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.
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.
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.
