Interim Recommendation IR19990114

Interim Recommendation issued to: Allied Signal Aerospace

Recommendation details
Output No: IR19990114
Date issued: 13 August 1999
Safety action status:
Background: Why this Interim Recommendation was developed

Output text

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:


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.


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.


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.


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

Initial response
Date issued: 21 October 1999
Response from: Allied Signal Aerospace
Action status: Closed - Accepted
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.


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".

Last update 01 April 2011