Interim Recommendation IR19990112

Interim Recommendation issued to: Allied Signal Aerospace

Recommendation details
Output No: IR19990112
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 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.

As a result of the 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 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 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.


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 agrees to audit the manufacture of TPE331 turbine wheel knife-edged seals by March, 2000. The audit will focus on the manufacture and inspection of the slot corner radii, including the applicable paperwork. However, it has not been our experience that these slot corner radii have exhibited excessive variation with respect to design intent.

Furthermore, AlliedSignal does not believe that the seal corner radius was the primary cause of the wheel separation of engine P-03214C. Material analysis performed at AlliedSignal indicated that the primary contributing cause was the cooling flow blockage resulting from improper repair of the second-stage stator assembly. This blockage resulted in higher than normal operating temperatures for the knife-edged seal, which we believe led to the uncoupling of the seal from the wheel and subsequent damage. This small fatigue crack observed was believed to be secondary to the uncoupling of the seal from the wheel. Additionally, this knife-edged seal design has accumulated over 30 million operating hours without any instances of fatigue separations.

Last update 01 April 2011