CHI07FA107

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.

History of flight

On April 7, 2007, about 2123 eastern daylight time, a Canadair CL-600-2B19 (CRJ), N77181, operated by Mesa Airlines as flight 7264, was substantially damaged when the thrust reverser translating cowling separated from the left engine during cruise flight and struck the horizontal and vertical stabilizers. The airplane was near the eastern shore of Lake Michigan, about 35 miles west-northwest of Grand Rapids, Michigan, when the accident occurred. The revenue air carrier flight was operating under 14 CFR Part 121 on an instrument flight rules (IFR) flight plan. Visual meteorological conditions prevailed in the vicinity at the time of the accident. The one passenger, one flight attendant, and two pilots on-board were not injured. The flight departed Capital City Airport (LAN), Lansing, Michigan, at 2054. The flight continued to the intended destination, Chicago O'Hare International Airport (ORD), Chicago, Illinois, and landed without further incident at 2136.

The captain noted that air traffic into ORD was under flow restrictions and the flight had to wait for its assigned release time. While holding for takeoff at LAN, the flight crew received a left thrust reverser unlock master caution and associated Engine Indicating and Crew Alert System (EICAS) indications. The captain contacted maintenance and cycled the reverser a few times in an attempt to clear the indications. He had decided to return to the gate when the messages cleared. He subsequently cycled the thrust reversers two or three more times and both appeared to be operating and stowing properly. Since the thrust reversers seemed to be operating normally, he elected to depart for ORD.

The captain reported experiencing a small vibration on climb out. The flight climbed to a cruise altitude of 16,000 feet mean sea level. The vibration persisted and the captain became concerned about the thrust reverser. He stated that about 35 miles west-northwest of Grand Rapids, 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 hand flew the airplane for a time. He ultimately elected to continue to ORD because the thrust reverser unlock messages had cleared and the vibrations had stopped. The flight subsequently landed uneventfully at ORD. The autopilot and trim system worked properly after the event. The missing left engine cowling and damage to the empennage were observed during the post flight inspection.

Personnel information

The pilot-in-command (captain) held an Airline Transport Pilot certificate with multi-engine and single-engine airplane ratings. His certificate included CL-65 and EMB-145 type ratings. He was issued a First Class Airman Medical Certificate without limitations or waivers on December 8, 2006. He had accumulated approximately 8,000 hours total flight time, with about 500 hours in CRJ aircraft. His most recent flight check was completed on February 22, 2007.

The second-in-command (first officer) held an Airline Transport Pilot certificate with multi-engine and single-engine airplane ratings. The single-engine rating was limited to commercial pilot privileges only. His certificate included a CL-65 type rating limited to second-in-command privileges only. He was issued a First-Class Airman Medical Certificate without limitations or waivers on December 8, 2006. He had accumulated approximately 5,600 hours total flight time, with about 2,920 hours in CRJ aircraft. His most recent flight check was completed on January 13, 2007.

Aircraft information

The accident airplane was a 1997 Canadair (Bombardier) CL-600-2B19, serial number 7181. It was certificated as a transport category airplane under Type Certificate A21EA. It was configured with 50 passenger seats. The airplane was powered by two General Electric CF34-3B1 turbofan engines, rated at 9,300 lbs. of thrust. At the time of the accident, the airplane had accumulated 24,805 hours flight time. The left engine, serial number GE-E872898, had accumulated 11,489 hours time in service. The right engine, serial number GE-E872265, had accumulated 22,856 hours time in service.

Review of the aircraft's maintenance records revealed a history of anomalies related to the left engine thrust reverser. On March 11, 2007, the aircraft maintenance log contained the discrepancy, "L Rev Unlock Caution." The entry was deferred in accordance with the Mesa Airlines CRJ Minimum Equipment List (MEL). On March 18, 2007, the left pneumatic drive unit was replaced; however, operational testing determined that the discrepancy was not resolved. The maintenance record noted binding in the drive assembly to the ballscrew actuator. On March 20, 2007, the left engine thrust reverser flex shafts were replaced. Again, the discrepancy was not resolved. On March 22, 2007, a ballscrew actuator and a cascade assembly were replaced. The maintenance record indicated that rigging and operational checks were satisfactory. The MEL item was closed at that time.

Routine maintenance was conducted on March 30, 2007, at which time the thrust reverser and ballscrew actuators were lubricated. No defects were noted in the records. Subsequent to the date the MEL was closed, there were no entries related to the left thrust reverser in the aircraft maintenance log.

The flight log indicated that on the day of the accident, the airplane flew from Columbia Metropolitan Airport (CAE), Columbia, South Carolina, to ORD. The airplane then flew to Harrisburg International Airport (MDT), Harrisburg, Pennsylvania, and back to ORD. The next flight, which was the flight prior to the accident flight, was from ORD to LAN. According to the log, that flight landed at 1943.

The Mesa Airlines CRJ MEL, dated July 14, 2006, stated that an airplane may be dispatched with one of the engine thrust reversers inoperative. The MEL required that the inoperative reverser be inspected for structural damage, deactivated, and stowed and locked in the forward thrust position.

Meteorological information

Surface weather conditions recorded at the Gerald R. Ford Airport (GRR), Grand Rapids, Michigan, at 2056, were: Winds from 280 degrees at 9 knots; 10 miles visibility; broken clouds at 5,500 feet above ground level (agl); overcast clouds at 7,500 feet agl; temperature and dew point -03 and -07 degrees Celsius, respectively; altimeter 29.93 inches of mercury.

Flight recorders

The NTSB Vehicle Recorders Laboratory downloaded the flight data recorder (FDR) and the data were reviewed. Elapsed time is noted in minutes and seconds (MM:SS) relative to the airplane becoming fully airborne (weight-off-wheels signals from all three landing gear). The data indicated that:

At 03:14 (MM:SS) after liftoff, as the airplane climbed through approximately 8,500 feet pressure altitude, the left engine thrust reverser unlock parameter indicated "On" for about one second.

At 05:42 into the flight, the left engine fuel flow began to decrease, followed by the engine speed (N1 and N2 parameters). The fuel flow decreased over a period of approximately 20 seconds before returning to the previous level. The engine speed followed that trend.

At 11:20, both engine fuel flows and engine speeds again began to decrease. Just prior to this, the left engine vibration parameter increased for approximately 2 seconds, before dropping below the left engine vibration level recorded earlier in the flight. As this fuel flow decrease was occurring, the airplane rolled right, left, and right again over about a 4-second period. The decrease in the left engine parameters lasted over 3 minutes. The decrease in the right engine parameters lasted for approximately 2 minutes before both began to increase again to normal levels.

At 14:46, the lef