NTSB Identification: NYC07MA073
Nonscheduled 14 CFR Part 135: Air Taxi & Commuter
Accident occurred Thursday, March 08, 2007 in Princeville, HI
Probable Cause Approval Date: 01/14/2009
Aircraft: AEROSPATIALE AS350BA, registration: N354NT
Injuries: 4 Fatal,3 Serious.
NTSB investigators traveled in support of this investigation and used data obtained from various sources to prepare this aircraft accident report.
The helicopter was returning from a sightseeing flight when the pilot contacted the dispatcher and announced, "I'm having hydraulic problems, and I'm probably going to have to do a run-on landing." The language he used to describe his situation changed from "hydraulic problem" to "hydraulic failure" as the flight proceeded. The pilot did not immediately land at the approach end of the runway; instead, he continued to the departure end, where the passenger terminal was located. The helicopter approached the mid-point of the runway, and as it neared the surface, the dispatcher heard the pilot state, "Okay we're done." Then, the sound of the rotor changed pitch and the helicopter impacted the ground. Postaccident examination of the helicopter revealed that the left lateral flight control servo became disconnected in flight at the transmission. Further examination revealed that 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 and torque testing of the accident servos revealed that the jam nut, washer, and piston rod were able to easily rotate off of the clevis if no torque was applied and with the locking tang missing. Under normal loads, the main hydraulic flight control servos experience slight twisting that could contribute to the servo rotating off the clevis bolt (without an intact and functioning locking washer installed). Therefore, the investigation revealed that the servo attachment nut was not torqued during maintenance about one month prior to the accident, and the lack of torque, coupled with the installation of a severely worn lock washer, allowed the servo to rotate off of the clevis during the accident flight, resulting in a complete disengagement of the servo from its mounting clevis on the transmission. Examination of the company maintenance program revealed that none of the mechanics at the helicopter's base had received factory training, and that 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 prescribed in the General Operator's Manual and the manufacturer's maintenance manuals (that included the replacement of the flight control servo), were not performed.
(This report was modified on June 16, 2009)
The National Transportation Safety Board determines the probable cause(s) of this accident to be: 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 a complete loss of helicopter control. Contributing to the accident was the operator's failure to ensure its maintenance program was being executed in accordance with Federal regulations. Full narrative available
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