Accident Overview
On June 4, 2015, at 1500 central daylight time, an Air Tractor Inc. AT-602, registration N5035R, impacted a crawfish pond during takeoff from the south runway at Mid South Flying Service Landing Strip/Fruge Airstrip in Branch, Louisiana. The runway measured 1,800 feet by 75 feet and was composed of grass/turf. The airplane sustained substantial damage upon impact, but the pilot was uninjured. The aircraft was registered to and operated by Richard Flying Service Inc. under 14 Code of Federal Regulations Part 137 as an aerial application flight, and it was not operating on a flight plan. Visual meteorological conditions prevailed at the time of the accident, which occurred during the originating local flight.
Pilot Account
The pilot stated that during the takeoff roll, engine power was at maximum when he observed a significant loss of indicated torque. Engine temperature and compressor speed reached redline, and torque decreased by 25 percent from full power. After lifting off, the airplane was unable to climb or sustain level flight. The pilot reported that there was insufficient time to dump the load of applicant. The airplane then experienced a left wing stall and impacted a pond at the departure end of the runway.
Investigation Findings
A Federal Aviation Administration Air Safety Inspector from the Baton Rouge Flight Standards District Office arrived at the accident site and noted that prevailing winds were from north to south. He observed an American Flag standing straight out from the north wind. The inspector asked the pilot what he thought happened; the pilot responded that the bleed valve failed during takeoff. When asked why, the pilot explained that the engine bleed valve had been replaced 80 hours prior to the accident flight. The inspector then inquired why the pilot took off downwind with 3,000 pounds of fertilizer and full fuel. The pilot became upset and stated that he had taken off from the same airstrip that morning without issues, adding, "This is a PT6A turbine engine, I have plenty of power to make that take-off."
The pilot later reported that there was a mechanical malfunction or failure of the engine compressor bleed valve or possibly the fuel control.
Weather data from nearby airports indicated: at St Landry Parish Airport (OPL), 15 nautical miles northeast, wind 360 degrees at 5 knots, temperature 86°F, dew point 66°F, altimeter 29.98 inHg; at Lafayette Regional Airport (LFT), 17 nautical miles southeast, wind 030 degrees at 6 knots, temperature 88°F, dew point 63°F, altimeter 29.98 inHg.
The airplane had a maximum gross weight of 12,500 pounds; at the time of the accident, it weighed 9,900 pounds with 165 gallons of Jet A fuel on board.
Engine Examination
The engine was removed and examined by Pratt & Whitney Canada under the supervision of a Transportation Safety Board of Canada Senior Investigator. The examination revealed minimal impact damage, including light deformation of the exhaust duct and water immersion damage. Circumferential contact signatures were found on the compressor turbine, first and second stage power turbine components, and the interstage abradable air seal. The interstage abradable air seal exhibited more severe rubbing and scoring with frictional heat discoloration and material smearing due to contact with the air seal rotor.
Post-accident functional testing of the high-pressure fuel pump, fuel control unit, compressor bleed valve, fuel heater, and fuel nozzles, along with disassembly inspection of the bleed valve, showed no conditions that would have precluded normal operation prior to impact. None of the engine components displayed any pre-impact anomalies or distress. The bleed valve had water immersion damage but no apparent impact damage; its lock wire and lead seal were in place, though the secondary exit orifice plug showed no appreciable torque. Functional testing revealed leakage at the piston damper pins, but the bleed valve closing point was normal. The fuel control unit also showed water immersion damage, but its functional testing was satisfactory with minor variations not affecting normal operation.
Performance Considerations
According to the airplane manufacturer, the AT-602 is a restricted category aircraft with frequently changing external equipment configurations, and no takeoff performance data is published in the aircraft flight manual. Based on flight test data for an AT-602 with a PT6A-60AG engine under conditions of 87°F, a takeoff weight of 9,900 pounds, and a field elevation of 50 feet mean sea level, the expected takeoff ground roll is about 1,980 feet, with a total distance of 3,300 feet to clear a 50-foot obstacle. This calculation assumes a liquid spray system on a flat, paved runway with no wind and conservative piloting technique. The manufacturer noted that a pilot could potentially achieve a shorter takeoff, but a turf runway and a 5-knot tailwind would increase the ground roll. Additionally, if a large fertilizer spreader was installed, takeoff roll would be extended due to increased drag.