Accident Summary
On June 20, 2023, at approximately 1445 eastern daylight time, an Air Tractor AT-802A, registration N3069S, was substantially damaged during a forced landing near Wellington, Florida. The pilot, who was the sole occupant, reported no injuries. The flight was conducted under Title 14 Code of Federal Regulations Part 137 as an aerial application operation.
Pilot Report
The pilot stated that he had been applying fertilizer to several fields and noted no anomalies with engine performance during the application. After completing the last application, he was returning to the landing strip at about 500 ft above ground level when the engine lost power. He executed a forced landing to a clearing.
Aircraft Damage
Examination of the wreckage by an FAA inspector revealed that the airplane came to rest upright. Substantial damage was noted 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 remained attached only by cables and torque tubes. The right elevator and horizontal stabilizer were bent downward at mid-span.
Engine Examination and Testing
The engine, a Pratt & Whitney Canada PT6A-65AG, was removed and sent to the manufacturer for further examination and testing. Initial examination did not reveal any anomalies. The engine was placed in a test cell, where it achieved ignition during the first four start attempts but did not reach ground idle speed. During troubleshooting, the fuel control unit (FCU) and fuel pump were removed and replaced with test units, after which the engine started and ran successfully.
Fuel Control Unit and Fuel Pump Findings
Further examination of the FCU revealed that the drive shaft splines and the corresponding fuel pump drive gear splines were worn sufficiently to allow disengagement of the FCU drive shaft from the fuel pump. The surfaces near 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 showed a similar residue on the gearbox interface and drive shaft.
Functional testing of the fuel pump indicated leakage at the FCU side carbon seal, consistent with the observed wetness. Additionally, fuel flows were found to be below minimum limits, which was consistent with wear of the pump gears. However, the fuel pump is designed to deliver more fuel than required for operation, with the excess bypassed by the FCU and returned to the pump inlet.
Disassembly of the fuel pump revealed pitting on the fixed bearing dams and on the housing gear bore inlet surfaces, both evidence of prolonged cavitation. The drive gear journals exhibited surface corrosion, and the drive and driven gear teeth showed a high level of wear on the working flanks.
Fuel Control Unit Bench Testing
Bench testing of the FCU permitted 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 by 34%, from a rate of 632 lbs per hour to 416 lbs per hour.
Manufacturer Information
According to the manufacturer, the FCU receives the engine gas generator rotational speed (Ng) signal via the drive shaft of the fuel pump, allowing fuel modulation in relation to Ng and other inputs. The engine manufacturer stated that the FCU drive shaft and corresponding fuel pump drive gear are “on condition” components and do not require specific inspection intervals.
Maintenance History
The airplane’s most recent annual inspection was completed on April 5, 2023. At that time, the aircraft had accumulated 4,588 total hours of operation, and the engine had 8,893 total hours of operation.