Recommendation R20070021

Recommendation issued to: Transport Canada

Recommendation details
Output No: R20070021
Date issued: 09 October 2007
Safety action status: Closed - Action Taken
Background: Why this Recommendation was developed

Output text

Safety Recommendation

The Australian Transport Safety Bureau recommends that Transport Canada require that Pratt & Whitney Canada incorporate measures to electrically isolate the starter-generator gear-shaft input coupling and the engine number-1 main shaft bearing of all Pratt & Whitney Canada PT6A series engines.

Initial response
Date issued:
Response from: Transport Canada response
Response text:

Transport Canada responded to this proposed safety recommendation (draft recommendation J) in the ATSB draft report by stating:

Larger PT6A-68 series engines now incorporate a "Vespel" (Polyimide composite) coupling to isolate/protect the number-1 bearing from electrical discharge damage.  Small PT6A-114 series engines (such as the one involved in the current investigation) have space limitations affecting the insertion of an insulating material for protection of the engine number-1 bearing from electrical discharge damage emanating from a malfunctioning starter-generator.

Modification of the starter-generator to electrically isolate the shaft has been previously identified as the terminating action to mitigate the issue of EDD.  Attached you will find a copy of a letter dated May 25, 2001 to the Federal Aviation Administration, New York Aircraft Certification Office (FAA NYACO) requesting a status update on the work done by the TRW Lucas and the FAA position on this matter.

This work included a modification to electrically isolate the shaft to mitigate this issue.  A copy of this letter was also forwarded to the FAA Engine Certification Office in October 2002.  TC suggests that the TSB or the ATSB contact the FAA in regards to this issue.

ATSB comment:

The ATSB does not accept Transport Canada's suggestion that either the Transportation Safety Board of Canada or the ATSB should contact the FAA in regard to this issue. As the responsible airworthiness authority, the ATSB believes that TC, in conjunction with the engine manufacturer, actively pursue any methods available to electrically isolate PT6A series engines to eliminate the possibility of EDD damage. As such, the ATSB now formally issues the safety recommendation as R20070021.

Further correspondence
Date issued: 15 November 2007
Response from: Transport Canada response
Response status: Closed - Partially Accepted
Response text:

While the suggested design change may be desirable, it has been demonstrated to be unfeasible to retrofit the existing fleet. To retroactively introduce such a major modification to the existing fleet of PT6A engines could introduce unforeseen issues, which could have a negative effect on the existing reliability record.

P&WC is presently developing further maintenance recommendations, the most significant of which is that, upon suspicion of an EDD event, the engine must be removed from service for inspection. The Original Equipment Manufacturers (OEM), Pilatus and Cessna, have been canvassed by P&WC to further expand the Instructions for Continued Airworthiness (ICA) relevant to their products to improve identification of an EDD event.

Based on present In-Service data, TCCA does not concur that a design change is necessary to further mitigate the risk of EDD to the PT6A engine family. TCCA believes that the primary failure causing the EDD is that of the starter-generator, an airframe component. The OEM is responsible for the installation of the starter-generator and, as such, in a better position to develop necessary requirements to mitigate this failure mode. It is our understanding that P&WC is working with these OEMs to meet this end.

TCCA will produce a Service Difficulty Advisory for all Civil Aviation Authorities and affected Canadian operators and maintainers to further bring the circumstances surrounding this event and the additional ICA under development to all stakeholders' attention.

Last update 05 April 2012