Engine failure due to bolt

The results of an investigation into an accident that occurred on 29 January this year in Australia has found that a number of failed bolts inside a gearbox caused the engine of an aircraft to stall, according to the Australian Transport Safety Bureau (ATSB). The Pilatus PC-12 aircraft, owned and operated by the Royal Flying Doctors Service (RFDS), was on a medical flight from Derby to Kununurra, Western Australia, with four people onboard when the pilot reported engine problems and turned back to base. The ATSB preliminary report into the incident states: “The engine subsequently failed and the pilot glided the aircraft to the aerodrome and landed safely with no reported injuries. A subsequent inspection confirmed that the engine propeller reduction gearbox had seized.” The investigation also found that a number of first-stage reduction gearbox carrier bolts had failed, and as a result of the findings, the engine manufacturer has reviewed a number of issues relating to engine overhaul practices and has recommended withdrawal from service of an engine from one aircraft in the Australian fleet for examination. The aircraft was fitted with a Pratt & Whitney PT6A-67B engine, which was manufactured in Canada in 1998 and had a total time in service of 5,619 hours. The engine’s power section had 1,120 hours’ time since last overhaul, which took place on 23 May 2007. Pratt & Whitney Canada (P&WC) has since advised the ATSB that between 2000 and 2010, there have been 27 Reduction Gearbox failures due to the first stage carrier bolt head distress on large PT6A engines. Of the 27 reported failures, 15 resulted in in-flight shutdowns – two of which were on PC-12 aircraft. In 2009, Pratt & Whitney initiated new overhaul processes for the engines. Since the incident in January, P&WC has advised the ATSB that it is undertaking a further detailed investigation of the PT6A-67 series of engines to better understand the carrier bolt distress mechanism. The preliminary information indicates that engine overhaul processes may be a contributing factor, and those processes, as well as several other identified issues, continue to be evaluated. The ATBS report can be downloaded here.