Tamarack UH-1H N175SF

Summary

On 5 October 2016 at about 16:00 local time, a Tamarack UH-1H registered N175SF, operated by Farm Ag Enterprises, was involved in an accident near Gila Bend, Arizona, United States. One person was on board and one had minor injuries. The aircraft was substantially damaged. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

5 October 2016 at 16:00

Classification

Accident

Location

Gila Bend, Arizona, United States

Coordinates

32.9756, -112.7286

Aircraft

Tamarack UH-1H

Registration

N175SF

Category

Helicopter

Year built

1969

Engines

1

Operator

Farm Ag Enterprises

Operating rule

Part 137: Agricultural

Purpose of flight

Aerial Application

Phase of flight

Not recorded

Route

Gila Bend → Gila Bend

Aircraft damage

Substantial

Weather

VMC

Light

Day

NTSB number

WPR17LA002

1 person had minor injuries.

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.

Quoted verbatim from the NTSB record. This site does not paraphrase or interpret it.

Read the full NTSB narrative

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.

Quoted verbatim from the NTSB record.