Accident Details
On February 5, 2015, at approximately 1510 central standard time, an Air Tractor AT-602, registration N602PB, collided with terrain shortly after departing Slaton Municipal Airport (F49), Slaton, Texas. The commercial-rated pilot was uninjured, and the airplane sustained substantial damage. The aircraft was registered to Neal Aircraft, Inc., Slaton, Texas, and operated by Frontier Ag Inc., Oakley, Kansas, under 14 CFR Part 91 as a cross-country flight. Visual meteorological conditions prevailed.
Pilot Report
The pilot reported that before departure, the airplane was filled with fuel. After engine start, he taxied to runway 18 and conducted an engine run-up with no anomalies. The initial takeoff appeared normal; however, about halfway down the runway, he noticed a "lack of airspeed" and the engine monitor displayed "several red lights." The airplane became airborne around that time. The pilot climbed to about 150 feet before noticing additional power loss. Despite advancing the throttle to full power, the airplane lost altitude. The pilot landed in a field south of the airport; the landing gear dug into soft dirt, causing the airplane to nose over and come to rest inverted. The pilot stated the engine never developed full power.
Post-Accident Examination
An FAA inspector examined the airplane and found substantial damage to the vertical stabilizer and wings, and the engine was fractured at the propeller reduction gearbox. The airplane was equipped with an AmSafe airbag system, but the airbags did not deploy. The airplane was recovered for further examination. The engine, a PT6A-65AG with 273 hours, was sent to Pratt & Whitney's Service Investigation Center. The engine pressure line (Py line) was retained for examination by Air Tractor. The Electronics International MVP-50 engine monitor, recently installed, was downloaded for review.
Engine and Component Examination
On April 1, 2015, Pratt & Whitney and Air Tractor, under FAA supervision, examined the engine. The gas generator module was deformed with contact signatures from internal components. Compressor and power turbine rotational signatures indicated rotation at impact, but the level of engine power could not be determined. Bench testing of the fuel control unit, fuel pump, and flow divider valve revealed no defects. The propeller governor also showed no anomalies.
The flexible air signal line (Py line), which provides reference pressure from the propeller governor to the Fuel Control Unit (FCU), was installed by Air Tractor under Supplemental Type Certificate (STC) #SE8652SW as a replacement for a rigid line. On September 9, 2015, Air Tractor, under NTSB supervision, pressurized the line to approximately 125 psi and submerged it in water; no leaks were detected. The engine examination found no pre-impact anomalies that would have precluded normal operation.
Engine Monitor Review
The MVP-50T engine monitor was installed on January 30, 2015, under STC # SA02135SE. The log entry noted ground runs and leak checks with no defects, but no completed setup or installation checklists were found in the records. The accident flight was the first flight after installation. The monitor's clock was set one hour earlier than local time, and the fuel flow calibration factor was off by a factor of 10, causing the monitor to indicate fuel flow ten times the actual value.
Data from the monitor showed 32 files from January 19 to February 5, 2015. Only two files contained engine starts: February 2, 2015, and the accident flight. The February 2 file logged a maximum propeller rpm of 950. For the accident flight, propeller rpm increased at 14:10:27 (monitor time) consistent with takeoff roll, then went to zero at 14:11:41, indicating a flight duration of about 1 minute 14 seconds. During the takeoff run, propeller rpm reached 1,730 rpm at an engine torque of 1,304 ft-lbs. About 21 seconds after power application, torque peaked at 1,492 ft-lbs—the maximum for the flight. Torque then dropped to about 1,200 ft-lbs for about 20 seconds before further reduction. Propeller rpm stayed at 1,730 rpm for about 32 seconds after maximum torque, then dropped below 1,700 rpm with torque at 736 ft-lbs. Torque then decreased into the 500 ft-lbs range, with a slight increase about 10 seconds before the crash followed by a decrease. The engine's maximum torque was 3,245 ft-lbs, with a propeller maximum limit of 1,700 rpm. The engine monitor's operating range for propeller rpm was correctly set at 1,700 rpm; readings above this would trigger a red warning light.
Pilot Information
The pilot held a commercial pilot certificate with single-engine land and instrument-airplane ratings. He reported 561 total flight hours, including 3 hours in the accident airplane make and model.