AD NUMBER: 2010-04-07

MANUFACTURER: Turbomeca Arriel 2S1 Turboshaft Engines
SUBJECT: Airworthiness Directive 2010-04-07

SUMMARY: This 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 digital electronic control unit (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 steps of the stepper motor controlling the fuel metering valve inside the Hydro-mechanical 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 issuing 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.

FMI: http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/0/3DF5619F22BDDD70862576CC0051031A?OpenDocument