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.