Tamarack Helicopters Inc UH-1H N175SF accident near Gila Bend, Arizona

Probable cause

The separation of the 90° tail rotor gearbox assembly as a result of fatigue to the tail rotor gearbox input quill assembly attachment studs due to the helicopter experiencing highly repetitive agricultural flight profiles for which it was not designed.

NTSB analysis

While the helicopter was maneuvering at low altitude after an agricultural application, its 90° tail rotor gearbox separated. The commercial pilot performed an autorotation, and the helicopter rolled over on impact.

The gearbox had been installed about 300 flight hours before the accident. Postaccident examination revealed that the gearbox separation was the result of fatigue fracturing of its six attaching studs, which, once failed, liberated the gearbox from the input quill sleeve. Fatigue is typically the result of insufficient preload in the fastener often due to insufficient torque during installation or by the loss of preload after installation. Fretting wear on the studs and in the holes indicated that there were small amounts of relative movement between these components and thereby also indicated insufficient preload. However, the interface between the input quill housing and the vertical fin showed no significant fretting or wear.

The locations of the main fatigue origins in the studs and the fretting location between the studs and fin holes were both consistent with normal torsional loads in the tail rotor drive system. The studs exhibited a large percentage of fatigue penetration before final stud separation and relatively small overstress zones. These signatures are indicative of a low mean stress and a large number of small tail rotor inputs (loads) consistent with repetitive aerial application flight profiles.

The helicopter, which was originally designed and manufactured for the military, was never made commercially available. As such, individual owners and companies were responsible for certification and continued airworthiness of this and other surplus military aircraft.

A review of National Transportation Safety Board records indicated two accidents involving similar failures on this helicopter type. Both were surplus military helicopters flying in the restricted category and used in highly repetitive cycle work that would require significant tail rotor inputs.

Text from the NTSB. Open the NTSB docket for WPR17LA002 for the full record.