CEN24LA193

The improper maintenance of the fuel control unit, which resulted in air leakage and a loss of control of fuel flow to the engine.

During an agricultural application flight, the pilot noticed a loss of thrust and adjusted engine power as needed. The subsequent application pass was normal; however, while turning the airplane around for another pass, he felt the “tail drop” and noticed that several engine gauges had exceeded limitations. The pilot attempted to reduce engine power but found that the engine was unresponsive to power control inputs and that the airplane was unable to maintain altitude. He chose to land on a nearby gravel road, during which the airplane impacted tree branches and ground looped into a ditch, resulting in substantial damage to both wings. After the airplane came to rest, the engine continued to run until the pilot cut off the fuel supply. Postaccident examination of the engine revealed an improperly torqued and safety wired P3 adapter on the fuel control unit (FCU). This allowed the two attachment screws, used to secure the P3 adapter to the main body of the FCU, to back off, which resulted in excessive air leakage and movement of the gasket between the adapter and the FCU main body. This leakage fluctuated when light finger pressure was applied to the P3 adapter that connected to the FCU main body. The P3 adapter’s seal with the FCU’s main body likely degraded over time because the screws backed off, allowing relative motion between the gasket and sealing surfaces. The intermittent air leakage compromised FCU fuel control capability, likely resulting in the FCU’s fuel flow metering valve to move to its minimum setpoint and the erratic engine behavior observed by the pilot. The date that the accident FCU was installed on the accident engine could not be determined based on maintenance records. The only entry indicating this FCU’s condition was on August 25, 2022, and noted that the FCU was already installed on the engine when received and was in serviceable condition during final inspection. The only maintenance performed on this FCU while installed on the accident airplane was changing the fuel filter.

On May 18, 2024, about 1930 central daylight time, an Air Tractor AT-502 airplane, N15189, was substantially damaged when it was involved in an accident near Covert, Kansas. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 agricultural flight. The pilot stated that, after arriving at the field and performing one application pass, he noticed a loss of thrust and adjusted engine power as needed. The second pass was normal; however, while turning around to perform the third pass, the airplane was “super sluggish.” He felt the “tail drop” and noticed that the engine torque, NG (gas generator speed), and propeller rpm gauges were all “over redline.” The interstage turbine temperature (ITT) gauge indicated high but had not exceeded redline. The pilot stated that the engine was unresponsive to power control inputs, and with he airplane unable to maintain altitude, he chose to land on a nearby gravel road. The pilot stated that he attempted to extend the wing flaps, but the flaps did not extend. During the landing, the airplane impacted tree branches and ground looped into a ditch, which resulted in substantial damage to both wings. The engine continued to run after the airplane came to rest until the pilot shut off the fuel supply. Postaccident examination of the airframe revealed no anomalies with the flap system. The system was functionally tested and operated as designed. The throttle, condition, and propeller lever flex cables were examined for the full length with no anomalies found; however, the start control and propeller levers were observed to be stiff, with the start control stuck in place. All three cables were tested for slide and breakaway force using a force gauge. The testing revealed that the start control and propeller control flex cables were found to be exceeding the design maximum force limit. A label was found on the propeller lever flex cable that showed a manufacture date of June 29, 2017. Examination of the engine revealed an improperly torqued and safety wired P3 adapter on the fuel control unit (FCU). The FCU (with flexible P3 air supply line) was removed from the engine and tested for external air leakage in accordance with manufacturer test specifications, which stated that only “slow bubbling” was allowed from specific FCU locations, including the joint between the drive body and P3 air adapter. The testing was performed twice and found to be 15 and 36 inches of mercury, both outside of limits. The FCU was subsequently removed from the test stand and disassembled. The P3 adapter attached to the FCU’s main body was loose and appeared to move with light finger pressure. Two screws securing the P3 adapter to the FCU’s main body were safety wired; however, the wire on both screws appeared to be in a neutral configuration. A digital screwdriver registered zero torque to unscrew one screw, while the other was removed by hand without the screwdriver. Both screws and their threaded holes appeared in good condition. Contact surfaces on the P3 adapter, aluminum gasket, and FCU main body appeared fretted. A light scratch was found between the Py restrictor and aft screw hole on the FCU main body. The P3 air adapter displayed small, metallic particles inside the Px air cavity (see figure 1). Figure 1. Fretted aluminum gasket (left) and FCU main body (right) The flexible P3 supply line appeared undamaged and clear of internal obstructions. The P3 line connects to a pressure port fitting on the P3 adapter and has a conical metering orifice to convert incoming P3 air into Px air. This orifice exhibited a concentric impression around the cone portion of the fitting. Multiple engine logbook entries had incorrect or missing engine operational hours and/or cycles. The date that the accident FCU was installed on the accident engine could not be determined based on maintenance records; however, according to the engine and airplane logbooks, the accident airplane flew with the accident FCU for at least 448.2 hours and 167 cycles with no reported engine anomalies. The only entry indicating this FCU’s condition was for maintenance completed on August 25, 2022. The entry noted that the FCU was already installed on the engine when received and was in serviceable condition during final inspection. The only maintenance performed on this FCU while installed on the accident airplane was changing the fuel filter.