Pilot Debrief

Bell UH 1H near Mesa, AZ — 2020-04-24

Final reportWPR20LA130
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Date
2020-04-24
Location
Mesa, AZ, USA
Aircraft
Bell UH 1H
Registration
N3276T
Category
Helicopter
Highest injury
Fatal
Fatalities
1

Probable cause

An in-flight separation of the tail rotor gearbox from the airframe due to fatigue failure of the gearbox attachment studs, resulting in a loss of directional control and subsequent ground impact. Contributing to the accident were 1) the improper application of paint on the clamping surfaces between the tail rotor gearbox and the vertical stabilizer that led to the initiation of fatigue fractures on the gearbox attachment studs and 2) the lack of a requirement to check the torque of the gearbox attachment hardware after installation of a gearbox onto the airframe.

NTSB narrative

Witnesses reported they observed the helicopter flying low with white smoke emitting from the rear rotor area. Suddenly, the tail rotor separated from the helicopter and landed in a dirt lot below. The helicopter continued northeast as it started to spin and impacted the ground. Postaccident examination of the engine revealed no preimpact anomalies. The top of the vertical stabilizer, tail rotor assembly, tail rotor gearbox, input quill assembly, and mounting studs were examined. Progressive crack growth was noted on the mounting studs within the tail rotor gearbox housing. The fracture surfaces were relatively rough, consistent with low-cycle fatigue or cyclic overstress crack growth under relatively high cyclic stresses. Additionally, multiple layers of primer, paint, and sealant were observed on the input quill assembly, including paint on the flange clamping surface where it had mated to the vertical stabilizer. Paint was also observed on the vertical stabilizer support casting where it had mated to the input quill as well as the washers for the tail rotor gearbox attachment hardware. In 2012, the gearbox was removed from another helicopter, repaired, painted, and installed on the accident helicopter. There was no other information regarding the installation. In January 2019, the helicopter was painted and photographs that were taken during this process indicated that the tail boom, tail rotor gearbox, and tail rotor assembly remained installed during the painting process. In addition, the tail rotor assembly and tail rotor gearbox were masked. Therefore, the paint found on the contact mounting surfaces was likely applied during the gearbox installation in 2012; however, the maintenance logs did not provide detailed information of the painting process. In April 2019, the tail rotor gearbox was removed from the vertical stabilizer for a corrosion inspection; no defects were noted in the maintenance logs. A retorque and/or torque stabilization check was not completed after the gearbox was installed, nor was it required to be according to applicable technical manuals. The presence of paint on the contact surfaces between the tail rotor gearbox and vertical stabilizer can lead to excessive relaxation of clamping forces due to compressive creep deformation of the paint. Therefore, the improper application of paint on the contact surfaces, which would have been disturbed during the last tail rotor gearbox removal about 39 flight hours prior to the accident, was likely a factor in the eventual reduction in clamping force of the tail rotor gearbox installation onto the vertical stabilizer. However, improper torque of the retaining nuts would also affect the clamping force between the tail rotor gearbox and the vertical stabilizer, but it could not be determined if the retaining nuts were properly torqued during the tail rotor gearbox’s reinstallation after the corrosion inspection in April 2019. The reduction in clamping force on the installation led to the initiation of fatigue fractures on the attachment studs and subsequent separation of the tail rotor gearbox from the vertical stabilizer in flight. The separation of the tail rotor gearbox resulted in a loss of directional control of the helicopter. Had a retorque and/or torque stabilization check of the gearbox retaining nuts been required, it could have identified the reduction in clamping force of the tail rotor gearbox’s installation onto the vertical stabilizer prior to the accident. The detected levels of oxycodone and its psychoactive metabolite oxymorphone in the pilot’s specimen are most consistent with some degree of tolerance to the effects of oxycodone, indicating regular use. Although it is likely that the pilot was impaired to some degree by the effects of oxycodone at the time of the accident, pilot impairment is unlikely to have contributed to the accident circumstances given that the tail rotor assembly separated in flight.

NTSB coding

Evidence available

  • Video
  • ADS-B / radar
  • Photos
  • 21 docket documents
View NTSB final reportView NTSB docket

Docket documents21

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