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Service Difficulty Advisory

No. AV-1990-06-R1
14 January 2008

ALLISON 250-C SERIES
ENGINES LOOSE “B” NUTS

Transport Canada Civil Aviation (TCCA) is revising this SDA as a result of the Canadian Transportation Safety Board investigation into a Bell 206L accident in 2006. As demonstrated by this and other accidents and incidents worldwide, loose B-nuts on a pneumatic or fuel delivery tubes on the Allison/Rolls-Royce 250 series engines continue to cause in-flight loss of engine power. These failures can result in substantial damage to the helicopter and death or serious injury to the occupants.

Engine manufacturers’ maintenance documents provide specific information on the proper assembly methods and torque values. These documents also prescribe the use of torque stripes, witness marks and lockwire so that a fitting or nut that has come loose will be readily noticeable.

To attract the maintainers’ attention and highlight the importance of proper maintenance procedures, the manufacturers use appropriately placed NOTES, WARNINGS and CAUTIONS in the text of the related documents. Rolls Royce has also produced Commercial Service letters (CSL) for each model affected, latest revision February 2007.

250-C18 Series - CSL A-169
250-C20 Series - CSL A-1166
250-C28 Series - CSL A-2113
250-C30 Series - CSL A-3117
250-C20R Series - CSL A-4036
250-B15G - TP CSL A-101
250-B17 Series - TP CSL A-1121
250-B17F Series - TP CSL A-2019

TCCA reminds AMEs of the requirements to adhere to all the latest manufacturers’ instructions even when performing what may seem a rudimentary task.  It is also recommended all owners/operators insure tasks outlined in the appropriate CSL above are immediately incorporated in the relevant maintenance schedule.

For more information, please contact your nearest regional transport Canada Center or district airworthiness office or call directly Mr. Paul Jones at 613-952-4431, jonesp@tc.gc.ca.

For Director, National Aircraft Certification

B. Goyaniuk
Chief, Continuing Airworthiness

Note: For the electronic version of this document, please consult the following Web address: http://www.tc.gc.ca/eng/civilaviation/certification/menu.htm