AD/MAKILA/9

Engine Control Unit - Comparator/Selection Board

7/2007

Applicability:

Makila 1A and Makila 1A1 turbo-shaft engines.

Note 1:  These engines are known to be installed in, but not limited to, AS 332 B, B1, C, C1, F1, L and L1 Super Puma helicopters.

Requirement:

Replace the Engine Control Unit (ECU) Selector-Comparator board with one incorporating Turbomeca modification TU250.  Replacement of the Selector-Comparator board and return of the Modification Compliance Certificate to Turbomeca must be performed in accordance with Turbomeca Mandatory Service Bulletin 298 73 0250 dated 23 March 2007.

Note 2:  A post-TU250 ECU and a pre-TU250 ECU can be installed in the same helicopter.

Note 3:  EASA AD 2007-0144 refers.

Compliance:

Before 30 June 2008.

This Airworthiness Directive becomes effective on 5 July 2007.

Background:

Modification TU203, applicable to Makila 1A and Makila 1A1 engines, introduced a control system backup law that fixes N1 (gas generator speed) at 65% when at least two of the three N2 (power turbine speed) signals are lost.  The intent is to limit the maximum speed attainable by the power turbine in the event of a failure of the shaft between the Engine and the Main Gearbox that could result in collateral damage to the N2 speed probes.

Recent occurrences of 65% N1 backup activation remain unexplained despite detailed investigation.  Although not substantiated, it is postulated that the events may have been due to corruption of the engine N2 speed signals by short perturbations, for example by EMI.  The potential therefore exists for a hazardous condition in which both engines on the same helicopter are simultaneously affected.

This Airworthiness Directive requires a hardware modification to the ECU that allows recovery from the 65% backup mode if the loss of N2 speed signals is determined to be temporary.  The modification applies to the Selector-Comparator board in the ECU.