Pilot Debrief

BELL 407 near Pecan Island, LA — 2015-06-08

Final reportCEN15LA265
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Date
2015-06-08
Location
Pecan Island, LA, USA
Aircraft
BELL 407
Registration
N501PH
Category
Helicopter
Highest injury
None
Fatalities
0
Phase of flight
Cruise

Probable cause

The fatigue fracture of the tail rotor gearbox attachment studs, which resulted in a loss of tail rotor control and a subsequent hard landing.

NTSB narrative

The operator reported that the helicopter air taxi flight was in cruise about 1,000 ft above ground level when the pilot felt an impact and a strong vibration. The pilot completed an instrument and functional control check and could not immediately identify any anomalies. The pilot stated that, as he slowed the helicopter for landing, he noticed a "heavy mechanical sound and strong vibration." The vibration worsened, and the pilot began to have difficulty controlling the helicopter; he subsequently initiated an autorotation and deployed the helicopter's floats. The helicopter touched down in a marshy area, and the pilot and passengers egressed. During the landing, the main rotor blades contacted the tail boom and one of the tail rotor blades, resulting in separation of the tail rotor gearbox (TRGB) support structure, which was subsequently located in the marsh. A postaccident examination and metallurgical analysis revealed that fatigue fractures on two of the four TRGB attachment studs likely existed before the accident flight. As the fatigue fractures grew larger through the first two attachment studs, their load-carrying capability lessened, and the additional load was transferred to the remaining attachment studs. The progressive failure of the fatigued TRGB attachment studs led to the vibrations felt by the pilot and, ultimately, the uncommanded right yaw and subsequent loss of tail rotor control. The reverse- bending, high-cycle fatigue fracture initiation mode observed on two of the TRGB studs suggests the fatigue fractures were a result of a loss of torque of the attachment stud nuts. The reason for the loss of torque could not be determined based on the available information.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Pilot could have identified mechanical issues earlier during control checks.

NTSB coding

Evidence available

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

Docket documents10

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