling of the event led to the aircraft operating in Alternate Law.
Findings
- The primary cause of the overspeed was a rapidly shifting wind direction that was unpredictable for the crew.
- The crew failed to follow the prescribed "Abnormal and Emergency Procedures / Misc / Overspeed Recovery" protocol.
- During the attempt to manage the overspeed, the pilot in command incorrectly deactivated two of the three Air Data Inertial Reference Units (ADIRU) as part of a misapplication of OEB No. 49. This action deactivated the autothrust, leaving the engines at idle thrust.
- The combination of manual pitch-up inputs, the resulting climb, and the lack of thrust led to a rapid loss of airspeed and the activation of the stall warning.
- The crew experienced a loss of situational awareness and failed to effectively coordinate during the recovery phase.
- The pilot monitoring did not adequately fulfill their supervisory role during the stall recovery.
- There were deficiencies in the airline's training, as specific overspeed recovery techniques and upset recovery training were not sufficiently emphasized in the simulator programs.