Branch, LA, USA
N5035R
AIR TRACTOR INC AT-602
The pilot was departing in tailwind conditions on an aerial application flight from a 1,800-foot-long turf runway with a full payload of fertilizer. The pilot stated that the engine lost power during initial climb and that he had insufficient time to dump the payload before impacting a pond off the end of the runway. Examination of the engine revealed no anomalies that would have precluded normal operation. According to the airplane manufacturer's calculations, the calculated takeoff distance given the conditions present at the time of the accident exceeded the runway length available; however, the airplane's performance specifications were not available in the POH, which precluded the pilot from conducting performance calculations before takeoff. It is likely that, as the airplane neared the end of the runway, the pilot lifted the airplane off the ground prematurely and attempted to climb at a steep attitude, which exceeded the airplane's critical angle of attack, which resulted in an aerodynamic stall.
On June 4, 2015, at 1500 central daylight time, an Air Tractor Inc. AT-602, N5035R, impacted a crawfish pond during takeoff from the south runway (1,800 feet by 75 feet, grass/turf) at Mid South Flying Service Landing Strip/Fruge Airstrip, Branch, Louisiana. The airplane received substantial damage on impact with the pond. The pilot was uninjured. The airplane was registered to and operated by Richard Flying Service Inc. under 14 Code of Federal Regulations Part 137 as an aerial application flight that was not operating on a flight plan. Visual meteorological conditions prevailed at the time of the accident. The local flight was originating at the time of the accident. The pilot stated that during takeoff roll, the engine power was at maximum and then "lost a significant amount of indicated torque." The engine temperature, the compressor speed went to "redline," and the engine torque decreased 25 percent from full power. After the airplane lifted off the ground, it was unable to climb or sustain level flight. The pilot stated that there was not sufficient time to dump the load of applicant. The airplane left wing stalled first and the airplane impacted a pond at the departure end of the runway. A Federal Aviation Administration Air Safety Inspector from the Baton Rouge Flight Standards District Office stated that upon his arrival at the accident site, he saw that the prevailing winds were from north to south and an American Flag, near the accident site, that was standing straight out from the north wind. The Inspector asked the pilot, who was standing with his father, what he thought happened, and the first thing the pilot said was "The bleed valve failed while taking off!" The Inspector asked the pilot why he thought that was the case. The pilot stated that the engine bleed valve was changed 80 hours prior to the accident flight. The Inspector then asked the pilot why he took off downwind with 3,000 pounds of fertilizer and full fuel. The Inspector stated that at this point the pilot got a little upset, and made the statement that he "took off from the same airstrip that morning and had no problems, this is a PT6A turbine engine, I have plenty of power to make that take-off". The pilot reported that there was a mechanical malfunction/failure of the engine compressor bleed valve or "possibly" the fuel control. The St Landry Parish Airport-Ahart Field (OPL), Opelousas, Louisiana located approximately 15 nautical miles northeast of the accident site, at an elevation of 75 feet mean sea level, recorded at 1455: wind – 360 degrees at 5 knots; temperature – 86 degrees Fahrenheit (F); dew point – 66 degrees F; altimeter setting – 29.98 inches of mercury. The Lafayette Regional Airport/Paul Fournet Field (LFT), Lafayette, Louisiana located approximately 17 nautical miles southeast of the accident site, at an elevation of 42 feet mean sea level, recorded at 1453: wind – 030 degrees at 6 knots; temperature – 88 degrees F; dew point – 63 degrees F; altimeter setting – 29.98 inches of mercury. The pilot reported that the airplane had a maximum gross weight of 12,500 pounds and at the time of the accident the airplane weight was 9,900 pounds. The pilot reported that the fuel on board at time of the accident takeoff was 165 gallons of Jet A. The engine was removed and shipped to Pratt & Whitney Canada where a post-accident examination of the engine was performed under the supervision of a Transportation Safety Board of Canada Senior Investigator. The examination revealed that the engine displayed minimal impact damage, with light impact deformation of the exhaust duct, and water immersion damage. Circumferential contact signatures were displayed by the compressor turbine, first stage power turbine vane ring, first stage power turbine shroud, first stage power turbine, second stage power turbine vane ring and interstage abradable air seal, second stage power turbine shroud, and second stage power turbine due to their making contact under impact loads and external housing deformation. The interstage abradable air seal displayed more severe circumferential 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, and disassembly inspection of the compressor bleed valve, showed no conditions that would have precluded normal operation prior to impact. None of the engine components displayed any indications of any pre-impact anomalies or distress. The bleed valve displayed water immersion damage and no apparent impact damage. The normal lock wire with lead seal to the secondary orifice plug was in place. During installation of the unit for functional testing it was noted that the secondary exit orifice plug showed no appreciable torque. Functional testing showed leakage at the piston damper pins, however the bleed valve closing point was normal. Disassembly and inspection showed the condition of all components to be satisfactory. The fuel control unit showed water immersion damage and no apparent impact damage. Functional testing of the unit was satisfactory with minor variations not affecting normal operation including full fuel flow and normal response to power lever inputs. According to the airplane manufacturer, the AT-602 is a restricted category aircraft and the external equipment configuration changes frequently; there is no takeoff performance data published in the aircraft flight manual. Based upon flight test data, the takeoff distance for an AT-602 with a PT6A-60AG engine and the following parameters: temperature - 87 degrees F (31 degrees C), takeoff weight - 9,900 pounds, and field elevation - 50 feet mean sea level, the expected takeoff for these conditions is about 1,980 feet ground roll and a total distance of 3,300 feet to clear a 50-foot obstacle. This is calculated for an aircraft with a liquid spray system and using a flat, paved runway with no wind and a conservative piloting technique. Depending on piloting technique, it is reasonable to believe that a pilot could take off a little shorter than this. However, with a turf runway and a 5-knot tailwind, this ground roll would be expected to increase. If he had a large fertilizer spreader installed, then the takeoff roll would also be extended due to increased drag.
An exceedance of the airplane's takeoff/climb capability, which resulted in a stall during initial climb at an altitude that was too low for recovery.
Source: NTSB Aviation Accident Database
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