The EMB-120ER Brasilia was cruising at Flight Level 190, en route from Darwin to Tindal. Just prior to top-of-descent, the crew reported that they heard a loud bang from the right side of the aircraft, and the aircraft simultaneously yawed to the right. Suspecting an engine failure, the pilot-in-command disconnected the autopilot and re-trimmed the aircraft, noting that a considerable amount of rudder trim was required to maintain directional control.
Observation of the engine instruments by the crew confirmed a right engine failure, with the right engine torque gauge indicating 1%. A burning smell and fine smoke then became evident in the cockpit, and the crew put on their oxygen masks. Communications between the two pilots proved difficult with the masks fitted, due to a faulty right oxygen mask microphone. The crew positioned the right engine power lever to flight idle and commenced a descent. The flight attendant also indicated to the crew that smoke had begun to enter the cabin area.
The smoke immediately began to dissipate, following the reduction in engine power. The crew then shut down the right engine in accordance with company operating procedures, and broadcast to Air Traffic Control requesting that emergency services be made available on arrival at Tindal. Air Traffic Control declared a distress phase.
The crew removed their oxygen masks as the smoke had dissipated from the cockpit, however, they still fitted the masks intermittently due to the residual strong burning smell.
By that time the flight attendant had completed the company emergency plan actions, and following approval from the pilots, conducted the emergency briefing. During the briefing, some of the passengers indicated that they were experiencing difficulty hearing the details of the pre-recorded instructions, and the flight attendant had to stop the presentation several times to repeat unheard information. After the flight attendant's briefing, the pilot in command used the public address system to inform the passengers of the right engine problem.
After the single-engine landing, the crew stopped the aircraft on the runway. Emergency services indicated that there was a fuel leak from the right engine cowling. As a precaution, the crew shut down the left engine and instructed the flight attendant to disembark the passengers onto the runway through the main cabin entrance door.
The operator conducted an initial inspection of the failed Pratt and Whitney PW118A engine. This inspection revealed extensive damage to the engine's power turbine and a jammed low-pressure compressor. The reduction gearbox also appeared to have decoupled from the engine, with internal damage to the reduction gearbox case. The engine oil filter bypass indicator had activated, with evidence of metal contamination in the oil filter and on the reduction gearbox chip detector.
The Australian Transport Safety Bureau (ATSB) then quarantined the engine, and delivered it to Pratt and Whitney Canada (PWC) for an investigation supervised by the Transportation Safety Board of Canada. The PWC investigation found that the in-flight shut down had occurred due to the decoupling of the reduction gearbox drive from the power turbine rotor. This was considered to be a direct result of the fracture of the reduction gearbox input shaft by torsional overload. (Refer to Annexes A and B attached to this report).
Maintenance
The following PWC Service Bulletins (SB's), were considered to be relevant to this investigation:
* SB 20246 - Replacement of reduction gearbox (RGB) oil transfer tube arrangement; modification of RGB housing set.
* SB 21323 - Replacement of RGB input shaft.
The aircraft was maintained in accordance with the operator's Civil Aviation Safety Authority (CASA) approved system of maintenance; the operator utilising the engine manufacturer's `high utilisation' periodic inspection requirements as detailed in the maintenance manual.
Listed in those periodic inspection requirements was an engine reduction gearbox input shaft (pinion) borescope inspection, which was required at an interval of 3,000 hours maximum since new, or total time since overhaul, or since input shaft (pinion) replacement. Thereafter, the shaft was to be subjected to an ongoing borescope inspection at a maximum interval of 600 hours. If `unacceptable spalling' was detected on the gear teeth surface, an inspection was to be carried out at an interval of 300 hours maximum. A description of `unacceptable spalling' was included in chapter 72-00-00 of the manufacturer's maintenance manual.
A `Note 5' that applied to the borescope inspection, had been placed in the maintenance manual by the manufacturer in May 1996. This note indicated that the borescope inspection was not required for `non floating layshaft' type reduction gearboxes with SB 20246 incorporated. `Note 5' applied to the reduction gearbox assembly fitted to the failed engine.
The first stage input shaft had been installed in the engine, during the last overhaul in May 1997, after having been assessed as serviceable in accordance with PWC's, Cleaning, Inspection and Repair manual requirements. The shaft had previously been removed from another engine, following the incorporation of SB 21323 into that engine. SB 21323 introduced a new input shaft to the system, with a larger tooth fillet radius and an increased carburised case hardening depth on the shafts' surface. This service bulletin had been issued following the fatigue failure of some reduction gearbox input shafts with insufficient carburised case depth. When the shaft was removed from the previous engine it had accrued 10,180 hours time in service.
At the time of failure, the shaft had accrued a further 2,565.2 hours. During that time, the operator reported that they had carried out all required oil filter servicings and completed daily operational checks of the engine and reduction gearbox chip detectors. No contamination had been noted during those checks.
The operator had been conducting a Spectrometric Oil Analysis Program (SOAP) on the engine. However, the program had been discontinued following the receipt of advice from a PWC field representative that it was not required.
The flight crew oxygen mask microphone could not be faulted during maintenance ground testing, however, as a precaution both masks were replaced.
Following this incident, the operator engaged a cabin safety specialist to conduct an audit into the cabin issues highlighted during the occurrence, and the operator's general aircrew procedures. The audit identified several areas for improvement, which were subsequently acted upon by the operator.