EASA AD No.: 2013-0171

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EASA AIRWORTHINESS DIRECTIVE

AD No.: 2013-0171

Date: 30 July 2013

Note: This Airworthiness Directive (AD) is issued by EASA, acting in accordance with Regulation 

(EC) No 216/2008 on behalf of the European Community, its Member States and of the 

European third countries that participate in the activities of EASA under Article 66 of that 

Regulation.

This AD is issued in accordance with EU 748/2012, Part 21.A.3B. In accordance with EC 2042/2003 Annex I, Part M.A.301, the 

continuing airworthiness of an aircraft shall be ensured by accomplishing any applicable ADs. Consequently, no person may operate an 

aircraft to which an AD applies, except in accordance with the requirements of that AD, unless otherwise specified by the Agency [EC 

2042/2003 Annex I, Part M.A.303] or agreed with the Authority of the State of Registry [EC 216/2008, Article 14(4) exemption].

Design Approval Holder’s Name:

TURBOMECA

Type/Model designation(s):

ARRIEL 2 engines

TCDS Number: EASA E.001

Foreign AD: Not applicable

Supersedure: This AD supersedes EASA AD 2009-0091 dated 04 May 2009.

ATA 73 Engine Fuel & Control – Hydro-Mechanical Metering Unit (HMU) 

Constant Delta Pressure Valve Diaphragm – Replacement

Manufacturer(s): Turbomeca S.A.

Applicability: ARRIEL 2B1 and 2B1A engines, all serial numbers.

These engines are known to be installed on, but not limited to, Eurocopter

AS 350 B3 and EC 130 B4 helicopters.

Reason: Several cases of rupture of the constant delta pressure (delta-P) valve 

diaphragm were reported on ARRIEL 2B1 engines, due to wear of the delta-P 

diaphragm fabric. 

This condition, if not corrected, can result in the loss of the automatic control 

mode of the helicopter, accompanied by a deterioration of the behaviour of the 

auxiliary back-up mode (emergency mode). On a single-engine helicopter, this 

could possibly prompt an emergency landing.

In order to reduce the probability of a diaphragm rupture due to fabric wear, 

EASA issued AD 2007-0006, requiring periodical replacement of all diaphragms 

having logged more than 2 000 operating hours.

Since that AD was issued, no further case of rupture of the constant delta-P 

valve diaphragm was reported on ARRIEL 2B1 engines. However, several 

additional diaphragms returning from service were inspected by Turbomeca, 

and some signs of wear were detected on diaphragms having logged less than 

2 000 hours. Based on the inspection results, it was decided to decrease the

limit from 2 000 hours to 1 500 hours, in order to further reduce the probability 

of delta-P diaphragm rupture. Prompted by these findings, EASA issued AD 

2007-0126, which superseded EASA AD 2007-0006, to require implementation 
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of the reduced limit.

After EASA AD 2007-0126 was issued, Turbomeca developed modification 

TU157, which consists in modifying the pressure relief valve of the HMU by 

introducing a damping device into the valve. Introduction of this device has 

demonstrated to decrease the pressure fluctuations in the system, thereby

significantly reducing the risk of wear of the delta-P diaphragm fabric. This 

removes the need for life limit of the delta-P diaphragm, which was before 

overhaul of the HMU. Modification TU157 was therefore considered as the 

terminating action for the requirements of EASA AD 2007-0126.

Consequently, EASA issued AD 2009-0091, retaining the requirements of

EASA AD 2007-0126, which was superseded, except that:

 In addition to ARRIEL 2B1 engines, the Applicability was expanded to 

include ARRIEL 2B1A engines, which share the same HMU design

 Applicability was reduced to exclude ARRIEL 2B1 and 2B1A engines which 

incorporate modification TU157.

Since issuance of EASA AD 2009-0091, an occurrence was reported on an 

ARRIEL 2B1 engine fitted with an HMU having a diaphragm that had been 

operated for less than 1 500 hours since installation. This engine had 

performed a large number of engine starts and shut-downs, and had an 

average of Power Turbine (C2) cycle consumption rate per hour higher than the 

usual rate known in service.

