Airworthiness Directives; Eurocopter France Model SA-365N, N1, N2, N3, and SA-366G1 Helicopters

This amendment supersedes an existing airworthiness directive (AD), applicable to Eurocopter France Model SA-365N, N1 and SA-366G1 helicopters, that currently requires repetitive inspections of the main gearbox (MGB) magnetic chip plug and oil filter if certain part number/ modification level MGB's are installed. This new action expands the helicopter model and MGB applicability to include the SA-365N2 and N3 helicopters and all variants of the MGB. It also requires installing a MGB planetary gear shaft (gear shaft) vibration level monitoring unit (VLMU); inserting procedures into the Rotorcraft Flight Manual (RFM) for a preflight vibration check using the VLMU and inserting a related emergency procedure and limitation for an inoperative VLMU into the RFM. This action is prompted by two occurrences of gear shaft cracks. The actions specified by this AD are intended to detect cracks in the MGB planetary gear shaft, which could lead to failure of the MGB and subsequent loss of control of the helicopter.