On 12 September 2013, a twin-engine Beech Aircraft Corporation, Model 200 Super King Air was conducting a passenger charter flight from Utirik Atoll to Marshall Islands International Airport (Majuro Atoll). About 40 minutes into the flight, while in cruise to the destination, the pilot observed abnormal oil pressure indications for the left engine and approximately 2 minutes later the engine failed. The crew secured the engine and elected to continue with the original flight plan. The aircraft landed at Majuro Atoll without further incident.
What the ATSB found
Failure of the left engine, a Pratt and Whitney Canada PT6A-41, resulted from fatigue cracking and breakdown of the first-stage sun and planet gears in the propeller reduction gearbox. The specific factors contributing to the initiation of the gear breakdown could not be positively identified; however, the ATSB identified certain PT6A-38, -41, -42 and -42A engines that, while in compliance with applicable maintenance requirements, could be operating with first-stage reduction gears in excess of the manufacturer’s recommended maximum 12,000 hour service life. This specifically related to engines that had not been overhauled since September 1999, when it became mandatory to replace the reduction gears during every engine overhaul. The engine manufacturer has indicated that, provided the engine is maintained in accordance with the applicable instructions for continuing airworthiness, the rate of engine failure associated with high‑time gearsets is extremely remote and that immediate action is not warranted in these situations.
It was also reported that the magnetic chip detector cockpit warning light did not illuminate and that the associated circuit breaker had popped. There have been similar reports of Beech 200 aircraft with popped circuit breakers accompanied by momentary or no chip detector light illumination, shortly preceding engine failure. A previous situation of accelerated engine failure, involving a large volume of liberated engine material bridging the chip detector terminals, was reported to have caused the circuit breaker to pop, precluding the illumination of the warning light. A similar scenario was considered likely in this occurrence.
Pratt and Whitney Canada PT6A-38, -41, -42 and -42A engines last overhauled prior to September 1999 may be operating with high-time first-stage reduction gears which have an increased susceptibility to deterioration. Current maintenance procedures may be effective in identifying gradual deterioration of reduction gears, but not necessarily impending rapid, catastrophic breakdown per the subject engine failure. The ATSB therefore encourages operators and maintainers of affected engines to review their maintenance records and give consideration to the replacement of high-time gearsets.
Additionally, pilots and operators should also be aware of the potential for reduction gearbox chip detector cockpit annunciator lights to only illuminate momentarily or not at all, due to short circuits caused by the rapid accumulation of liberated gearbox material, in situations of accelerated engine failure. Even a momentary indication could be an indicator of engine deterioration and therefore should be noted for subsequent maintenance attention.