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Output Number
IR19990115
Approval Date
Published Date Time
Recommendation type
Interim recommendation
Mode
Aviation
Date released

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.



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.





IR19990114



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.





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
Civil Aviation Safety Authority
Response Text

CASA has reviewed Air Safety Interim Recommendation IR 999015
(sic). The cause of the failure is agreed to be rubbing wear of the
wheel near the transition from hub to web. This wear is believed to
have occurred from contact with the second stage seal. BASI has
advanced a reason for failure of this seal, but we note that the
reason advanced elsewhere by the engine manufacturer differs. Any
further advice from the manufacturer, subsequent to issue of your
report, would be appreciated.



The recommendations in the report are agreed, and CASA will review
the regulatory requirements related to recording the operational
history of life limited components.

ATSB Response

The following correspondence was forwarded to the Civil Aviation
Safety Authority on 3 November 1999:



I refer to your letter BE99/253 dated 25 October 1999 in which you
requested further advice the ATSB may have from the manufacturer,
AlliedSignal in relation to the ATSB's safety recommendations
relating to the Pel Air TPE-331 uncontained engine failure.



The ATSB has received information from AlliedSignal stating that
they agree that the fracture of the seal fitted to the second stage
turbine wheel caused the fracture of the wheel and the release of
fragments that penetrated the engine nacelle and aircraft
fuselage.



The cause of the seal fracture has not been determined with
certainty. ATSB analysis identified fatigue cracking as a possible
cause, with variations in the size of slot corner radii acting as a
local stress raiser. AlliedSignal consider that an increase in
operating temperature of the seal was the cause of fracture. They
base this hypothesis on the presence of blockages in the cooling
air flow holes in the second stage stator asembly (sic). The
blockages are attributed to an improper repair of the second stage
stator.



It appears that AlliedSignal consider that the overheating of the
seal resulted in an overstress fracture. It is possible that engine
operation over a number of flights, with the seal being heated to a
higher than normal temperature, may lead to thermal fatigue.



At present there is a lack of hard evidence to support either
hypothesis.



In recognition of the lack of evidence, AlliedSignal, in response
to the recommendations, have agreed to perform a sensitivity study
on turbine wheel knife-edged seals with respect to fatigue crack
initiation life. The analysis will be completed by March 2000. They
have also specified in-service inspection requirements for the
seal. Fluorescent penetrant inspections of the second stag turbine
assembly include the seal. The inspection acceptance criteria is
"no cracks allowed".



AlliedSignal will inform the ATSB of the results of the testing
program.



In the last paragraph of your letter you state that CASA will
review the regulatory requirements related to recording the
operational history of life limited components. With respect to
this review, please advise when the ATSB can expect the
results.

Date Received
Organisation
Civil Aviation Safety Authority
Response Text

An audit of the subject matter has identified a need to review
Civil Aviation Regulation (1988) 42W (5)(iii). The regulation
requires that time in service or the number of cycles be recorded.
The Certificate of Approval holder who installed the second-stage
turbine wheel had complied with CAR 42W in regard to the recording
of time in service. On this occasion, the recording of merely the
time in service meant that the operational history of the subject
turbine wheel could not be determined with certainty.



The matter has been raised with CASA Maintenance Standards Group,
Airworthiness Branch, for review and action as necessary.