Pilot Debrief

BELL 407 near Aberdeen, SD — 2012-04-14

Final reportCEN12LA242
Sign in to save
Date
2012-04-14
Location
Aberdeen, SD, USA
Aircraft
BELL 407
Registration
N509MT
Category
Helicopter
Highest injury
Minor
Fatalities
0
Phase of flight
Initial climb

Probable cause

The fatigue failure of two turbine engine blades due to vibration.

NTSB narrative

The pilot reported that, shortly after departure, the helicopter yawed left and that he heard a "loud howling/grinding sound" emanating from the upper area of the helicopter. He subsequently reduced power and performed a minimum power descent to an open area. During the landing, the helicopter touched down hard. A postimpact examination of the engine found that two third-stage turbine wheel blades had cracked and separated due to high-cycle fatigue at the trailing edge during operation of one or more natural frequencies of the wheel. Four additional blades displayed fatigue cracks. Metallurgy examinations revealed that the blades had no evidence of material anomalies and met the material hardness and composition specifications. The observed high-cycle fatigue initiation and propagation characteristics were consistent with operating in a condition conducive to vibration. The engine manufacturer plans to release design changes that will increase the operating speed natural frequency margin and improve the vibration margin.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Pilot could have aborted the flight after noticing abnormal engine sounds.

NTSB coding

Evidence available

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

Docket documents6

Related mishaps