12 of September 2013, Blog #535
A Beech Aircraft Corporation model 200 Super King Air, powered by two P&W PT6A-41 engines, was operating a charter flight from Utirik Atoll to Marshall Islands International Airport on the Majuro Atoll (Both on the Marshall Islands). A crew of two was operating the flight with one passenger on board.
Left - contaminated magnetic chip detector of the reduction gearbox
Right top - second-stage power turbine disk Right bottom - damaged first-stage planet gear in situ within the gear carrier (Source ATSB)
After an uneventful take-off and climb the flight and those onboard had settled in cruise flight at FL 230. When approximately 155 nm (287 km) from their destination the captain noticed that the oil pressure indication for the left engine was fluctuating widely. No other engine indications were abnormal and the engine operated normally.
Two minutes later that changed, when the left engine failed. The flight crew completed the relevant checklist and secured the engine and propeller. After assessing their situation the crew decided that the safest option was to continue to their planned destination as per the filed flight plan. As per the performance requirements for continued flight on one engine, they descended to FL 180. The remainder of the flight was uneventful and a single-engine landing was performed without problems at Marshall Islands International Airport. There were no injuries to the occupants of the aircraft.
PT6A-41 engine diagram highlighting the general arrangement of the
first-stage gears within the reduction gearbox (Source; ATSB)
A post-flight inspection revealed that the engine failure was contained and there was no further damage than the failed engine. It also revealed extensive internal damage to the turbine section of the engine. The circuit breaker for the left engine reduction gearbox magnetic chip detector had popped. The failed engine was removed from the aircraft at Majuro Atoll by the aircraft operator and freighted back to Australia. The engine was subsequently disassembled and inspected at an approved maintenance, repair, and overhaul facility in Brisbane, Australia, in the presence of ATSB (Australian Transport Safety Bureau) investigators and a representative from the engine manufacturer.
The following significant findings were made during the disassembly and inspection;
Most of the blades from the second-stage power turbine disk had fractured and released at their base at the blade platform. The fracture surfaces of the remnant blade stubs were consistent with failure by overstress with no evidence of any pre-existing defects or progressive crack growth. The power turbine housing successfully The second-stage power turbine (© ATSB) contained the released blades.
Extensive damage to the reduction gearbox, ranging from worn away teeth on the sun gear, damage to the first stage planet gears, and a large amount of debris captured by the reduction gearbox magnetic chip detector and sump screen
The ATSB published an extensive report into the engine failure on the 11th of December 2014. In their report the following contributing factors are presented;
"The engine failure resulted from fatigue cracking and accelerated breakdown of the first-stage sun and planet gears in the propeller reduction gearbox that led to a subsequent loss of load and consequential Overspeed of the second-stage power turbine."
Example of fatigue cracking (arrowed) on a first-stage planet gear tooth (Source; ATSB)
Other factors that increased the risk were;
Pratt and Whitney Canada PT6A-38, -41, -42, and -42A engines, last overhauled prior to September 1999, may be operating with first-stage reduction sun and planet gears in exceedance of the 12,000-hour life limit required for the manufacturer’s time between overhaul (TBO) extension program, while still complying with the requirements of Civil Aviation Safety Authority Airworthiness Directive AD/ENG/5 Amdt 9 and the manufacturer’s baseline TBO program.
The reduction gearbox chip detector cockpit warning light circuit breaker had popped, preventing the flight crew from receiving a visual warning prior to the engine failure.
The ATSB report, on which this blog is based, can be accessed for the readers' reference by clicking on the .pdf file below;