Accident Overview
On September 10, 2024, at approximately 0843 central daylight time, an Air Tractor AT-802, registration N802JF, was substantially damaged during a forced landing near Hickory Valley, Tennessee. The airline transport pilot was not injured. The airplane was operating under Title 14 Code of Federal Regulations Part 137 as an aerial application flight.
Pilot's Account
The pilot reported departing his base airport to spray fields located about 6 miles to the southwest. After making multiple passes over the target fields, he noticed the fuel flow was lower than his set value. He advanced the power lever forward to increase power, but the engine began to lose power. The pilot then moved the power and condition levers to the forward stops in an attempt to regain power, but the engine continued to “roll back.” He jettisoned the aerial application payload from the hopper and initiated a forced landing. The airplane impacted a hay field, bounced on a slight uphill section, and came to rest.
Postaccident Examination
Examination of the airplane revealed substantial damage to the engine mounts and fuselage. Continuity of flight controls was confirmed from the cockpit to the respective control surfaces. The throttle control also showed continuity from the cockpit to the actuator on the fuel control unit, which moved freely from stop to stop.
Fuel Control Unit Findings
The pilot stated he departed with 176 gallons of Jet-A fuel. Data from the engine monitoring system showed that about 26 minutes after recording began and approximately 10 minutes after takeoff, the fuel flow started dropping from about 82 gallons per hour to about 11 gallons per hour over 24 seconds. Torque, propeller rpm, and gas generator speed declined concurrently with the reduced fuel flow until the end of the data.
The fuel control unit (FCU) and engine-driven fuel pump were removed, tested, and examined. The engine-driven fuel pump performed within specifications. Testing of the FCU revealed that fuel flow in the P3 schedule and the start/acceleration schedule was below the component maintenance manual minimum limit for P3 pressures under 100 psia. Partial disassembly of the FCU showed external surface contamination and corrosion on the bellows. A leakage check indicated an increase in bellows length, suggesting a potential leak. When the bellows were internally pressurized and submerged in fluid, leakage was observed at the solder joint between the bellows and end cap. Materials analysis found a silver deficiency at the solder joint interface, consistent with oxidation from corrosion. The engine manufacturer reported that a bellows leak would allow the bellows to expand, moving the FCU fuel valve to minimum fuel flow and preventing further power modulation, matching the recorded engine data.
Service Bulletin and Maintenance History
Woodward Service Bulletin (SB) 83212-73-023, issued October 13, 2014, provides instructions to measure bellows length for various models. Based on the bellows part number from the accident airplane, which lacked an “L” length reference, the SB states the bellows were manufactured before January 1993 and must be replaced, and that this should be accomplished whenever the unit is returned for service.
Manufacturer records showed the FCU was originally shipped on May 11, 1988, and last shipped on April 27, 1993, with no returns recorded since. Maintenance records indicated the FCU was overhauled in 2019 and repaired in 2021 and 2023 by a licensed repair service facility (LRSF) for the FCU manufacturer. The repair facility reported that the service bulletin was not complied with during those maintenance events. The FCU manufacturer stated they expect LRSFs to perform services in strict accordance with the component maintenance manual, service bulletins, Federal regulations, special program announcements, and any agreements. The manufacturer also confirmed that a copy of the SB had been sent to the repairing facility.
Corrosion Considerations
The operator stated the airplane was primarily used for spraying and fertilizing operations. FAA Advisory Circular 43-4B notes that chemicals used in dusting and spraying are corrosive and can accelerate deterioration of materials, emphasizing the importance of corrosion prevention measures.