AD/Arriel/30 Digital Engine Control Unit Software (Cth)

AIRWORTHINESS DIRECTIVE

For the reasons set out in the background section, the CASA delegate whose signature appears below issues the following Airworthiness Directive (AD) under subregulation 39.001(1) of CASR 1998.  The AD requires that the action set out in the requirement section (being action that the delegate considers necessary to correct the unsafe condition) be taken in relation to the aircraft or aeronautical product mentioned in the applicability section: (a) in the circumstances mentioned in the requirement section; and (b) in accordance with the instructions set out in the requirement section; and (c) at the time mentioned in the compliance section.

Turbomeca Turbine Engines - Arriel Series

AD/ARRIEL/30 Digital Engine Control Unit Software 3/2009

Applicability:	Arriel 2S1 turboshaft engines, all serial numbers.
Note 1:  These engines are known to be installed on, but not limited to, Sikorsky S-76C+ series twin-engine helicopters.
Requirement:	1.    Perform an upgrade of the digital engine control unit (DECU) software to version 11.01 to implement modification TU109 in accordance with the accomplishment instructions of Turboméca Mandatory Service Bulletin (MSB) 292 73 2109 Version E dated 17 September 2008 or later version approved by the European Aviation Safety Agency (EASA).
Previous upgrade of the DECU software to version 11.01 accomplished before the effective date of this Directive, in accordance with the accomplishment instructions of MSB 292 73 2109 Version D, dated 30 November 2007, satisfy this Requirement.
Warning:	Never mix on a twin engine helicopter a post TU109 (DECU software at version 11.01) engine with a pre TU109 engine (DECU software at version 8.08 or earlier).
2.    Arriel 2S1 turbo shaft engines may not be installed on any aircraft unless its DECU has software at version 11.01 or higher.
Note 2:  EASA AD 2009-0010 refers.
Compliance:	For Requirement 1 - No later than 31 August 2010.
For Requirement 2 - After 31 August 2010.
This Airworthiness Directive becomes effective on 12 March 2009.
Background:	During acceleration up to One Engine Inoperative (OEI) 30-second rating, one event of in-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 steps of 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 lead 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).

It has been determined that the Arriel 2S1 is the only engine variant of the Arriel 2 family affected by this potential unsafe condition.

For the reason stated above, this Directive requires, on DECUs of Arriel 2S1 engines, the application of modification TU109 - which is a software modification to version 11.01 - that increases the tolerance to loss of steps of the control system.  This modification reduces significantly the risk of an in-flight loss of full automatic control due to loss of steps of the stepper motor, notably during engine accelerations up to OEI 30-second rating.

James Coyne
Delegate of the Civil Aviation Safety Authority

27 January 2009