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.
