ENSTROM F 28F near Glendale, AZ — 2017-08-22

Probable cause

An internal failure of the turbo-compressor, which resulted in oil leaking into the turbo-compressor’s exhaust.

NTSB narrative

The flight instructor and student pilot were practicing hovering autorotations when the cockpit filled with smoke. The helicopter had just touched down, so the flight instructor secured the cockpit and both he and the student egressed the helicopter with the engine still running. About 5 minutes later, the engine stopped. Postincident examination of the helicopter revealed no fire or other related damage. Maintenance personnel reported that the smoke appeared to have originated from the failure of an internal oil seal within the turbocharger-compressor that leaked oil into the turbocharger-compressor's exhaust system. No further examination or repair was accomplished and the turbocharger was replaced. Further examination revealed damage to the turbocharger-compressor v-clamp assembly that was installed on the exhaust. The strap portion of the v-clamp assembly contained cracks on the outer face that emanated from three of the four spot welds. The edge of the strap portion exhibited evidence of bending deformation on both sides of the assembly. In the deformed areas, the distance between the edge of the retainer and the edge of the strap was smaller when compared to other, non-deformed areas. Further, microscopic examination of the largest crack revealed a fatigue crack emanating from the outer surface of the v-clamp assembly strap. It is likely that the v-clamp assembly bolts were tightened to an extent that caused the deformation in the strap. The damage found to the v-clamp was not related to the source of the smoke.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Instructor could have aborted the maneuver when engine sounded rough.

NTSB coding

• Fire/smoke (non-impact) · Autorotation

Evidence available

• 3 docket documents

Docket documents3

• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Record of Emailother
• Materials Lab Factual Report No. 17-074report

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