What aircraft was involved and where did it happen?

The accident on 2007-11-20 involved a AS332L2 Super Puma, registration G-CHCF, at Aberdeen Airport, Scotland, GB.

What was the probable cause of the accident?

The incident was caused by the failure of the right engine freewheel unit due to excessive wear on the shaft ramps, which led to an engine overspeed and subsequent automatic shutdown.

Engine Freewheel Failure During Training Flight at Aberdeen

An AS332L2 Super Puma experienced a right engine overspeed and subsequent shutdown during a simulated engine failure training exercise, leading to a firm touchdown.

What happened

On 20 November 2007, an AS332L2 Super Puma, registration G-CHCF, was conducting a training flight at Aberdeen Airport. The flight was intended for standardisation training and an Operational Proficiency Check. The crew was utilizing a Training Idle System (TIS) to simulate a left engine failure during a rejected takeoff profile.

As the helicopter accelerated along Runway 16, the commander activated the TIS at approximately 28 kt. While the pilot was performing the rejected takeoff, the right engine experienced a sudden overspeed. The engine's overspeed protection system responded by shutting down the engine. This caused the main rotor speed (Nr) to decay rapidly. The commander took control, adopting a landing attitude and raising the collective to its maximum limit to cushion the descent. The helicopter touched down firmly on the runway, and the crew noted what appeared to be smoke or vaporised fuel. No injuries were sustained by the 3 crew members on board, and there was no damage to the aircraft.

The investigation

The AAIB investigation focused on the mechanical failure of the engine components and the accuracy of the training system's documentation. Engineering analysis of the main rotor gearbox revealed that the right engine freewheel unit had suffered severe damage. The investigation found that the roller cage had moved into the adjacent trough, causing the freewheel shaft to disengage from the gearbox.

Further examination of the freewheel unit showed that the rollers had overridden the ramps due to excessive wear. Metallurgical testing confirmed no material abnormalities in the components. The investigation also reviewed the manufacturer's maintenance records, noting that the right engine input shaft was subject to higher wear rates than the left. Additionally, the investigation examined the flight simulator modelling, finding that the simulator's representation of the helicopter's behaviour during an operating engine failure while using the TIS was inconsistent with actual flight characteristics.

Findings

Excessive wear to the freewheel shaft ramps caused the rollers to override the ramps, disengaging the engine from the main rotor gearbox.
The failure of the operating engine freewheel unit occurred during a phase of the takeoff profile where recovery was possible.
The prompt corrective action by the commander, specifically lowering the collective to prevent over-pitching and flaring to cushion the touchdown, was critical to the safe outcome.
The Flight Manual Supplement (FMS) contained inaccurate information regarding the technique required to restore rotor speed following an engine failure while the TIS was active.
The flight simulator used for training did not accurately model the handling qualities and performance of the aircraft during an operating engine failure in TIS mode.