N622AW – Air Tractor AT-602 – 2024-07-17 NTSB Accident Report

HISTORY OF FLIGHTOn July 17, 2024, about 1453 central daylight time, an Air Tractor AT-602 airplane, N622AW, was substantially damaged when it was involved in an accident near Oakland, Nebraska. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 agricultural flight.

The pilot reported that there was a loss of engine power while the airplane was in an aerial-application spray turn. The airplane’s aft fuselage was substantially damaged during the subsequent forced landing in a cornfield.

A review of the airplane’s engine monitor data revealed that the engine was started at 1343:13. At that time, the left- and right-wing fuel tanks contained 105 gallons and 75 gallons, respectively.

At 1348:40, the gas generator speed (Ng), propeller speed (Np), engine torque, and fuel flow increased to values consistent with takeoff power levels. After takeoff, the initial 30-gallon fuel imbalance between the wing fuel tanks decreased to about 15 gallons while enroute to the field to be sprayed, as shown in Figure 1. According to the Air Tractor AT-602 airplane flight manual (AFM), the wing fuel tanks are interconnected by a 2-gallon header tank and cannot be individually selected.

According to the recorded fuel level in each wing tank, as the pilot conducted aerial-application spray passes, there was an overall decrease of fuel in the right-wing fuel tank and a minimal decrease of fuel in the left-wing fuel tank. About 57 minutes 27 seconds after engine startup, the fuel level in the right-wing fuel tank decreased below 11 gallons and the engine monitor’s master caution parameter activated, as shown in Figure 2. The AT-602 AFM states that the low fuel warning light in the cockpit illuminates when either fuel tank contains less than 11 gallons. About 24 seconds after the master caution parameter activated, the recorded fuel level in the right tank increased above 11 gallons and the master caution parameter reverted to an inactive status.

Over the next 3 minutes 23 seconds, the engine monitor’s master caution parameter cycled active/inactive two additional times before the level of fuel in the right tank remained less than 11 gallons and the master caution parameter remained activated for the remainder of the recorded data.

Figure 1. Plot of engine gas generator speed, fuel flow, fuel tank levels, and fuel imbalance for the entire flight.

At 1 hour 3 minutes 29 seconds after engine startup, the fuel level of the right-wing tank decreased to 0 gallons. About 6 minutes later, at 1452:53, the engine monitor recorded a sudden loss of fuel flow and corresponding loss of engine power. During the 70 seconds before the loss of engine power, the fuel level of the right-wing tank fluctuated between 0 gallons and 3.8 gallons. According to the airplane owner’s manual, the fuel tank sender units do not sense the last 2 gallons in either wing tank. Additionally, the 2 gallons of fuel contained in the header tank are also not indicated.

The loss of engine power occurred 1 hour 9 minutes 39 seconds after the engine startup. About 15 seconds later, at 1453:07, the engine monitor ceased recording data. At the end of the recorded data, the recorded fuel level of the left- and right-wing tanks were 79.8 gallons and 2.5 gallons, respectively.

Figure 2. Plot of engine gas generator speed, fuel flow, fuel tank levels, and fuel imbalance at the end of the flight. AIRCRAFT INFORMATIONThe airplane was equipped with a 117-gallon fuel tank in each wing. The wing fuel tanks are interconnected by a header tank and cannot be individually selected. Including the 2 gallons of fuel in the header tank, the airplane’s total fuel capacity was 236 gallons, of which 230 gallons were considered usable.

According to the Air Tractor AT-602 Owner’s Manual, each wing fuel tank was equipped with two fuel quantity sending elements that are connected in series electrically. The inboard sending element measures fuel level when fuel is present in the lower third of the tank. The outboard sending element measures the fuel level when fuel is present in the upper third of the tank. Both sending elements are active when the fuel level is in the mid-tank range.

As per the AT-602 AFM, the airplane was equipped with a placard installed on the instrument panel that stated:

Avoid skidding turns which may result in fuel migration from one tank to the other. The engine may quit when either tank becomes empty. Monitor the fuel level in each tank frequently when the fuel level is less than ½ tank.

The AT-602 AFM further states that while conducting agricultural flying:

In addition to being hazardous, a skidding turn can transfer fuel from one tank to another, which will result in flameout if one tank runs dry. Monitor the fuel level in each tank when the fuel level reaches ½ tank. For aircraft with a single fuel gauge, leave the selector switch on the low tank. Fuel transfer can occur when flying a racetrack pattern if the turns are not coordinated.

According to the AT-602 AFM, the airplane is equipped with a low fuel warning system that consists of two float switches, one in each wing tank, connected to a red warning light in the instrument panel. Either float switch can activate the warning light independent of the other. While in coordinated flight, the floats are positioned so that the warning light activates with about 22 gallons usable fuel remaining (11 gallons usable fuel per tank). However, a fuel load imbalance due to uncoordinated flight, flying all turns in one direction, or other cause, may result in fewer than 22 gallons usable at the time of warning light illumination. The AT-602 AFM stipulates that the fuel gauges are the primary instruments for fuel management, and the pilot should not use the warning light system as a means of determining the amount of fuel remaining.

