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Output Number
IR19990114
Approval Date
Published Date Time
Recommendation type
Interim recommendation
Mode
Aviation
Date released

The Bureau of Air Safety Investigation recommends that the
engine manufacturer AlliedSignal consider, during engine design and
the formulation of continuing airworthiness instructions, the
possibility that turbine wheel seal fatigue failure may result in
hazardous modes of engine failure.



As a result of investigation into this occurrence, the Bureau
simultaneously issues the following interim recommendations and
safety advisory notice:





IR19990112



The Bureau of Air Safety Investigation recommends that the engine
manufacturer AlliedSignal audit the process employed to manufacture
TPE 331 turbine wheel knife-edged seals to determine those factors
that may lead to excessive variations in slot corner radii.





IR19990113



The Bureau of Air Safety Investigation recommends that the engine
manufacturer AlliedSignal determine the sensitivity of turbine
wheel seals to the initiation of fatigue cracks from slot corners,
as a function of slot corner radii.





IR19990115



The Bureau of Air Safety Investigation recommends that the Civil
Aviation Safety Authority Australia conduct an audit of that part
of the aviation safety system that establishes the operational
history of life-limited components, to establish why the
operational history of second-stage turbine wheel, p/n 868272-1,
s/n P03214C, could not be determined with certainty.





SAN19990116



The Federal Aviation Administration should note the safety
deficiency identified in this document and take appropriate action
as considered necessary.

Organisation Response
Date Received
Organisation
Allied Signal Aerospace
Response Text

Please find attached AlliedSignal's planned actions and
AlliedSignal's Materials Analysis (MA) of the 2nd stage turbine
wheel from the subject incident.



Based on the MA and review of the engine hardware, AlliedSignal
believes that an improper repair of the 2nd stage stator assembly
resulted in blocked cooling passages which resulted in reduce
cooling flows and higher operating temperatures in the inner
support region. This resulted in the separation / uncoupling of the
curvic seal from the 2nd stage wheel followed by the uncontained
separation of the 2nd stage turbine wheel.



Upon completion of the planned actions associated with the interim
recommendations, I will forward any results and/or process
changes.



If you have any comments or questions, feel free to contact me via
E-mail Thank you for the opportunity to respond to these
recommendations.



IR19990114



AlliedSignal considers in due course of engine design the various
failure modes that may result in hazardous conditions. AlliedSignal
recognises that seal separation may result in hazardous modes of
engine failure, including wheel separations. As stated above in the
response to IR19990112, AlliedSignal does not believe that the seal
corner radius was the primary cause of the wheel separation of
engine P-03214C. However, if the analysis performed in accordance
with IR19990113 above supports further action, appropriate steps
regarding the design and manufacture of the turbine wheel
knife-edged seals will be taken.



To prevent hazardous conditions from occurring, AlliedSignal has
specified in-service inspection requirements for the knife seal
(Refer to Engine Maintenance Manual 72-00-98, Section 72-50-07,
Page 212 and Inspection/Repair Manual 72-IR-10). In-service
inspections include visual and FPI (fluorescent penetrant
inspection) plus the following dimensional inspections. The knife
seal diameter is inspected, and the concentricity (72-IR-10 only)
is checked with respect to the curvic couplings. The axial, wheel
to seal gap is also checked. The slot radii would not normally be
dimensionally checked as the radii would not be expected to change
in service, however, the assembled turbine wheel visual and FPI
inspections would cover the slot radii. FPI acceptance criteria is
"no cracks allowed".