N802JF

On September 10, 2024, about 0843 central daylight time, an Air Tractor AT-802, N802JF, was substantially damaged when it was involved in an accident near Hickory Valley, Tennessee. The airline transport 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 departed his base airport to spray fields about 6 miles to the southwest. He had made multiple passes over the target fields when he noticed the fuel flow was lower than he had set. He pushed the power lever forward to increase power and the engine began to lose power. The pilot 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 jettisoned the aerial application payload in the hopper and initiated a forced landing. The airplane impacted a hay field and bounced on a slight uphill section before coming to rest.

Postaccident examination of the airplane 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 respective 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 reported 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 coincident 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 engine-driven fuel pump tested within parameters and was not examined further. Testing of the FCU indicated that fuel flow in the P3 schedule and the start and acceleration schedule was lower than the component maintenance manual minimum limit for P3 pressures below 100 psia. 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; the bellows 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 October 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 it 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 (LRSF) for the manufacturer of the FCU. They reported that the service bulletin was not complied with during the overhaul or during any of the repairs. The FCU manufacturer reported that they expect the LRSF 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 LRSF. 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.”