CEN24LA272

The pilot’s failure to maintain coordinated flight during aerial-application operations, which resulted in fuel migration between the interconnected wing fuel tanks, and his failure to adequately monitor fuel levels during the flight which resulted in a loss of engine power due to fuel starvation when the right-wing tank became depleted of usable fuel.

The pilot was conducting an agricultural flight when there was a sudden loss of engine power during an aerial-application spray turn. The airplane’s aft fuselage was substantially damaged during the subsequent forced landing in a cornfield. According to engine monitor data, as the aerial-application flight progressed, there was an overall decrease in fuel in the right-wing fuel tank and a minimal decrease of fuel in the left-wing fuel tank. The airplane’s wing fuel tanks are interconnected through a header tank and cannot be individually selected. As the flight progressed, the fuel imbalance increased until the right-wing tank was depleted of usable fuel and there was a sudden loss of engine power due to fuel starvation. The left-wing tank contained nearly 80 gallons of fuel at the time of the accident. Examination of the airplane’s fuel system did not reveal any preimpact anomalies that would have precluded normal distribution of fuel from the fuel tanks to the engine. According to the airplane flight manual, fuel can migrate from one tank to the other if the airplane is flown uncoordinated or if all turns are made in the same direction. A postaccident examination confirmed that the airplane was equipped with a required cockpit placard that instructed pilots avoid skidding turns, which may result in fuel migration from one tank to another, that the engine may lose power when either tank becomes empty, and to monitor the fuel level in each tank frequently when the fuel level is less than ½ tank. During the final 12 minutes of the flight, the engine monitor’s master caution parameter activated multiple times when the fuel level in the right-wing tank decreased below 11 gallons. The master warning parameter can be activated by several factors; however, at that time the recorded fuel level in the right-wing tank (less than 11 gallons) was the only parameter that would have activated the master caution. Additionally, the airplane’s low fuel warning system functioned normally when tested after the accident. As such, it is likely the pilot failed to notice or acknowledge the low fuel warnings that were presented in the cockpit during the flight. Based on the available information, the pilot likely did not maintain coordinated flight during the aerial-application flight, which resulted in fuel migrating from the right-wing tank to the left-wing tank. Additionally, the pilot did not adequately monitor fuel levels during the flight which resulted in fuel starvation to the engine when the right-wing tank was depleted of usable fuel.

The 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. On 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. Examination 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 t