BELL 206B near St Louis, MO — 2013-05-24

Probable cause

The loss of engine power due to the separation of compressor blades, which resulted from fatigue cracking.

NTSB narrative

The flight instructor reported that, shortly after he and the pilot-rated student departed on a night training flight in the turbine-powered helicopter, they heard a loud bang, and the helicopter yawed left. The flight instructor chose to perform an autorotation with forward airspeed onto the airport's taxiway. Examination of the turbine engine revealed several holes in the compressor case, consistent with engine blades exiting the engine. Further examination revealed that the stages 3 through 6 compressor blades had separated at the roots. Examination also revealed that the 3rd-stage wheel blade root had fractured due to progressive cracking, consistent with fatigue; 54 percent of the fracture surface exhibited fatigue features. However, the fracture initiation surface was destroyed, so it was not possible to determine the cause of the fatigue failure. Indications of fatigue were also found on the stages 3 through 6 blades' fracture surfaces. The engine's compressor had 13,874 hours time since new (TSN) and the compressor rotor stages 1 through 6 wheels had 1,185.3 hours TSN and were Parts Manufacturer Approval replacement parts.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

The decision to continue the takeoff after the loud bang could have been reconsidered.

NTSB coding

• Uncontained engine failure · Maneuvering

Evidence available

• 4 docket documents

Docket documents4

• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Release of Aircraft Wreckage, NTSB Form 6120.15form
• Materials Lab Factual Reportreport
• Weather Reportreport