Fairchild Metro, Swearingen Aviation Corp SA226-TC, VH-EEQ

As a result of this investigation the following interim
recommendations were issued.

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 should be
audited 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.

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.

IR19990116

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

The uncontained failure of the second-stage turbine wheel was
caused by the progressive reduction of the wheel cross-section,
during operation, near the transition from the hub to web. No
material anomalies or regions of pre-existing crack growth were
associated with the separation of wheel segments.

Analysis of the recovered turbine components revealed that the
reduction in wheel cross-section was caused by the effects of
sliding contact with sections of the fractured second-stage wheel
knife-edged seal. The web was reduced from original approximately
13 mm to approximately 8.5 mm.

Examination of the remnants of the second-stage seal indicated
that the fracture of the seal had occurred as a result of fatigue
crack growth from the radii at the corners of the slots formed in
the forward edge of the seal.

A significant variation in the radii of the slot corners was
noted when a comparison was made between the fractured seal and the
seal from another second-stage wheel assembly. The one intact slot
corner of the fractured second-stage wheel seal was sharper and did
not display the smoothly rounded characteristic of the corners of
other seals. A decrease in radius would create an increase in
stress concentration and increase the likelihood of fatigue crack
initiation.

The right engine of a Fairchild Metro II aircraft, VH-EEQ,
failed at the top of descent to Launceston. The aircraft
subsequently completed an uneventful single-engine landing. On-site
inspection revealed that the engine failure was caused by the
disintegration of the second-stage turbine wheel and that segments
of the wheel had penetrated the engine nacelle. One fragment
penetrated the fuselage and was found lodged in the cabin
lining.

The AlliedSignal TPE331-3U-304G engine, serial number P03214C,
was reported to have completed 6,017.4 hours and 6,273 cycles since
new.

The second-stage wheel was identified as part number 868272-1,
serial number 1-01345-1313. The recorded history of the second
stage wheel indicated that the wheel had been in service for a
total of 2,972.84 hours and 2,878 cycles at the time of failure on
5 March 1999. The wheel was installed new in engine s/n P90105 on
11 Jan. 1982. It was removed from engine s/n P90105 on 25 Sept.
1992 with a recorded service life of 564.74 hours and 508 cycles.
The wheel was installed in engine s/n P03214C on 30 Aug. 1994 with
a recorded history of 564.74 hours and 508 cycles in service. There
remain, however, uncertainties in the recorded service history of
the part.