On July 23, 2012, at a total time since new of 3,752.3 hours total time, the engine was placed onto the Maintenance on Reliable Engines (MORE) STC, which extended the TBO from 3,600 hours to 8,000 hours, which was subsequently approved by the FAA. The MORE STC literature states that the MORE Instructions for Continued Airworthiness (ICA) is not a stand-alone document but is only a supplement to the PW&C engine maintenance program requiring that all the inspection and maintenance specified in the PW&C manual must be performed in addition to inspections and maintenance specified by the MORE program, such as a more frequent borescope inspection of the hot section, periodic inspection of the compressor and exhaust duct areas, and periodic Power Plant Adjustment/Tests.
In conflict with the P&WC guidance, which requires engine condition trend monitoring (ECTM), a method of continuously monitoring engine health, the MORE STC does not recommend ECTM. The operator elected to follow the MORE program guidance and did not conduct ECTM.
According to the airframe and engine records, when the engine was placed onto the MORE STC, an engine hot section inspection (HSI) was performed. Guidance for managing and performing an HSI is contained in the P&WC applicable engine maintenance manual and service bulletin (SB), No. 1703 titled “Operating Time Between Overhauls and Hot Section Inspection Frequency Recommendations”. Revision 6 of the P&WC SB 1703 was applicable at the time. The maintenance manual and SB required a compressor turbine (CT) blade metallurgical evaluation, which consisted of selecting two random pre-SB1669 CT blades, sectioning them and evaluating the samples in a materials laboratory. The tests destroy the two samples. No documentation was located to show that this inspection was complied with.
According to the operator, the combined guidance documentation between the MORE literature, P&WC Maintenance Manual and the P&WC SB was confusing. They further stated that they interpreted the P&WC guidance recommending the destructive blade testing if a TBO extension was being requested from P&WC, however, since the TBO extension was granted via the MORE STC, which did not contain blade sectioning instructions, they deemed that this task was not necessary because they believed the increased inspection intervals required in MORE STC guidance, would effectively manage the CT blades.
Two other HSI tasks, a combustion liner cooling ring gap check and a trim thermocouple verification, which are recommended by the P&WC maintenance manual, but not required under the MORE STC, were not accomplished. The MORE STC documentation also does not reference the P&WC publication service information letter (SIL) PT6A-116R3 “Borescope Inspection in Conjunction with Fuel Nozzle Check” which better illustrates the borescope examination of the trailing edges of the CT blades.
At the time of the accident, the engine had been operating under the MORE STC for about 1,137.3 hours. The most recent inspection performed on the engine prior to the accident was conducted 3.9 hours prior, and consisted of the MORE STC 100, 200, and 400-hour inspections. The MORE STC program 400-hour inspection included a hot-section inspection, which only required inspection of the compressor blades via a borescope.