ERA23LA272

The pilot was returning to land following an aerial application flight when the engine lost power. The pilot performed a forced landing in a clearing, during which the airplane sustained substantial damage to the fuselage and empennage. Examination and a test run of the engine revealed that the fuel control unit (FCU) drive shaft splines and the corresponding fuel pump drive gear splines were worn sufficiently to allow for disengagement of the FCU drive shaft from the fuel pump. The surfaces in the vicinity of the FCU-to-fuel-pump interface were wet with a brown residue, similar to iron oxide. Removal of the fuel pump from the accessory gearbox revealed a similar residue on the accessory gearbox interface and drive shaft. Functional testing of the fuel pump showed leakage at the FCU side carbon seal, consistent with the wetness observed on the surfaces. Testing also revealed that the fuel flows were below minimum limits, consistent with wear of the pump gears; however, the fuel pump is designed to deliver more fuel to the engine than required for operation (most of the fuel is bypassed by the FCU and returned to the fuel pump inlet). Disassembly of the fuel pump showed pitting on the fixed bearing dams and on the housing gear bore inlet surfaces, both evidence of exposure to prolonged cavitation. The drive gear journals exhibited surface corrosion. The drive and driven gear teeth showed a high level of wear on the working flanks of the teeth. Bench testing of the FCU permitted a simulation of a loss of speed input condition. When the input rpm was decreased from 6,500 rpm to 0 rpm, the output fuel flow decreased 34%, from a rate of 632 lbs per hour to 416 lbs per hour. According to the manufacturer, the FCU receives the engine gas generator rotational speed (Ng) signal via the drive shaft of the fuel pump. This allows fuel modulation in relation to Ng, among other inputs. The presence of iron oxide on the FCU fuel inlet strainer suggests possible fuel contamination, which may have reduced the fuel lubricity and caused the observed corrosion, wear, and cavitation on the fuel pump’s internal components. It is probable that this lower fuel lubricity may have introduced vibrations in the drive gear, which induced the wear observed on the FCU and fuel pump splines. Based on the available evidence, the disengagement of the fuel control unit drive shaft from the fuel pump resulted in a decrease in fuel flow and a subsequent loss of engine power.

On June 20, 2023, about 1445 eastern daylight time, an Air Tractor AT-802A, N3069S, was substantially damaged when it was involved in an accident near Wellington, Florida. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. The pilot reported that he had been applying fertilizer to several fields, during which he noted no engine performance anomalies. After finishing the last application, he was returning to the landing strip about 500 ft above ground level when the engine stopped producing power. He performed a forced landing to a clearing. Examination of the wreckage by an FAA inspector revealed that the airplane came to rest upright with substantial damage to the fuselage and empennage. The fuselage was crushed behind the cockpit to the tail section. The rudder and vertical stabilizer were fractured off the tail section and attached by cables and torque tubes. The right elevator and horizontal stabilizer were bent mid-span downward. The engine, a Pratt & Whitney Canada PT6A-65AG, was sent to the manufacturer for further examination and testing. Initial examination of the engine did not reveal any anomalies, and the engine was placed in a test cell. During the first four start attempts, the engine achieved ignition, but would not reach ground idle speed. While troubleshooting, the fuel control unit (FCU) and fuel pump were removed and replaced with test units, after which the engine started and ran successfully. Further examination of the FCU revealed that the drive shaft splines and the corresponding fuel pump drive gear splines were worn sufficiently to allow for disengagement of the FCU drive shaft from the fuel pump. The surfaces in the vicinity of the FCU to fuel pump interface were wet with a brown residue, similar to iron oxide. Removal of the fuel pump from the accessory gearbox revealed a similar residue on the accessory gearbox interface and drive shaft. Functional testing of the fuel pump showed leakage at the FCU side carbon seal, consistent with the wetness observed on the surfaces. Testing also revealed that the fuel flows were below minimum limits, consistent with wear of the pump gears; however, the fuel pump is designed to deliver more fuel to the engine than required for operation (most of the fuel is bypassed by the FCU and returned to the fuel pump inlet). Disassembly of the fuel pump showed pitting on the fixed bearing dams and on the housing gear bore inlet surfaces, both evidence of exposure to prolonged cavitation. The drive gear journals exhibited surface corrosion. The drive and driven gear teeth showed a high level of wear on the working flanks of the teeth. Bench testing of the FCU permitted a simulation of a loss of speed input condition. When the input rpm was decreased from 6,500 rpm to 0 rpm, the output fuel flow decreased 34%, from a rate of 632 lbs per hour to 416 lbs per hour. According to the manufacturer, the FCU receives theengine gas generator rotational speed (Ng) signal via the drive shaft of the fuel pump. This allows fuel modulation in relation to Ng, among other inputs. The airplane’s most recent annual inspection was completed on April 5, 2023, at which time the airplane had 4,588 total hours of operation and the engine had 8,893 total hours of operation. According to the engine manufacturer, the fuel control unit drive shaft and the corresponding fuel pump drive gear are “on condition” components and do not require specific inspection intervals.