NTSB Identification: CEN09FA094
Scheduled 14 CFR Part 121: Air Carrier operation of Mesa Airlines, Inc
Incident occurred Monday, December 15, 2008 in Chicago, IL
Probable Cause Approval Date: 09/30/2009
Aircraft: BOMBARDIER CL-600-2C10, registration: N511MJ
Injuries: 32 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.

During the landing approach, the landing gear "Gear Disagree" warning message appeared. The flight crew performed the appropriate emergency checklists without success. An emergency landing was conducted with a fully extended nose and right main landing gear, but the left main landing gear was not extended. The pilot reported that he was able to keep the left wing up for only a few seconds before it contacted the runway. The airplane skidded to a stop with the left wing supported by the wing flaps and the outboard leading edge slats. The inspection of the airplane revealed that the landing gear belly fairing was fractured. The inspection further indicated that the left main landing gear door had intruded over the belly fairing and weather seal at the fracture area, and had become "hung up" and was not able to extend to the down position. The clearance between the left landing gear door and the belly fairing was measured while the landing gear was in the stowed position. The investigation revealed that the manufacturer’s Aircraft Maintenance Manual (AMM) did not specify the clearance measurement for zone “E”, which was the zone that corresponded to the fractured belly fairing. A review of the airplane’s maintenance records indicated that the left main landing gear door had been replaced at the manufacturer's heavy maintenance facility in September 2007. A maintenance review revealed that the door was rigged and checked with the use of the aircraft hydraulic pumps instead of the external hydraulic power unit as required by the AMM.

As a result of the investigation findings, the following safety actions were taken:

1. On December 23, 2008, Bombardier issued Service Letter CRJ700/900-SL-32-039, “Body Fairing Seal Inspection Common to MLG Inboard Door” to provide operators with guidelines for inspecting the body fairing seal common to the main landing gear inboard doors at the next convenient maintenance opportunity (i.e., 72 hour or 100 flight hours).

2. On December 24, 2008, Bombardier issued Reference Instruction Letter (RIL) No. 1583, "Clarification of clearance for MLG door installation." It stated that it was permissable to use the clearances shown in AMM 32-12-01-220-801-A02 Fig 502 during utilization of AMM 32-12-01-220-801-A01 Figure 501.

3. On January 28, 2009, the Federal Aviation Administration (FAA) issued a Special Airworthiness Information Bulletin (SAIB) No. NM-09-06. The SAIB stated that the FAA “strongly recommends that the affected operators comply with Bombardier Service Letter CRJ700/900-SL-32-039 at the earliest opportunity.”

4. Mesa Airlines conducted a fleetwide inspection of their aircraft and found no other door seal anomalies. Bombardier plans to issue a revised AMM procedure to indicate the proper clearances for zone E. The FAA plans to issue an AD once Bombardier makes the revision to the AMM.

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

The failure of the left landing gear to extend due to improper maintenance when the landing gear door was replaced. Contributing to the incident was the manufacturer's inadequate maintenance manual instructions.

Full narrative available

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