The AT-602 AFM states that when the low fuel warning light illuminates there is a low fuel condition in either or both wing fuel tanks. To resolve the situation, a pilot should fly the airplane straight and level, monitor the fuel gauges, and land as soon as practical. However, if fuel gauges indicate a fuel imbalance with an adequate amount of fuel remaining, the flight may be continued after allowing fuel to transfer and equalize between the wing tanks. The AT-602 AFM states that an intermittent, or flickering, low fuel warning light should be treated the same as a steady warning light.

The AT-602 AFM states that the indications of an engine flameout include a sudden drop in interstage turbine temperature, engine torque, and gas generator speed (Ng). Additionally, the engine running out of fuel or a temporary interruption of fuel to the engine can result in a flameout. AIRPORT INFORMATIONThe airplane was equipped with a 117-gallon fuel tank in each wing. The wing fuel tanks are interconnected by a header tank and cannot be individually selected. Including the 2 gallons of fuel in the header tank, the airplane’s total fuel capacity was 236 gallons, of which 230 gallons were considered usable.

According to the Air Tractor AT-602 Owner’s Manual, each wing fuel tank was equipped with two fuel quantity sending elements that are connected in series electrically. The inboard sending element measures fuel level when fuel is present in the lower third of the tank. The outboard sending element measures the fuel level when fuel is present in the upper third of the tank. Both sending elements are active when the fuel level is in the mid-tank range.

As per the AT-602 AFM, the airplane was equipped with a placard installed on the instrument panel that stated:

Avoid skidding turns which may result in fuel migration from one tank to the other. The engine may quit when either tank becomes empty. Monitor the fuel level in each tank frequently when the fuel level is less than ½ tank.

The AT-602 AFM further states that while conducting agricultural flying:

In addition to being hazardous, a skidding turn can transfer fuel from one tank to another, which will result in flameout if one tank runs dry. Monitor the fuel level in each tank when the fuel level reaches ½ tank. For aircraft with a single fuel gauge, leave the selector switch on the low tank. Fuel transfer can occur when flying a racetrack pattern if the turns are not coordinated.

According to the AT-602 AFM, the airplane is equipped with a low fuel warning system that consists of two float switches, one in each wing tank, connected to a red warning light in the instrument panel. Either float switch can activate the warning light independent of the other. While in coordinated flight, the floats are positioned so that the warning light activates with about 22 gallons usable fuel remaining (11 gallons usable fuel per tank). However, a fuel load imbalance due to uncoordinated flight, flying all turns in one direction, or other cause, may result in fewer than 22 gallons usable at the time of warning light illumination. The AT-602 AFM stipulates that the fuel gauges are the primary instruments for fuel management, and the pilot should not use the warning light system as a means of determining the amount of fuel remaining.

The AT-602 AFM states that when the low fuel warning light illuminates there is a low fuel condition in either or both wing fuel tanks. To resolve the situation, a pilot should fly the airplane straight and level, monitor the fuel gauges, and land as soon as practical. However, if fuel gauges indicate a fuel imbalance with an adequate amount of fuel remaining, the flight may be continued after allowing fuel to transfer and equalize between the wing tanks. The AT-602 AFM states that an intermittent, or flickering, low fuel warning light should be treated the same as a steady warning light.

The AT-602 AFM states that the indications of an engine flameout include a sudden drop in interstage turbine temperature, engine torque, and gas generator speed (Ng). Additionally, the engine running out of fuel or a temporary interruption of fuel to the engine can result in a flameout. WRECKAGE AND IMPACT INFORMATIONExamination of the airplane fuel system did not reveal any preimpact anomalies that would have precluded normal distribution of fuel from the fuel tanks to the engine. The fuel system vent system was clear of any blockage. The fuel filler caps were found properly installed, and their O-ring seals were in good condition. All fuel filters were clear of significant debris. The fuel quantity sending elements functioned as designed when tested. The low fuel warning switches functioned as designed when tested, and the incandescent lightbulb associated with the low fuel warning light illuminated when connected to an external battery power source.

The airplane was equipped with a cockpit placard that advised pilots to avoid skidding turns that may result in fuel migration from one tank to another, that the engine may quit when either tank becomes empty, and to monitor the fuel level in each tank frequently when the fuel level is less than ½ tank.

At the time of the examination the rudder trim wheel in the cockpit was in the full-nose-left position. However, due to the impact-related damage to the aft fuselage and subsequent removal of the empennage during wreckage recovery, it was not possible to determine the preimpact position of the rudder trim. TESTS AND RESEARCHAdditional review of the airplane’s engine monitor data revealed evidence of fuel imbalances during the previous three flights preceding the accident flight. During those flights, the recorded fuel level in each tank fluctuated by at least ± 10 gallons during each spray pass. However, unlike the accident flight, decreases in fuel level observed in either wing tank were subsequently followed by increases in fuel level in that same tank, as shown in Figure 3. Such increases were observed in both the left- and right-wing fuel tanks during separate portions of the previous three flights reviewed.

Particularly, when the fuel level in one wing tank decreased, the opposite wing tank exhibited either only a minimal decrease or a net increase in fuel level as the flight progressed. This nonuniform fuel depletion was primarily observed during aerial-application spray passes. During other portions of the flight, likely during transit to and from application areas, the fuel load tended to become more balanced.

Figure 3. Fuel tank levels and fuel imbalance during one of the three flights preceding the accident flight.