Event Summary
While holding for takeoff, the flight crew received a left thrust reverser unlock indication. The captain cycled the reverser and initially decided to return to the gate, but when the messages cleared, he elected to take off. During climb out, the captain reported a small vibration, followed shortly by a loud bang and a pitch, yaw, and roll to the left. The autopilot disengaged and the left thrust lever moved to idle. The first officer ran the checklist to stow the reverser. The captain continued to the intended destination as the thrust reverser messages cleared and vibrations ceased. The flight landed uneventfully.
Flight Data Recorder Analysis
FDR data showed elevated left engine vibration levels during the flight. Approximately three minutes after takeoff, the left engine thrust reverser unlock parameter changed to "On" for about one second. At about 11 minutes and 20 seconds after takeoff, the left engine vibration level increased momentarily, and both left and right engine speeds and fuel flows began to decrease. As fuel flows decreased, the airplane rolled right, left, and right again over about four seconds. The decrease in engine parameters lasted over three minutes before returning to normal. During that period, the left engine thrust reverser unlock parameter indicated "On" for about four seconds. After the event, left engine vibration levels decreased below previous levels, approximately matching right engine vibration. The vibration and engine speed variations suggested separation of the left engine translating cowling at that point. The left engine thrust reverser deploy parameter remained "Off" for the entire flight. FDR data also revealed that left engine vibration levels were elevated and the left thrust reverser did not deploy on the two previous flights.
Post-Accident Inspection and Maintenance History
Post-accident inspection revealed that the left engine translating cowl had separated from the engine assembly and damaged the empennage. Damage to the thrust reverser components was consistent with prior operation with the reverser out of alignment and jamming of the translating structure. Review of maintenance records showed a history of anomalies related to the left engine thrust reverser. About one month before the incident, the aircraft maintenance log contained the discrepancy "L Rev Unlock Caution." The entry was deferred per the minimum equipment list (MEL). The left pneumatic drive unit and left thrust reverser flex shafts were replaced, but the discrepancy was not resolved. Eleven days later, a ballscrew actuator and cascade assembly were replaced, and rigging and operational checks were satisfactory, closing the MEL item. No further entries related to the left thrust reverser appeared in the log.
Regulatory and Procedural Context
The MEL authorized dispatch with one thrust reverser inoperative, requiring inspection for structural damage, deactivation, and stowage and locking in forward thrust position. In the event of an in-flight unlock or deploy indication, the flight manual instructed the crew to manually stow the reverser. No guidance existed for a ground indication. Bombardier initiated a revision to the flight manual to include instructions for thrust reverser unlock messages received during ground operations.