Final Report: Maintenance Error Resulted in Asymmetric Flaps

 - March 16, 2007, 10:58 AM

MITSUBISHI MU-2B-35, HILTON HEAD, S.C., AUG. 1, 2001–The NTSB determined that the probable cause of the accident was the improper maintenance/installation and inadequate inspection of the airplane’s flap torque-tube joints during routine maintenance by company maintenance personnel. This 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 impact with a tree during approach to land.

Mitsubishi N1VY was on final approach to land at Hilton Head Airport (HXD) when it was seen by witnesses to suddenly roll to the right, descend and hit trees at about the 70-foot level, killing the pilot. A fire broke out when it hit the ground. N1VY’s debris field extended about 370 feet.

Examination of the wreckage revealed the left-wing flap actuator and jack nut measurements were consistent with the wing flaps being extended to 40 degrees, and on the right wing the flap jack nut and actuator measurements were consistent with the right flap being extended to about 20 degrees. The right-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, or as specified in AD 88-23-01. Instead, the cotter pin had missed the male spline on the flap motor. The flap coupler on the opposite side of the flap motor was also found without a cotter pin installed.

Company maintenance records showed that on April 3, 2001, about 87 flight hours before the accident, the airplane was inspected per Airworthiness Directive AD 88-23-01, which required the disassembly, inspection and reassembly of the flap torque-tube joints. On July 9, 2001, the airplane was given a phase 1 inspection and Bankair records showed that a company-authorized technician performed the applicable maintenance items and certified the airplane for return to service.