Pilot Debrief

HILLER UH12E near Hanford, CA — 2011-06-23

Final reportWPR11LA282
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Date
2011-06-23
Location
Hanford, CA, USA
Aircraft
HILLER UH12E
Registration
N99579
Category
Helicopter
Highest injury
None
Fatalities
0
Phase of flight
Initial climb

Probable cause

The separation of the tail rotor drive gear shaft due to torsional fatigue of its threaded portion, which resulted in a loss of tail rotor control.

NTSB narrative

The pilot reported that during a preflight inspection he observed a small oil leak originating where the tail rotor drive shaft attaches to the transmission. He said that, following a discussion with his boss, they decided to reposition the helicopter from a vehicle-mounted platform onto the ground so they could perform a more detailed examination of the oil leak. The pilot stated that following a normal engine start, he began to lift the helicopter off the platform and heard a "growling" sound originating from the transmission followed by an immediate loss of tail rotor control. The pilot said that the helicopter immediately began to rotate to the right as he rolled the throttle off and lowered the collective. Subsequently, the helicopter landed hard onto the ground adjacent to the truck and platform. Postaccident examination of the helicopter revealed that the tail rotor gear shaft was separated from the tail rotor drive shaft coupling. Metallurgical analysis of the fracture surface of the tail rotor gear shaft revealed that the fracture was consistent with fatigue cracking that emanated from multiple origins at the root portion of a thread on the threaded portion of the shaft. The fatigue crack origin exhibited signatures consistent with being open for an extended period of time compared to the other regions of the fracture face; the amount of time could not be determined. The fatigue crack eventually propagated around the gear shaft as torsional fatigue. No maintenance records for the helicopter were recovered during the course of the investigation, and therefore the amount of flight time on the tail rotor gear shaft was undetermined.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Pilot could have conducted a more thorough inspection before flight.

NTSB coding

Evidence available

  • 5 docket documents
View NTSB final reportView NTSB docket

Docket documents5

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