What happened
On 2 December 2003, an AS355F1 helicopter, registration G-XCEL, was conducting a post-maintenance test flight near Hurstbourne Tarrant, Hampshire. The flight was intended to verify engine torque balance following recent maintenance work. During the 17-minute flight, witnesses observed the aircraft making unusual, loud mechanical noises and shaking violently. Shortly thereafter, the tail boom appeared to fold forward toward the cabin, and the main rotor detached. The aircraft subsequently struck the ground and caught fire. The accident resulted in 3 fatalities, including the pilot and two passengers.
The investigation
The AAIB investigation focused on the mechanical integrity of the transmission system. Examination of the wreckage revealed that the main rotor gearbox (MRGB) and combining gearbox had detached from the airframe prior to impact. Metallurgical analysis of the combining gearbox showed significant distress in the left freewheel, with evidence of wear and overheating on the ramps and rollers.
Investigators also examined the manufacturing history of the freewheel rollers. It was discovered that a batch of rollers had been processed with an incorrect chemical coating. While the manufacturer intended to apply a zinc phosphate coating, the rollers were actually treated with a manganese phosphate process. This error occurred because the person preparing the manufacturing instructions failed to recognize the significance of the 'Bonderite' notation.
Findings
- The primary cause of the accident was a series of freewheel slippages under load, followed by aggressive re-engagements of the mechanism.
- These slippages were caused by an incorrect manganese phosphate coating on the rollers, which provided insufficient friction compared to the required zinc phosphate specification.
- The alternating engagement and disengagement of the two freewheels created high-frequency, high-torque oscillations that compromised the structural integrity of the tail boom.
- A secondary contributing factor was the failure to verify the chemical composition of the newly supplied rollers during the quality assurance process.