While en route from Darwin to Gove, the master warning system of
the BAe 146 chimed and the crew noticed that the engine vibration
indication for the number four engine was fluctuating between 0.6
and 2.0 inches per second. All other engine indications appeared
normal. The pilot in command switched off the thrust management
system, retarded the number four engine power lever to idle and
switched off the associated bleed air. At the same time, a loud
series of thuds were heard and the master warning system again
chimed, alerting the crew to low oil pressure indications for this
engine. The pilot in command and the first officer then noticed
smoke in the cabin when they looked back through the flight deck
door. The crew donned oxygen masks and completed the "smoke or fire
in cabin" and "engine fire or severe damage" checklists. ATC was
notified and a return to Darwin requested. With the diversion
approved and a distress phase declared, the subsequent return and
one-engine-inoperative landing were uneventful. Company maintenance
inspection found the number four-engine could not be rotated and
had incurred severe internal damage.
The manufacturer inspected the engine and found that the number
1 bearing pack had failed. The pinion gear retention nut backed
off, and had misaligned the gears, causing metal contamination in
the oil supply from the improperly meshed gear teeth. The failure
of the bearing pack in turn caused major internal damage to other
parts of the engine as the high-pressure compressor shaft was
allowed to orbit within the engine. The reason the pinion gear nut
lost tension could not be determined.
The manufacturer indicated that the incidence of failure of the
pinion gear retention nut is very low, with a mean time between
failure of 2.9 million fleet hours. As a result, the ATSB does not
believe further safety action is necessary.
The company investigation also identified several other issues
associated with this occurrence that are being addressed:
- The quality of communications between cabin crew and cockpit
crew using the dedicated emergency-in-cabin (EIC) call
facility; - The clarity of audio when the crew were wearing oxygen masks;
and - Formalising procedures for medical examinations and counselling
after the event.
The operator will advise the ATSB if any safety action is
taken.
The cabin crew reported minor and temporary eye irritation and
sinus discomfort from the smoke during the incident. The medical
examinations the following week did not find anything notable. The
pilot in command and first officer did not seek medical attention
after the incident.