N5007RAir Tractor AT-602 · 1998

On June 1, 2022, about 1700 central daylight time, an Air Tractor AT-602 airplane, N5007R, sustained substantial damage when it was involved in an accident near Allport, Arkansas. The pilot sustained minor injury. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight.

The pilot reported he was on the 35th flight of the day and the airplane had been performing well all day with no issues noted. The airplane was refueled two flights before the accident flight and at the time of the accident each wing fuel tank was about half full of Jet A fuel. The airplane’s 6,500 lb hopper was loaded with 3,600 lbs of urea. For the accident flight, the pilot intended to fly to a rice field to apply fertilizer. Immediately after the takeoff from the private airstrip, the pilot heard a “loud pop” noise emit from the turboprop engine and he observed flames emit from the left side of the airplane. The pilot confirmed that a loss of engine power occurred, he made sure the flaps were down, and he began to scan the area to perform a forced landing.

During the forced landing, the pilot reported that it felt like there was a “slight recovery” of engine power, but then the engine ceased producing power. The pilot retarded the throttle and landed on a flat field consisting of grass, dirt, and mud. During the landing, the airplane nosed over and came to rest inverted. The pilot was able to egress from the airplane without further incident. The airplane sustained substantial damage to the left wing, the fuselage, and the empennage.

Examination of the airframe established flight control continuity. Fuel was found in the fuel lines at the fuel pump, to the fuel selector, the header tank, and forward to the airframe fuel filter. The airframe fuel filter assembly on the firewall was removed and FOD of unknown origin was found between the airframe fuel filter pleats and in the fuel filter bowl.

Examination of the engine revealed FOD contamination of the engine fuel filter and various fuel-wetted accessories. Bench testing of the fuel control unit, the flow divider/dump valve, and fuel nozzles indicated that they performed within expected parameters. The engine exhibited rotational contact signatures on the compressor turbine and power turbine components consistent with the engine producing power at the time of impact. The engine power output at the time of impact could not be determined.

Examination of the propeller revealed no mechanical malfunctions or failures that would have prevented normal operation. Marks on the piston, cylinder, and blade damage indicated that the propeller was operating at low blade angles with low or no power at impact.

Chemical testing of the FOD samples from the engine and the airframe revealed a composition consistent with an unknown, fertilizer-type material.

A review of the airplane’s maintenance records revealed that the airframe fuel filter was most recently inspected and cleaned on August 12, 2021, at 1,285.5 hours Hobbs time. The accident occurred at 1,429.5 hours Hobbs time. The Air Tractor AT-602 Maintenance Manual stated that the airframe fuel filter was to be inspected and cleaned every 100 hours. Engine maintenance was performed on May 23, 2022, and the engine was successfully test run, but the associated maintenance logbook entry did not show any tasks related to the fuel system.

The maintenance manual discussed cleaning the airframe fuel filter and provided a caution that stated , “if any contamination is found in either fuel filter, investigate the cause and rectify the problem.”

The operator was unable to provide fuel or chemical loading records. The operator additionally reported that after the accident, the other Air Tractor AT-602 airplane that they operate did not have any mechanical issues and that both airplanes received fuel from the same fuel truck.

Air Tractor published Service Letter #148, Fuel Contamination, on June 24, 1996. This document discusses fuel contamination with Air Tractor airplanes and states in part:

As we all know, it is of the utmost importance to maintain a clean fuel system in your aircraft. It is also just as important to acquire clean fuel and maintain clean storage facilities.

The National Agricultural Aviation Association published a Fly Safe Message, “Avoid Engine Fuel System Contamination,” on May 1, 2023. This document discusses preventing engine fuel system contamination for aerial application aircraft and states in part:

Ag aircraft are subject to numerous situations that could result in contamination to the engine fuel system such as unclean fuel storage facilities, aircraft washing, loading, refueling, and maintenance; and/or operating/storing the aircraft outside in condensation/rain. Contamination could be from a variety of sources such as chemicals, debris from the aircraft and maintenance work, or dirt, dust, microbial growth, rust, sand, and water. Contamination of an engine fuel system could result in either a partial or a complete loss of engine power.

Turbine engine components that could be adversely affected by contamination include the fuel control unit and the fuel nozzles. According to Federal Aviation Administration Advisory Circular 20-43, turbine engine fuel controls and pumps are more sensitive than piston engine fuel systems. Their fuel feed and pumping systems must work harder, tolerances are closer and fuel pressures are higher. Fine contaminates may block fuel supply systems and erode critical parts of engine and fuel control systems. Due to the nature of aerial application work, operators should consider increasing the inspection frequency of turbine engine fuel system components. Always follow the maintenance manual when performing your work. Consult with the airframe, the engine, and the engine accessory manufacturers on any additional service documents regarding the topic of fuel contamination.