Air Tractor Inc AT-502 N15189 accident near Covert, Kansas

Probable cause

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

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for CEN24LA193 for the full record.