AD/Makila/10 Time Limits Maintenance Checks (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 - Makila Series

AD/MAKILA/10 Time Limits - Maintenance Checks 3/2008

Applicability:

MAKILA 2A turboshaft engines, all serial numbers, except those incorporating modification TU 38.

Note 1:  These engines are known to be installed on, but not limited to, Eurocopter Model EC 225 LP helicopters.

Requirement:

Amend the engine maintenance programme to include the following limitation:

Limitation on the cumulative use of the 2-minute OEI rating:

Mandatory maintenance requirements associated with cumulative use of the 2-minute OEI rating, specified in the MAKILA 2A Maintenance Manual, no. 298 N7 450 1, Section 05-10-10, Task 200-801-A01 are revised as follows:

-      After any use of the 2-minute OEI rating as a result of single-engine operations, before next flight, replace module M03;

-      In cases of incursion into the 2-minute OEI rating range during twin-engine operations (a transient condition known to occur and appropriately recorded by the OEI usage counter), module M03 must be replaced when 5 minutes at the
2-minute OEI rating has been accumulated.

For additional details, refer to TURBOMECA Mandatory Service Bulletin (MSB) no.298 72 2806.

Note 2:  EASA AD 2007-0298 dated 11 December 2007 refers.

Compliance:

Within 30 days after the effective date of this AD.

This Airworthiness Directive becomes effective on 13 March 2008.

Background: There have been three occurrences of MAKILA 2A centrifugal compressor blade loss during acceptance testing of new engines.  The preliminary results of the investigations suggest that the rupture of the blades could have resulted from high frequency vibrations.  The fatigue crack initiation at the origin of the rupture and subsequent propagation may occur at a blade resonant frequency.  This potential resonance could occur within a small band of gas generator speeds (N1) at the high end of the rated N1 speed range, depending on the particular characteristics of each centrifugal compressor.  The centrifugal compressors thought most likely to be susceptible to blade loss are from a limited manufacturing batch.  These compressors have been identified, and have been, or will soon be, removed from service.  A new compressor has been designed in which the potential resonance is eliminated.  This compressor, when available, will be incorporated through modification TU 38.  The occurrence of compressor blade loss in the remainder of the current fleet cannot be ruled out, however.  As compressor blade loss could lead to an uncommanded or commanded in-flight engine shut-down, this AD requires the introduction of a limitation on the cumulative use of the 2-minute OEI rating, until modification TU 38 has been incorporated.

Charles Lenarcic
Delegate of the Civil Aviation Safety Authority

21 January 2008