Plane & Pilot
Saturday, May 1, 2004

Known And Unknown Deficiencies

It’s both the pilot’s and mechanic’s responsibility to find faulty equipment

During the examination of the wreckage, investigators found that the flap torque tube assembly between the flap motor and the flap stop assembly had disconnected. The flap torque tube assembly’s female coupler, which attaches to the male spline end of the flap motor and flap stop assembly, was found with a cotter pin installed through the female coupler of the flap stop assembly. The cotter pin had not been placed through the spline and the coupler, consistent with normal installation, as per Mitsubishi’s maintenance manual and as specified in Service Bulletin 189. The cotter pin missed the male spline on the flap motor. In addition, the flap coupler on the opposite side of the flap motor did not have a cotter pin installed.

Measurements taken during the wreckage examination revealed that the flap on the left side was fully extended to the 40-degree position, while the flap on the right side had been extended to only 20 degrees.

The NTSB determined that the probable cause of the accident was improper maintenance/installation and inadequate inspection of the airplane’s flap torque tube joints during routine maintenance by company maintenance personnel, which resulted in the right flap torque tube assembly coupler becoming detached and the flaps developing asymmetrical lift when extended, which resulted in an uncontrolled roll, a descent and an impact with a tree during the approach to land.

Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other news concerning the National Transportation Safety Board. To subscribe, write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, NY 10602-0831.


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