ATSB: AS355F-1 engine failure followed rapid fatigue crack propagation

A fatigue crack in a turbine blade spread rapidly, causing a helicopter’s engine failure, an ATSB investigation has found.

Rolls-Royce is redesigning the third-stage turbine wheel to improve its tolerance to fatigue cracking and operation at responsive wheel modes. ATSB Photos
Rolls-Royce is redesigning the third-stage turbine wheel to improve its tolerance to fatigue cracking and operation at responsive wheel modes. ATSB Photos
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The incident occurred when a twin-engined Airbus Helicopters AS355F-1 helicopter, registered VH-SEV, was completing an air-taxi from a hangar to a maintenance facility at Sydney’s Bankstown Airports. On board were the pilot and one passenger.

As the skids touched the ground on landing, the pilot heard a loud “squeal” from the helicopter’s right-hand side. The pilot scanned the instrument panel and observed the engine gas generator speed (Ng) drop to 55 percent and the right-hand engine chip light illuminate. A few seconds later, the pilot observed smoke coming from that side of the aircraft and immediately shut down the right-hand engine.

Mechanics from a nearby workshop quickly extinguished the engine fire. There was substantial damage to the engine, and minor heat damage to the surrounding structure of the helicopter.

“Any pilot, regardless of their level of experience, can find themselves confronted with an unexpected failure,” said ATSB director of transport safety Stuart Macleod. “The pilot’s quick response to the unexpected failure helped ensure the best possible safety outcome.”

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The ATSB investigation found that single third-stage turbine wheel blade failed due to fatigue cracking, resulting in secondary damage to the engine, and total engine failure.

“This incident reinforces that it is important for pilots to monitor aircraft performance parameters continuously for abnormal indications,” said Macleod.

Engine manufacturer Rolls-Royce advised the ATSB that, with the Rolls-Royce 250 enhanced power turbine engine, a dwell of just a few seconds can be enough to initiate damage and propagate a crack to failure.

In response, Rolls-Royce is redesigning the third-stage turbine wheel to improve its tolerance to fatigue cracking and operation at responsive wheel modes.

The report AO-2018-021, provides important advice for operators of aircraft with Rolls-Royce 250 enhanced power turbine engines.

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