AAIB investigation to Airbus Helicopters AS355 F1, G-BOSN
Engine fire due to exhaust clamp failure, Hedsor House, Bourne End, Buckinghamshire, 2 March 2021.
Summary:
Whilst hovering at 20 ft agl smoke was observed coming from the engine exhaust. A member of the operational team on the ground informed the pilot, who then landed immediately. A fire warning subsequently illuminated, and the pilot activated the fire extinguishing system. The fire was determined to have been caused by the loss of retention of the right engine inboard exhaust nozzle, which was released because of the failure of its securing clamp. The released nozzle had blocked the overboard exhaust outlet and allowed hot exhaust gases to impinge on the engine cowlings leading to local overheating.
The clamp failure was attributed to a combination of an incorrect locking washer being fitted during maintenance and elevated engine vibration which caused the clamp to loosen. A crack then propagated in low-load high-cycle fatigue until final rupture of the clamp.
The helicopter manufacturer is taking safety action to amend the Aircraft Maintenance Manual (AMM) highlighting the correct installation of the clamp.
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