An external examination by the operator's engineers found damage
to the left engine turbine blades and shrapnel damage to the
exhaust nozzle. The operator removed the engine and sent it to an
approved workshop for examination and repair. The Australian
Transport Safety Bureau (ATSB) did not attend the examination, but
requested that all damaged components be forwarded to the ATSB for
detailed examination.
After the engine was disassembled, the ATSB was advised that the
engine failure was precipitated by a failure of the turbine first
stage disc rotating air seal. The rotating air seal's outer rim was
missing and the downstream turbine components received damage
consistent with fragments of the rotating air seal passing though
the turbine. The failed rotating air seal, the first, second and
third stage turbine wheels and nozzle guide vane assemblies were
forwarded to the ATSB for further examination.
Engine and component history
The Allied Signal TPE331-12UHR-701G turboprop engine, serial
number P-70210, had accumulated 9,139.8 hours and 15,585 cycles
since new and 3,066.9 hours since overhaul. In May 1997, it had
Allied Signal service bulletin TPE331-72-2002 incorporated, which
detailed replacement of the inner baffle with a new inner baffle
part number 3108039-2. Service bulletin TPE331-72-2030, that
detailed replacement of the compressor interstage seal assembly
support, was incorporated in July 1999, at 6,073 hours, during
engine overhaul, After overhaul, the engine was installed into DMI
where it remained until the failure.
The rotating air seal, part number 3103839-3, serial number
2-23315-945, appeared to have been installed in the engine since
new as its time and cycles since new were identical to those
applicable to the engine. During the engine overhaul in 1999, it
was inspected in accordance with the requirements current at that
time and found serviceable.
The ATSB was advised that cracking of the rotating air seals had
occurred in the past, but that it was rare to see a cracked
rotating air seal on engines that have the requirements of the
engine manufacturer service bulletins TPE331-72-2002 and
TPE331-72-2030 incorporated.
Rotating air seal examination
The examination of the rotating air seal and other components
from the failed engine is detailed in the ATSB's technical analysis
report number 40/01. The examination revealed that the entire outer
rim of the rotating air seal had separated from the flanged
section. About seventy percent of the rim circumference was
recovered and most material from the outer ten millimetres of the
plate flange was lost.
One location, where the loss of material was substantially
greater, exhibited a short length of fracture showing evidence of
fatigue crack propagation. Heat tinting over the area of fatigue
indicted that it was present prior to the event failure. The seal
had no evidence of material or manufacturing anomalies.
Examination of the turbine components
The turbine disks and nozzle guide vane assemblies showed
evidence of random impact damage to the blade leading edges. The
damage was consistent with the separated pieces of the failed
rotating air seal passing through the turbine.
A copy of the ATSB's technical analysis report, number 40/01, is
available on the ATSB web site at or from the ATSB on request.
Engine manufacturer's action
The engine manufacturer reported a number of documented
in-flight shutdowns due to separation of the rotating air seal
plate rim. Their investigation into the events concluded that
cracking in the rim area was due to elevated rim operating
temperatures, primarily due to hot gas leakage from deteriorated
first stage stator assembly hardware. To alleviate the problem, the
manufacturer introduced service bulletins TPE331-72-2002 and
TPE331-72-2030 and revised the engine maintenance manual to improve
inspection of the relevant components at hot section inspection
with the intention of preventing hardware prone to gas path leakage
from returning to service.
On 12 August 2001,