What happened
On 14 November 2019, a DHC-8-402, registration G-FLBE, was operating a commercial passenger flight from Newquay to London Heathrow. During the second sector of the day, the aircraft encountered strong winds and turbulence. Shortly after climbing to 1,000 feet, the co-pilot noticed the aircraft struggling to maintain a right turn and observed that the handwheel was significantly offset to the right to keep the wings level.
The commander took control and confirmed the handwheel was displaced by approximately 30° to 40°. As the aircraft continued its climb, the required deflection increased with airspeed. The crew decided to terminate the climb at FL200 and diverted to Exeter Airport. During the descent, the crew noted that the handwheel deflection decreased as the outboard spoilers became active at lower speeds. The aircraft landed safely at Exeter without further incident.
The investigation
Post-flight inspections of G-FLBE revealed that the lower left aileron cable had snapped just outboard of the engine where it passed over a pulley. The investigation also examined the aircraft's flight data and control systems. It was noted that the operator's fleet of aircraft featured non-reversible filters on the Flight Data Recorder, which prevented the full reconstruction of actual control positions. Additionally, the investigation found that the right aileron on this aircraft and others in the fleet occasionally failed to respond to handwheel movements.
Findings
- The primary cause of the control imbalance was a broken left aileron cable located along the wing rear spar.
- The crew's decision to divert was appropriate given the control difficulties and the lack of a specific relevant checklist in their version of the Quick Reference Handbook.
- The use of non-reversible filters in the FDR made it difficult to accurately assess the true position of the control inputs during the event.
- The aircraft's right aileron had shown intermittent unresponsiveness to handwheel inputs prior to the cable failure.