What happened
On 15 April 2004, an Airbus A320-232, registration G-TTOA, was performing a passenger flight near Malaga, Spain. During the descent, the aircraft was operating in a Mach speed control mode. As the aircraft descended through FL220, it was descending at a rate of 4,800 ft/min while maintaining a speed between M 0.78 and M 0.79. Due to the nature of Mach speed maintenance during descent, the calibrated airspeed (CAS) increased significantly, eventually approaching the maximum operating speed (VMO).
Upon noticing the speed trend, the crew selected a lower Mach number. Shortly thereafter, the autopilot disconnected. In an attempt to prevent an overspeed, both pilots simultaneously applied aft stick inputs of approximately 10 degrees. This dual input caused the aircraft to experience a sudden surge in normal acceleration, peaking at 2g. The sudden movement caused three of the four cabin crew members to be thrown to the floor, resulting in one broken ankle and two minor injuries. Additionally, two passengers reported stiff necks.
The investigation
The investigation examined the flight control settings and the crew's response to the approaching speed limit. It was established that the co-pilot had manually changed the speed target from SPD to MACH mode, believing the aircraft had inadvertently reverted to SPD mode earlier in the descent. The investigators also analyzed the mechanics of the side-stick controllers, noting that both pilots continued to provide simultaneous inputs for eight seconds without using the side-stick take-over buttons.
Findings
- The aircraft was descending in MACH mode, which caused the airspeed to increase as altitude decreased.
- The crew did not realize the aircraft was still in MACH mode and therefore did not account for the rising CAS relative to VMO.
- The primary cause was the simultaneous application of aft stick inputs by both pilots, which led to the sudden increase in g-load.
- The pilots failed to use the side-stick take-over buttons or communicate control of the aircraft.
- Fatigue was noted as a factor, as both pilots had recently completed a demanding duty pattern.