Airworthiness Directives; Eurocopter France Model AS355E, F, F1, F2, and N Helicopters

This amendment supersedes an existing airworthiness directive (AD) for the specified Eurocopter France (ECF) model helicopters. That AD currently requires replacing certain main or combiner gearboxes with airworthy gearboxes. Further investigation has shown that the main gearbox is not affected, and this amendment requires replacing a certain combiner gearbox with a modified airworthy gearbox. This amendment is prompted by a report of a freewheel unit slipping resulting in an engine overspeed and shutdown. Also, this amendment is prompted by the conclusion of the investigation, which finds the freewheel slippage is due to the surface treatment applied to certain freewheel rollers in the combiner gearbox. The actions specified by this AD are intended to prevent an engine overspeed, an engine shutdown, and subsequent loss of control of the helicopter.