On 28 July 2004, a British Aerospace PLC, BAe 146 aircraft,
registered VH-NJA, was in cruise flight at flight level (FL) 280,
on a scheduled passenger service from Brisbane to Adelaide. At 1245
EST, the flight crew (crew) felt and heard a light to moderate
rumbling and grinding noise.
A check of the aircraft and engine instruments indicated that
the number-4 engine had a rapidly decreasing N11 RPM and an N22 RPM of less than 10%. The turbine gas
temperature (TGT) for that engine appeared to indicate zero and the
thrust management system (TMS) showed an error message and a 'Test
Fail' indication.
The crew shut the engine down in accordance with the operator's
abnormal checklist procedures and turned off the TMS. A fire bottle
was discharged into the number-4 engine cowling area as a
precaution.
The crew informed the cabin crew of the engine failure, with one
of the flight attendants reporting having seen sparks coming from
the engine.
The crew contacted air traffic control (ATC) requesting a
descent to FL150 to start the auxiliary power unit (APU). The
passengers were also advised that a generator failure had occurred.
The air traffic controller later contacted the crew asking if
operations were normal. At that time they informed ATC that the
engine had failed and that they were continuing to Adelaide. The
crew advised that they would not be declaring an emergency.
After starting the APU, the crew climbed the aircraft to FL240
and continued to Adelaide.
The failed ALF502R-5(-103A) engine, serial number LF05932, was
removed from the aircraft and forwarded by the Australian Transport
Safety Bureau (ATSB) to the engine manufacturer in the US. An
investigation of the failed engine was carried out by the engine's
manufacturer in August 2004, with a representative of the US
National Transportation Safety Board present on behalf of the
ATSB.
The investigation found that the engine failure resulted from
the failure of the high pressure compressor rotor, number-1
bearing, due to high temperature damage resulting from insufficient
lubrication. The number-1 bearing was located in the stub frame
mount in the engine's gas producer module (see diagram).
Lubricating and cooling oil was supplied to the bearing via two
jets; one each for the front and rear of the bearing.
Oil was ported to the two oil jets through galleries located in
the number-3 bearing support assembly. The oil supply galleries and
jets were sealed by two 'O' ring packings that were positioned on
the outside diameter of the number-3 bearing support assembly.
Examination of the number-3 bearing package support assembly
found that both 'O' rings on the outside diameter of the support
had multiple 'nibbles' (small missing pieces) around their
circumference. The forward 'O' ring had also been cut completely
through. Sectioning of the number-3 bearing support found that the
oil jet that supplied lubrication to the front of the number-1
bearing was also blocked with 'O' ring material. Analysis of that
material found it was consistent with the missing pieces of 'O'
ring from the outside of the number-3 bearing package support.
The engine manufacturer estimated that leakage past the damaged
'O' ring and the blocked oil jet had reduced the lubricating and
cooling oil supply to the number-1 bearing by approximately 50 to
60 percent. The report further indicated that all other engine
damage noted had resulted from the failure of the number-1
bearing.
The ATSB determined that the number-3 bearing package support
was last removed from the engine by the aircraft operator during
maintenance on 24 December 2003. The bearing package support had
been removed to facilitate the fitment of new carbon seals to the
number-1 and number-3 bearings. During that maintenance, both 'O'
rings on the support were replaced. At the time of the engine
failure, the engine had completed 975 hours and 706 cycles since
that maintenance. Information from the operator indicated that
there had been no history of excessive oil usage or excessive
vibration on the trend graphs for the engine.