Aerospatiale AS 350-BA, Princeville, Hawaii, March 8, 2007–The NTSB determined the probable cause of the AS 350 crash was the failure of maintenance personnel to properly tighten (torque) the flight control servo lower attachment clevis and reinstall a functioning lock washer, which resulted in a flight control disconnect and complete loss of helicopter control. The operator’s failure to ensure its maintenance program was being executed in accordance with federal regulations contributed to the accident, in which the ATP-rated pilot and three passengers were killed and three passengers were seriously injured.
The pilot was returning from a sightseeing flight and called the dispatcher to report a hydraulic problem. He then called it a hydraulic failure,
flew to the end of the runway, where the passenger terminal was located, and the helicopter hit the ground.
Examination of the helicopter revealed that the left lateral flight control servo had disconnected at the transmission in flight. The lower clevis of the left lateral servo was still attached to the transmission case, but was no longer attached to the servo.
The threads on the clevis, as well as the threads on the inner diameter of the servo, appeared undamaged. The jam nut, lock-nut washer and safety wire were still attached to the clevis threads and rotated freely. The lock washer was severely worn
and the locking tang was missing.
Metallurgical testing, torque testing and interviews with company mechanics revealed that the jam nut had been improperly tightened to a torque value prescribed in a service bulletin for the upper clevis, which was lower than
the prescribed torque value cited in the maintenance manual for the lower clevis. Improper installation of the servo attachment nut, coupled with the worn lock washer, allowed the servo to rotate off the clevis.
None of the mechanics at the helicopter’s base had received factory training, and the maintenance manuals they used were three revisions out of date. Mandatory quality-control inspections of maintenance actions, as well
as post-maintenance test flights, were not performed.