What happened
While holding for takeoff, the flight crew received a left thrust reverser unlock indication. The captain cycled the reverser, and after the messages cleared, the crew decided to proceed with the takeoff. During the climb out, the captain noted a small vibration followed by a loud bang, which caused the aircraft to pitch and yaw/roll to the left. During this event, the autopilot disengaged and the left thrust lever moved to idle.
The first officer performed the checklist to stow the reverser. Because the thrust reverser messages had cleared and the vibrations had ceased, the captain elected to continue to the intended destination. The flight concluded with an uneventful landing.
The investigation
Flight data recorder analysis showed that left engine vibration levels were elevated throughout the flight. Approximately three minutes after takeoff, the left engine thrust reverser unlock parameter changed to "On" for about one second. At approximately 11:20 into the flight, the left engine vibration increased momentarily, and both engine speeds and fuel flows decreased. During this period, the airplane experienced a series of rolls over a four-second duration. The decrease in engine parameters lasted over three minutes before returning to normal levels.
Post-accident inspection revealed that the left engine translating cowl had separated from the engine assembly, causing damage to the empennage. Damage to the thrust reverser components was consistent with prior operation involving a misaligned and jammed translating structure. Maintenance records for the Bombardier aircraft showed a history of anomalies regarding the left engine thrust reverser, including a previous "L Rev Unlock Caution" that had been deferred under the minimum equipment list (MEL). Although the pneumatic drive unit, flex shafts, a ballscrew actuator, and a cascade assembly were replaced, the underlying issue persisted.
Findings
- The separation of the left engine translating cowl caused the observed vibration and flight path deviations.
- Maintenance records indicated a history of thrust reverser discrepancies that were not fully resolved by previous repairs.
- Thrust reverser misalignment and jamming of the translating structure contributed to the component failure.