What happened
On 28 September 2005, a DHC-8-311, registration G-BRYW, was conducting a scheduled passenger flight from Aberdeen to Manchester. While cruising at FL230, the pilot discovered that the elevator trim hand wheels could not be moved forward, restricting the ability to apply nose-down trim.
Upon encountering an 'ELEVATOR MISMATCH' annunciation at FL170, the crew disconnected the autopilot and attempted to follow the Quick Reference Handbook (QRH) procedure for an elevator manual trim failure. During this process, they found that the standby electric trim system was also inoperative and could not move the trim wheels in either direction. Despite the restriction, the out-of-trim forces remained manageable, and the crew continued the flight to its destination. As the aircraft descended through FL150, the manual trim operation improved, allowing for some nose-down input.
The investigation
The investigation focused on the cause of the trim restriction and the failure of the standby electric trim system. Post-flight inspections of the aircraft identified the presence of water within the elevator trim screwjacks.
Reviewing the operator's fleet history, the AAIB noted several similar occurrences of pitch trim restrictions on other DHC-8 aircraft. These included incidents where the trim wheels were stiff or jammed, often occurring in flight conditions involving moisture or icing. In some previous cases, the standby electric trim had also failed to function.
Findings
- The primary cause of the trim restriction was moisture accumulation within the elevator trim screwjacks, which froze at altitude.
- The standby electric trim system was found to be inoperative during the flight.
- The aircraft had previously experienced several similar pitch trim restriction events within its fleet.
- While modifications had been implemented to add drain holes and grease fittings to the screwjacks, they remained susceptible to moisture ingress.