What happened
On 27 July 2016, an Airbus A320-214, registration G-EZWX, was operating a commercial passenger flight from Heraklion, Greece, to London Gatwick. While cruising at FL340, the aircraft experienced oscillations in pitch and normal acceleration. This instability triggered a 'stab jam' ECAM message, causing the autopilot to disengage and the flight control law to degrade to 'Alternate Law'.
During the event, the flight crew managed the situation by performing required checklists. While the crew noted that the manual trim was used, they did not move the stabiliser significantly as the aircraft appeared to be largely in trim. The aircraft briefly climbed 100 feet above its assigned altitude before the crew returned it to level flight. The flight continued to Gatwick without further incident, eventually transitioning to 'Direct Law' during the landing approach.
The investigation
Investigators analysed flight data recorder (FDR) information, which confirmed a period of pitch and acceleration oscillations. The data showed a handover between elevator aileron computers (ELAC) and a subsequent loss of autopilot. The investigation established that the stabiliser had settled in a position 0.6° more nose-down than its original state, while the elevator compensated with a 2° nose-up position.
Upon inspection of the trimmable horizontal stabiliser actuator (THSA), technicians found that the mini reduction gear assembly was no longer driving the COM transducer. Crucially, approximately 2 ml of water was discovered inside the mini reduction gear liner/cavity. Further examination of the COM transducer revealed additional water and evidence of corrosion within the housing.
Findings
- The primary cause of the malfunction was water ingress into the THS command transducer, which then migrated into the mini reduction gear.
- The presence of water in the gear assembly likely froze during flight, causing mechanical damage to the gears and preventing the transducer from sensing the commanded stabiliser position.
- The discrepancy between the commanded and sensed position triggered the system logic to cease all electrical commands to the stabiliser.
- No external source of water could be identified on the aircraft, and inspections of similar components in the fleet showed no similar issues.