Accident Summary
On July 18, 2017, at approximately 1710 central daylight time, an Air Tractor AT-802A airplane, registration N816FF, was substantially damaged during a forced landing near Reyno, Arkansas. The commercial pilot was not injured. The flight was conducted as a Title 14 Code of Federal Regulations Part 135 aerial-application operation.
Pilot Report
The pilot reported that after an uneventful spray flight, while maneuvering to land on a private airstrip, he observed an increase in engine torque shortly after reducing power. The torque rose to about 6,000 ft-lbs and propeller speed dropped to approximately 710 rpm. The pilot stated he pushed the propeller lever fully forward, but insufficient thrust remained to maintain altitude, necessitating a forced landing in an open soybean field. During the landing, the left wing struck the ground, causing the airplane to skid sideways. The right wingtip, right aileron, and right elevator were substantially damaged.
Postaccident Examination
Examination at the accident site confirmed flight control continuity. The power lever was at idle, the propeller lever was in the feather position, and the start control (condition lever) was in cut-off. Engine control continuity was verified from the cockpit levers to their respective engine components, and the controls moved smoothly through their full range. The bleed air reference line between the fuel control unit (FCU) and the constant speed unit (CSU) was properly installed and secured. A fuel sample from the header tank showed no contamination and was consistent with Jet-A fuel. Both wing tanks contained fuel; the engine monitor indicated 55.5 gallons in the left tank and 142 gallons in the right. Airframe fuel lines were intact with no leaks. The FCU filter housing contained fuel. The propeller remained attached to the engine and appeared undamaged. The propeller beta system was properly rigged.
Engine and Component Examination
The engine was removed and sent to the manufacturer for teardown inspection, which revealed no evidence of mechanical malfunction or failure that would have prevented normal operation. Clearances between the propeller shaft and sleeve met design limits, and dimensions between the shaft and internal oil transfer tube/oil jet were within specifications. Bench testing of the engine-driven fuel pump, flow divider, FCU, compressor bleed valve, CSU (propeller governor), and overspeed governor found no anomalies that would have precluded normal engine operation.
Engine Monitor Data Analysis
Analysis of the engine monitor data (see figure 1) showed that toward the flight's end, between 1708:44 and 1709:19, propeller speed (Np) decreased at three distinct rates, consistent with the cockpit propeller lever being moved aft at three different rates. This aft movement increased propeller pitch, reducing Np. As Np decreased, engine torque correspondingly increased. From 1708:45 to 1708:58, Np fell from 92% to 84%, with a 10% thrust decrease. From 1708:58 to 1709:14, Np continued decreasing from 84% to 65%, with another 10% thrust decrease.
At 1709:14, Np reached 62% (1,060 rpm), at which point the CSU feather valve dumped oil pressure from the propeller, causing it to feather. Postaccident bench testing confirmed the feather valve would dump oil pressure at 1,060 rpm. With the propeller feathered, engine torque rapidly exceeded the 5,100 ft-lbs limitation. At 1709:25, fuel flow and torque increased sharply, consistent with the pilot advancing the power lever fully forward while the propeller remained feathered. At 1709:28, torque peaked at 6,920 ft-lbs. Gas generator speed (Ng) and fuel flow remained relatively constant, with torque above 6,600 ft-lbs until the engine was shut down after the accident.