Turbomeca analysis shows that:

 the pressure pulsations in the fuel system, which are a significant contributor 

to the delta P diaphragm wear, mainly occur when the engine is stopped.

 the C2 cycle number is the most likely indicator of the number of engine 

starts and stops.

For the reasons described above, this AD, which supersedes EASA AD 2009-

0091, reintroduces periodical replacement of the HMU, with the possibility to 

account for HMU C2 cycle consumption.

Effective Date: 13 August 2013

Required Action(s) 

and Compliance 

Time(s):

Required as indicated, unless accomplished previously:

(1) For engines which do not incorporate modification TU157, within the 

compliance time specified in Appendix - Table 1 of this AD, as applicable, 

and, thereafter, at intervals not to exceed the values specified in Appendix 

- Table 1 of this AD, as applicable, replace the HMU with a serviceable 

HMU in accordance with the instructions of Turbomeca Mandatory Service 

Bulletin (SB) 292 73 2818 version D.

(2) Modification of an engine in accordance with the instructions of Turbomeca

SB 292 73 2157 (modification TU157) constitutes terminating action for the 

requirements of paragraph (1) of this AD.

(3) Compliance with the requirements of paragraph (1) of this AD can be

demonstrated by:

(3.1) Revising as follows the approved Aircraft Maintenance Programme 

(AMP) and standard practices on the basis of which the operator or 

the owner ensures the continuing airworthiness of each operated 

helicopter:

Incorporate the HMU operating limits described in the Appendix -

Table 1 of this AD 

and

(3.2) Complying with the approved AMP described in paragraph (3.1) of

this AD.
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(4) From the effective date of this AD, installation of a pre-mod TU157 HMU 

on an engine, or an engine with a pre-mod TU157 HMU on a helicopter is 

allowed, provided that the HMU is in compliance with the requirements of 

paragraph (1) of this AD.

Ref. Publications: Turbomeca MSB 292 73 2818 version D dated 24 June 2013.

The use of later approved revisions of this document is acceptable for 

compliance with the requirements of this AD.

Remarks: 1. If requested and appropriately substantiated, EASA can approve 

Alternative Methods of Compliance for this AD.

2. This AD was posted on 04 July as PAD 13-095 for consultation until 25 July 

2013. The Comment Response Document can be found at 

http://ad.easa.europa.eu.

3. Enquiries regarding this AD should be referred to the Safety Information 

Section, Executive Directorate, EASA. E-mail: ADs@easa.europa.eu.

4. For any question concerning the technical content of the requirements in 

this AD, please contact:

Turbomeca S.A., ARRIEL 2 Customer Support

40220 Tarnos, France

Fax: +33 5 59 74 45 15, or your usual or nearest TURBOMECA technical 

representative (refer to http://www.turbomeca-support.com).
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Appendix - Table 1 - Compliance Times

HMU 

Condition

Accumulated 

HMU operating 

hours (1) or C2 

cycles, on the 

effective date of 

this AD

Initial 

Replacement

Interval for repetitive replacement

HMU 

operating 

hours 

associated 

with an 

equivalent 

number of C2 

cycles

Less than 900 

HMU C2 cycles

Before 

accumulating 1 000 

HMU C2 cycles or 

1 500 HMU 

operating hours, 

whichever occurs 

first

1 000 HMU C2 cycles or 1 500 HMU 

operating hours, whichever occurs first

900 HMU C2 

cycles or more

Within 100 HMU 

C2 cycles after the 

effective date of 

this AD

HMU 

operating 

hours not 

associated 

with an 

equivalent 

number of C2 

cycles

Less than 1 100 

HMU operating 

hours

Before 

accumulating 1 200 

HMU operating 

hours

1 200 HMU operating hours

1 100 HMU 

operating hours or 

more

Within 100 HMU 

operating hours

after the effective 

date of this AD

Note (1): HMU operating hours are to be counted since new, overhaul, application of Turbomeca SB 292 

73 2105 any version or application of Turbomeca SB 292 73 2818 any version, whichever occurs later.