NTSB Identification: CHI07FA107
Scheduled 14 CFR Part 121: Air Carrier operation of Mesa Airlines, Inc. (D.B.A. United Express)
Accident occurred Saturday, April 07, 2007 in Lake Michigan, MI
Probable Cause Approval Date: 12/24/2008
Aircraft: Canadair CL-600-2B19, registration: N77181
Injuries: 4 Uninjured.

NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.

The flight crew received a left thrust reverser unlock indication while holding for takeoff. The captain cycled the reverser, and had decided to return to the gate when the messages cleared. With the issue apparently resolved, he elected to takeoff. The captain reported experiencing a small vibration on climb out and, a short time later, he heard a "loud bang" and the "aircraft pitched and yawed/rolled to [the] left." The autopilot disengaged and the left thrust lever moved to idle during the event. The first officer ran the checklist to stow the reverser. The captain decided to continue to the intended destination because the thrust reverser messages had cleared and the vibrations had stopped. The flight subsequently landed uneventfully. The flight data recorder (FDR) indicated that the left engine vibration levels were elevated during the accident flight. Approximately 3 minutes into the flight, the left engine thrust reverser unlocked parameter changed to "On" for about 1 second. About 11:20 (MM:SS) after takeoff, the left engine vibration level increased momentarily, and both the left and right engine speeds and fuel flows began to decrease. As the decrease in fuel flows was occurring, the airplane rolled right, left, and right again over about a 4-second period. The decrease in engine parameters lasted over 3 minutes before returning to normal. The left engine thrust reverser unlock parameter indicated "On" for about 4 seconds during that time. The left engine vibration levels decreased after the event, below the levels previously recorded during the flight, and approximately to the level of the 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 in the "Off" state for the entire flight. Additionally, the FDR data revealed that the left engine vibration levels were elevated and left engine thrust reverser did not deploy on the two previous flights. The 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 the aircraft's maintenance records revealed a history of anomalies related to the left engine thrust reverser. About one month prior to the accident, the aircraft maintenance log contained the discrepancy, "L Rev Unlock Caution." The entry was deferred in accordance with the minimum equipment list (MEL). The left pneumatic drive unit and the left thrust reverser flex shafts were subsequently replaced; however the discrepancy was not resolved. Eleven days after the initial write-up, a ballscrew actuator and a cascade assembly were replaced. The maintenance record indicated that rigging and operational checks were satisfactory, and the MEL item was closed. There were no further entries related to the left thrust reverser in the aircraft maintenance log. The MEL allowed the airplane to be dispatched with one of the engine thrust reversers inoperative. It required that the inoperative reverser be inspected for structural damage, deactivated, and stowed and locked in the 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. There was no guidance in the event of an indication on the ground. Bombardier has initiated a revision to the flight manual to include instructions in the event of a thrust reverser unlock message received during ground operations.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

In-flight separation of the left engine thrust reverser translating cowling due to intermittent binding and jamming of the reverser on the accident flight and on previous flights. Contributing factors were the inadequate maintenance action by the operator due to their failure to properly resolve the prior reverser malfunctions, the failure of the pilots of previous flights in not referring earlier reverser deployment failures for maintenance action, and incomplete company/manufacturer's procedures because they did not address anomalous reverser indications during ground operations.

Full narrative available

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