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.
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