Summary
We propose to adopt a new airworthiness directive (AD) for the products listed above. This proposed AD results from mandatory continuing airworthiness information (MCAI) issued by an aviation authority of another country to identify and correct an unsafe condition on an aviation product. The MCAI describes the unsafe condition as: During acceleration up to One Engine Inoperative (OEI) 30-second rating, one event of flight loss of full automatic control occurred on an Arriel 2S1 engine. The selection of OEI 30-second rating on engine 1 was triggered by the automatic detection of an OEI situation further to a transient deceleration of engine 2. The transient deceleration of engine 2 was caused by the untimely reset of its DECU. Once this reset was completed, engine 2 resumed its nominal operation. Afterwards the aircraft then continued its flight safely with its engine 1 operating in manual control mode. The loss of full automatic control of engine 1 was caused by loss of stepsof the stepper motor controlling the fuel metering valve inside the Hydromechanical Unit (HMU). It has been found that high accelerations, notably up to OEI 30-second rating, increase the risk of loss of steps of the HMU stepper motor. Therefore, this event has led to the consideration of the following unsafe condition at aircraft level: In- flight loss of full automatic control of the engine induced by the loss of steps of the stepper motor during acceleration up to OEI 30-second rating, further to an actual OEI situation on the other engine (such as a power loss event). We are proposing this AD to prevent loss of full automatic control of the engine during acceleration up to the OEI 30-second rating. This condition could result in reduced controllability of the helicopter.