Final Report: GEARBOXES BLAMED In Eurocopter accident
Eurocopter AS 355F1, Andover, England, Dec. 2, 2003–G-XCEL was on a test flight after it had been fitted with overhauled rotor drive gearboxes. Witnesses heard a “loud screeching mechanical noise,” then saw the tailboom appear to fold forward alongside the cabin and saw the helicopter fall to the ground. The pilot was killed, as were the two maintenance engineers on board. Early models of the AS 355 were fitted with Rolls-Royce Allison 250-C20s with conventional control systems; later models, including G-XCEL, were fitted with Turbomeca Arrius engines that incorporated FADEC.
Five earlier failures had occurred with the Arrius combination. Due to a misunderstanding of the problem, there was a delay between the first accident and the AD that grounded the gearboxes at risk. In 1995 Eurocopter ordered new rollers for the freewheel system with a coating of zinc phosphate. During the investigation it was found that a manganese phosphate had been used, which might have affected the degree of slippage.
The AAIB concluded that the accident was likely caused by slippage by the right freewheel, transferring the load to the left freewheel. That slipped, and it is possible that both entered a routine of slip/engage that caused an oscillation in yaw, inducing a harmonic failure in the tailboom.