Accident Details
On July 25, 2014, at approximately 0823 central daylight time, a Gulfstream-Schweizer G-164B, registration N425LS, collided with trees and the ground during a forced landing near Bonifay, Florida. The commercial pilot was not injured, and the airplane was substantially damaged. The airplane was registered to a private individual and operated by Croom Aviation under 14 Code of Federal Regulations as a Part 91 positioning flight. Visual meteorological conditions prevailed. The flight originated at Donalsonville Municipal Airport, Donalsonville, Georgia, about 0800, and was destined for Tri-County Airport, Bonifay, Florida.
According to Federal Aviation Administration records, the airplane was manufactured in 1979 and operated with a restricted category airworthiness certificate for agriculture and pest control purposes. It was originally equipped with a Pratt & Whitney R985-AN-14B radial engine but was subsequently modified with the installation of a Pratt & Whitney Canada PT6A-27 turboprop engine.
Pilot Account and Forced Landing
The pilot stated that before departure, he added 1 quart of oil to the engine and departed with about 60 to 70 gallons of fuel on board. The flight proceeded toward the destination airport. With the airport in sight, as he reached to reduce power, the propeller began windmilling, and the engine "went to screaming, pieces of metal [were] coming out [of the] right exhaust [duct] and down [the] right side of [the] aircraft." He immediately applied full power and adjusted the propeller and fuel controls, but got no response. The airplane struck trees during the ensuing forced landing and came to rest on the ground.
Engine Examination
Following recovery, the engine was sent to the manufacturer's facility in Montreal, Canada, for disassembly examination with oversight from a representative of the Transportation Safety Board of Canada.
Examination revealed the gas generator case housing interior contained two holes of varying sizes and numerous outward dimples and pockmarks. The 1st and 2nd stage compressor blades exhibited circumferential rubbing at the blade tips, while the 3rd stage blades showed heavy rubbing and tearing due to contact with the shroud and adjacent stator. Nine blades and root sections were displaced from the disk; two were not recovered. The No. 1 ball bearing supporting the compressor rotor showed no operational distress, but the No. 2 roller bearing elements and cage were disintegrated; the cage was fractured into multiple sections with severe oxidation, mechanical damage, and deformation. Twelve of sixteen rolling elements were recovered and displayed severe heat oxidation and mechanical damage. The bearing air seal rotors had heavy circumferential scoring and machining from radial contact with the air seal rotor knife edges. The No. 2 bearing inner and outer races were coated with heat-fused melted material. The No. 2 bearing oil nozzle was removed and flow tested, revealing it was obstructed. X-ray inspection showed material buildup to the top of the nozzle jet.
Components of the No. 2 bearing and the blockage from the oil nozzle were submitted to the Pratt & Whitney Canada Materials Engineering Laboratory. Examination showed the bearing races and cage material were consistent with drawing requirements, but the hardness of the outer race and rolling elements could not be determined due to heat damage. The blocked material was consistent with the material used in the bearing races and rollers. Additionally, the compressor turbine displayed circumferential rubbing with frictional heat discoloration and material transfer from contact with the shroud. The disk downstream side had circumferential rubbing from contact with the power turbine guide vane ring. The oil filter and reduction gearbox chip detector were clean.
Maintenance History
The Pratt & Whitney Canada PT6A-27 engine, serial number PCE-40160, was installed per FAA supplemental type certificate SA1377GL. On April 6, 2012, at an engine total time of approximately 18,045 hours, the engine was removed due to a reported failure of the No. 1 bearing. The repair included installing a new No. 1 bearing and main oil filter, replacing the No. 2 bearing, cleaning all oil-wetted passages, and replacing the oil cooler. The engine was reassembled, ground run with no discrepancies, and approved for return to service by an airframe and powerplant mechanic. It was installed on the accident airplane on April 13, 2012. A subsequent 100-hour inspection on August 27, 2012, included cleaning the oil filter and checking oil strainers and chip detectors, which found no metal.
On March 7, 2013, at an engine total time of approximately 18,773 hours, the engine was disassembled due to reports of vibration and unusual noise. The repair involved removing and replacing bearings Nos. 1 through 5 with overhauled bearings; the No. 6 bearing was inspected and found serviceable. After repair, the engine was installed on June 14, 2013. The maintenance record entry did not indicate whether oil strainers, oil filter, or chip detectors were checked. About two months later, the engine was removed and reinstalled following replacement of the compressor turbine blades. Subsequent routine maintenance and a hot section inspection were noted; the hot section inspection occurred on July 9, 2014. There was no record that any metal was detected in checks of oil strainers or chip detector at that time. As of the last hot section inspection, the engine had accrued approximately 287 hours since the overhauled No. 2 bearing was installed, and total time was 19,059 hours.
According to the engine maintenance manual, if oil system contamination is suspected, a reverse flush of the oil system including the oil-to-fuel heater must be performed, and the oil cooler must be rejected. The manual also indicated that identification of the contaminated material by analysis determines subsequent steps, which may require engine overhaul.