On April 2, 2024, about 1310 central daylight time, a Schweizer G-164B airplane, N75099, was substantially damaged when it was involved in an accident near Kaplan, Louisiana. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 137 agricultural flight.
The pilot reported that he departed a private airstrip with the engine torque set to 50 pounds per square inch gauge (psig) and the propeller speed at 2,200 rpm. After an uneventful takeoff, the pilot reduced engine torque to 40 psig at 2,000 rpm and continued north toward the field to be seeded with rice. About 1 mile from the intended field, the engine lost partial power. The pilot was unable to regain engine power to maintain altitude, which resulted in a forced landing in a nearby crawfish pond. The airplane came to rest inverted in the pond, resulting in substantial damage to the fuselage, upper wings, and empennage.
The Pratt & Whitney Canada (P&WC) PT6-15AG turboprop engine, serial number PCE-14007, was examined at the manufacturer. The engine did not exhibit evidence of preimpact fire or radial uncontainment. All external engine surfaces were covered in dirt and pond debris.
Multiple accessories mounted to the accessory gearbox had impact-related damage, including the fuel control unit (FCU), which was fractured and separated from the fuel pump. The exhaust case displayed 360° impact deformation and contained residual pond water. The propeller governor remained attached to the engine, but the speed setting lever had impact damage that rotated it past the max stop. The beta components (beta arm, retaining pin, carbon block) were deformed and/or out of normal position. The P3 and Py line b-nut connections were all safety wired with run-on torque.
Engine disassembly revealed no evidence of internal failure or other anomalies that would have precluded normal engine operation. Rotational scoring/rubbing was observed at multiple places in the engine, most notably the PT rotor disk blade fir trees/outer platform and the CT rotor blade platform. Light 360° circumferential rubbing was also documented on the compressor impeller and shroud.
The reduction gearbox magnetic chip detector plug exhibited a small metal sliver on one of the poles. The oil and fuel filters were in good condition and free of obstruction. The P3 air filter housing/filter was filled with pond water, but the filter was in otherwise good condition without obstruction.
The FCU could not be bench tested because the housing was fractured during impact. The FCU-to-fuel pump coupling shaft was intact, indicating there was continuity between the components. The FCU air inlet adapter Px air fitting exhibited an oily buildup that covered the surface downstream of the fitting orifice. The Py adapter fitting also appeared discolored, but the fitting was not blocked. The Py metering orifice in the air inlet adapter was clear. A materials laboratory examination of the debris found on the FCU air inlet adapter Px air fitting indicated it was a combination of environmental debris and ester-based compound (oil). The FCU governor bellows did not exhibit any anomalies when leak checked. The FCU metering valve was intact and undamaged. The FCU was partially disassembled, and no other internal anomalies were identified that would have precluded normal operation during the accident flight.
The fuel pump mount flange to the accessory gear box was fractured. The driveshaft coupling was also fractured, and the drive gear shaft was bent, which prevented rotation by hand. A vise grip plier was used to rotate the bent shaft, which confirmed the gear set was intact. The fuel pump was not further disassembled.
The propeller governor had grass and dirt debris on its external surfaces. The speed setting lever was displaced beyond the high-speed stop during impact. The customer-installed hardware was removed, and the unit was bench tested in accordance with the manufacturer’s Test Specification Procedure (TSP). According to the manufacturer, all test points were consistent with similar service run propeller governor units. Minor deviations from the specification were typical of customer adjustment.
The overspeed governor had grass and dirt debris on external surfaces. The governor receptacle was bent but all wiring remained intact. The overspeed governor was bench tested in accordance with the manufacturer’s TSP. The maximum speed was recorded above specification. According to Woodward, the recorded value was typical of customer field adjustment. All other test points were within TSP specification.
There were 3 bolts missing from the PT containment ring. The remaining containment ring bolts were not safety wired consistent with the engine maintenance manual. During engine disassembly, the 3 missing bolts were recovered from the combustor large exit duct. However, there was no evidence of upstream impact damage to indicate these loose bolts were ingested through the compressor. A materials laboratory examination established that the bolt dimensions were consistent with the power turbine containment ring bolts, and the bolts showed varying degrees of oxidation and hardness changes consistent with exposure to the gas temperatures of an operating engine.
The accident engine was a P&WC PT6A-15AG with a PT6A-27 power section. The engine maintenance documentation included a May 2023 logbook entry from when the PT6A-27 power section was installed. According to P&WC, the PT6A-27 and PT6A-15AG power sections are identical both mechanically and thermodynamically, but the PT6A-15AG power section has additional corrosion protection and hardware changes for agricultural operations. According to P&WC, there are no service bulletins that support mating a PT6A-15AG gas generator with a PT6A-27 power section. As such, the configuration did not meet the P&WC PT6A-15AG instructions for continuing airworthiness.
The CT vane ring was marked “STI W/O No. 0941R09 STI 72-50-3017 Rev. 02”, which indicated a repair completed by Southwest Turbine in Phoenix, Arizona. According to the engine manufacturer, the PT6A CT vane repairs performed by Southwest Turbine are not in the P&WC overhaul manual and are not a P&WC approved repair; however, the FAA has approved the Southwest Turbine CT vane repair procedure.