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PC12, en-route, near Derby Western Australia, 2010

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Summary
On 29 January 2010, a Pilatus PC-12/45 being operated on an aerial work basis on a Medevac flight from Derby to Kununurra Western Australia and climbing through FL180 in night VMC experienced signs of engine malfunction and initiated an air turnback during which the engine failed completely. The subsequent 6nm glide descent and approach culminated in a successful landing with the aircraft undamaged and all four occupants unhurt.
Event Details
When January 2010
Actual or Potential
Event Type
Airworthiness, Loss of Control
Day/Night Night
Flight Conditions VMC
Flight Details
Aircraft PILATUS PC-12 Eagle
Operator Private
Type of Flight Aerial Work
Origin Derby Airport
Intended Destination East Kimberley Regional Airport
Actual Destination Derby Airport
Take off Commenced Yes
Flight Airborne Yes
Flight Completed Yes
Flight Phase Climb
ICL / ENR
Location En-Route
Origin Derby Airport
Destination East Kimberley Regional Airport
Location
Approx. 11 km NE of Derby Airport, Western Australia
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General
Tag(s) Inadequate Airworthiness Procedures
LOC
Tag(s) Loss of Engine Power
AW
System(s) Engine - General
Contributor(s) Damage Tolerance,
Component Fault in service
Outcome
Damage or injury No
Causal Factor Group(s)
Group(s) Aircraft Technical
Safety Recommendation(s)
Group(s) None Made
Investigation Type
Type Independent

Description

On 29 January 2010, a Pilatus PC-12/45 being operated on an aerial work basis on a Medevac flight from Derby to Kununurra Western Australia and climbing through FL180 in night Visual Meteorological Conditions (VMC) experienced signs of engine malfunction and initiated an air turnback during which the engine failed completely. The subsequent 6nm glide descent and approach culminated in a successful landing with the aircraft undamaged and all four occupants unhurt.

Investigation

An Investigation was carried out by the Australian Transport Safety Bureau (ATSB). An inspection confirmed that the engine propeller reduction gearbox had seized. Further investigation established that four of the six first-stage bolts on the propeller reduction gearbox had failed due to fatigue cracking at the head to shank fillet radius.

Figure reproduced from the Official Report showing the first stage reduction assembly and the location of the failed bolts

It can be seen from the illustration that, in addition to the six carrier bolts, the splined adapter was aligned and held in position by three locator pins adjacent to the carrier bolts. Further examination of the four failed bolts confirmed the mode of failure had been reverse bending fatigue below the head of each one.

The investigation found that between 2000 and January 2010, there had been 27 failures of the larger variants of PT6A series engines due to similar failure of the first stage carrier bolts at the head-to-shank fillet radius. During that period, there had been a total of 2029 of these engines in service worldwide. Between January 2010 and October 2010, there were two further PT6A reduction gearbox failures, bringing the total number of failures since 2000 to 29.

In the light of this finding and in the context of a history of previous similar failures, Pratt & Whitney Canada, the engine manufacturer found that a quantity of in-service first stage reduction assembly carrier bolts fitted during overhaul had not undergone cold rolling of the head-to-shank fillet radius during manufacture. A series of Service Bulletins were issued which identified other affected gearboxes and provided recommended maximum elapsed times for the replacement of the potentially faulty carrier bolts.

A review of the relevant SAE specification AS7477D found that it was ambiguous in respect of the need to cold roll the head-to-shank fillet radius of the failed carrier bolts and a revised version of the specification was published by the SAE in October 2011 to require cold rolling of the head-to-shank fillet radius of such bolts.

Although a night medical evacuation flight using a single-engine aeroplane was not covered by the regulatory approval process for public transport operations administered by the Civil Aviation Safety Authority (CASA), the existence of a process for Approved Single-Engine Turbine-Powered Aeroplane (ASETPA) operations was noted. It was considered that the failure being investigated was relevant to the consideration of the reliability of the PT6A-67B engine for the ASETPA program. It was advised that the CASA requirements for ASEPTA were “based on a similar philosophy to that required for extended range operations for twin-engine aeroplanes (EROPS). At the time of the Investigation, the only two aircraft types approved for ASETPA operations in Australia were the Cessna 208 and the Pilatus PC-12 aircraft.

It was considered that the application by the operator of most of the requirements of the CASA ASETPA program to its operations (the incident flight was being conducted Instrument Flight Rules (IFR) which is not allowed at night under ASETPA) and the exposure of the incident pilot in the management of practice engine failures for other pilots had optimised the chances of a safe landing after the total power loss.

The Final Report Aviation Occurrence Investigation AO-2010-006 was released on 17 November 2011.

Safety Action taken by Pratt and Whitney Canada and the SAE to re-specify the manufacturing standard for the failed bolts and by CASA to proactively improve the monitoring of specific engine failure types were considered as satisfactory closing actions for the Investigation and therefore no Safety Recommendations were made.

Further Reading