Airworthiness Directives; Eurocopter Deutschland GmbH Model EC135 P1 and EC135 T1 Helicopters

This amendment adopts a new airworthiness directive (AD) for Eurocopter Deutschland GmbH (Eurocopter) Model EC135 P1 and EC135 T1 helicopters. This action requires, before further flight, adding a copy of this AD or a statement to the Emergency Procedures section of the Rotorcraft Flight Manual (RFM) to inform the pilot to reduce power and land as soon as practicable if a thump-like sound followed by unusual vibration occurs during flight. This action also requires visually inspecting for a crack or a break in certain main rotor drive torque strut (strut) assemblies at specified time intervals and recording details of the inspections in the historical or equivalent record. This AD also requires re-marking and relocating the strut as appropriate and replacing any unairworthy strut assembly with an airworthy strut assembly before further flight. Also, this AD establishes a life limit of 1000 hours time-in-service (TIS) for certain struts with an additional 1000 hours TIS for struts re-marked right-hand (RH) or left- hand (LH) before installing in the new location. This amendment is prompted by a report of a thump-like sound heard during flight followed by unusual vibrations due to failure of the RH strut between the main transmission and the fuselage. The actions specified in this AD are intended to prevent failure of a strut, failure of a worn or ineffective back-up emergency stop, and subsequent loss of control of the helicopter.