The pilot departed his base airport to spray fields about six miles to the southwest. He had made multiple passes over the fields when he noticed the Air Tractor AT-802’s fuel flow was lower than he had set.
He pushed the power lever forward to increase power, but the engine began to lose power. He then pushed the power and condition levers to the forward stops in an attempt to regain power, but the engine continued to “roll back,” he told investigators.
He jettisoned the aerial application payload in the hopper and initiated a forced landing near Hickory Valley, Tennessee. The plane hit a hay field and bounced on a slight uphill section before coming to rest.
Post-accident examination found substantial damage to the engine mounts and fuselage.
The examination also confirmed continuity of controls from the flight controls in the cockpit to the flight control surfaces. Continuity of the throttle control was also established from the control in the cockpit to the actuator on the fuel control unit, which moved from stop to stop.
The pilot told investigators that he departed with 176 gallons of Jet-A aviation fuel.
Review of data downloaded from the engine monitoring system showed that about 26 minutes after the recording began and about 10 minutes after takeoff, the fuel flow began to drop. It dropped from about 82 gallons per hour to about 11 gallons per hour over 24 seconds. The torque, propeller rpm, and gas generator speed all began to drop with the decreased fuel flow until the end of the data.
The fuel control unit (FCU) and engine-driven fuel pump were removed, tested, and examined. The pump tested within parameters and was not examined further.
The FCU was partially disassembled, and the bellows, when removed, showed signs of external surface contamination and corrosion. A leakage check was performed on the bellows that showed an increase in length, indicative of a potential leak. The bellows were then internally pressurized and submerged in a fluid. They showed leakage at the solder joint between the bellows and end cap.
Materials analysis found a silver deficiency at the interface of the solder joint, which is consistent with oxidation due to corrosion of the bellows.
The engine manufacturer reported that a leak in the bellows would have allowed the bellows to expand, subsequently moving the FCU fuel valve to the minimum fuel flow output and preventing further power modulation, consistent with what was observed in the engine data recorded at the time of the event.
Woodward Service Bulletin (SB) 83212-73-023, Engine Fuel and Control-Main Engine Control (MEC)-CDP Bellows Assembly Inspection, was issued Oct. 13, 2014. It includes instructions to measure the bellows length (“L” dimension marked on the part) for various models. Based on the part number of the bellows from the accident airplane, which was not marked with an “L” length reference, the SB states that the bellows were manufactured before January 1993 and that they shall be replaced. It also states this should be accomplished anytime the unit is returned for service.
Manufacturer records for the FCU showed it was originally shipped on May 11, 1988, and the most recent shipment from the manufacturer was on April 27, 1993, with no returns recorded since then.
A review of maintenance records and reports from the FCU repair shop showed that the FCU was overhauled in 2019 and repaired in 2021 and 2023. The repair facility was a licensed repair service facility for the FCU manufacturer. It reported that the service bulletin was not complied with during the overhaul or during any of the repairs.
The FCU manufacturer told NTSB investigators that they expect the repair facility to perform its services in strict accordance with the component maintenance manual, SBs, federal regulations, special program announcements, and any agreements between the manufacturer and the facility. The manufacturer also confirmed that a copy of the SB had been sent to the facility that repaired the FCU.
The operator was asked about the types of operation the airplane is used for and stated that the airplane was mainly used for spraying and fertilizing. FAA Advisory Circular 43-4B states “practically all chemicals used in dusting and spraying operations are corrosive by nature and hasten deterioration of fabric, metal, and wood. It is essential to safe operation that precautions be taken to prevent corrosion and deterioration of wood, metal, and fabric.”
Probable Cause: A partial loss of engine power due to a leak in the fuel control unit bellows, which resulted in the fuel valve moving to minimum fuel flow.
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This September 2024 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.
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