Pilot Debrief

HILLER UH-12E near Millen, GA — 2021-09-29

Final reportERA21LA386
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Date
2021-09-29
Location
Millen, GA, USA
Aircraft
HILLER UH-12E
Registration
N430NR
Category
Helicopter
Highest injury
Minor
Fatalities
0
Phase of flight
Cruise

Probable cause

The improper reassembly of the tail rotor gearbox following maintenance, which resulted in fatigue failure of the input pinion gear shaft, and the subsequent loss of tail rotor drive.

NTSB narrative

During cruise flight, the pilot heard a “pop” sound followed by the engine accelerating to a high rpm. According to the pilot, the helicopter began spinning counterclockwise at a high rate; the cyclic stick would not move; and collective and pedal control inputs had no effect. The pilot continued his attempts to counteract the helicopter’s rotation as it descended to ground impact. Postaccident examination of the wreckage revealed that airframe damage was consistent with ground impact and that main rotor blade deformation and damage were consistent with powered impact with terrain. Damage to the tail rotor blades was consistent with the tail rotor not being under power at impact, and the tail rotor gearbox input pinion bevel gear was found fractured from its gear shaft. Metallurgical examination of the input pinion gear shaft revealed that it had fractured from fatigue. The fatigue cracking initiated at gouge marks on the outer surface of the cylindrical shaft and propagated through nearly the entirety of the shaft cross-section. The length of the fatigue cracks and the fine spacing of the striations were indicative of high cycle fatigue crack propagation. Review of maintenance records revealed that the tail rotor gearbox was disassembled, visually inspected, and reassembled about 330.3 hours before the accident. Based on the high cycle fatigue crack propagation and the presence of fatigue cracking on the majority of the fracture cross-section, it is likely the gouge marks on the input pinion gear shaft were created during reassembly of the tail rotor gearbox during this maintenance action. A loss of drive to the tail rotor would result in a clockwise, nose-right yaw of the helicopter due to the torque effect on the fuselage of the counterclockwise rotation of the main rotor. The sudden loss of tail rotor drive due to the fracture of the input pinion bevel gear would unload the powertrain of the power being used by the tail rotor, leading to a temporary increase of rotor rpm as well as engine rpm, consistent with the pilot’s report. Although the pilot reported that the helicopter began to spin counterclockwise, it is likely he misidentified the direction of yaw.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Pilot could have conducted a thorough preflight inspection to identify potential issues.

NTSB coding

Evidence available

  • Photos
  • 11 docket documents
View NTSB final reportView NTSB docket

Docket documents11

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