Accident Overview
On July 22, 2011, at about 1400 central daylight time, an Air Tractor Inc AT802/A, registration N396LA, encountered engine surging and a subsequent loss of engine power after completing an aerial application of a field near Ullin, Illinois. The pilot executed a forced landing to a field. The certificated commercial pilot received minor injuries, and the airplane sustained substantial damage. The aircraft was operated by Thompson Flying Service LLC under 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed. The flight had originated from Flora Municipal Airport in Flora, Illinois.
Pilot's Account
According to the pilot, while returning from an aerial application near Flora, Illinois, the engine began to surge and felt like a “clutch was going out in a truck.” Power would fade and then return. No “popping” or “unusual” noises were heard. The torque and propeller speed gauge indications remained steady. Engine gauge readings showed: torque pressure at 30 psi, propeller speed between 1,400 and 1,500 rpm, gas generator speed at 95 percent, and inter-turbine temperature between 740 and 750 degrees Celsius. Normal cruise airspeed was about 150 mph. Oil temperature was hotter than normal but within limits.
The first power loss occurred at about 1,500 feet above ground level, with terrain elevation around 340 feet mean sea level. The pilot attempted to climb three times, but each effort resulted in altitude loss. He activated the auxiliary fuel pump and selected continuous ignition as per emergency procedures, but these actions had no effect on the engine surge or power. The propeller was rotating and was feathered before landing in a field with approximately 4 inches of standing water.
Engine and Maintenance History
The airplane was powered by a Pratt & Whitney PT6A-65AG engine, serial number 32254. The last maintenance logbook entry, dated July 13, 2011, recorded: time since new 5,020.8 hours; cycles since new 5,020; time since overhaul 2,527.7 hours; cycles since overhaul 2,527. That entry also noted the gas generator was partially disassembled for a suspected bearing failure, with overhauled number 1 and 2 bearings installed. Overhauled accessory gear box bearings were installed, and the accessory gears, fuel pump, pressure pump gear, and associated hardware were demagnetized. The gas generator section was reassembled with one serviceable thermocouple.
Post-Accident Examination
The engine was removed and shipped to Pratt & Whitney Canada for disassembly examination under the supervision of the Transportation Safety Board of Canada. The examination found no anomalies associated with the reported power surge. However, a fracture of the reduction gearbox front case at the A-flange and a fracture of the second stage carrier housing at the web were noted, characteristic of engine power at the time of impact.
The Woodward overspeed governor, part number 210960 B, serial number 16517050, was tested and disassembled at Woodward Governor Company under the supervision of the National Transportation Safety Board. Visual inspection revealed non-Woodward safety wire and torque seal on the governor, as well as external contamination consistent with sand or dirt. Discoloration consistent with corrosion/rust was present on the drive gear. Testing indicated that minimum and maximum speed settings were approximately 0.5 percent high at both points. Disassembly showed internal corrosion/rust and a sticking reset piston, attributed to the internal corrosion/rust.