GRYGLA, MN, USA
N8673S
AIR TRACTOR AT-301
THE PILOT HAD COMPLETED THREE SPRAY RUNS OVER A FIELD OF SUNFLOWERS. WHILE FLARING IN PREPARATION FOR THE FOURTH SPRAY RUN THE AIRPLANE DESCENDED INTO THE SUNFLOWERS AND COLLIDED WITH THE GROUND. ON-SCENE INVESTIGATION REVEALED NO MECHANICAL ANOMALIES WITH THE AIRFRAME OR ENGINE THAT WOULD PREVENT NORMAL FLIGHT. DURING DISCUSSIONS WITH THE PILOT IT WAS REVEALED THAT THE SUNFLOWER FIELD WAS WET, AND THAT IT WAS SURROUNDED BY PASTURES AND A HAYFIELD THAT HAD RECENTLY BEEN CUT. THE OUTSIDE AIR TEMPERATURE WAS 90 DEG F. THE APPROACH TO THE FIELD WAS BORDERED BY A LINE OF 30-FT HIGH TREES.
On July 12, 1995, at 1815 central daylight time (cdt), an Airtractor AT-301, N8673S, operated by the Holte Flying Service, Incorporated, of Grygla, Minnesota, and piloted by a commercial pilot, was substantially damaged when it collided with the ground at the start of its aerial application spray run. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 137 flight was not operating on a flight plan. The pilot reported no injuries. The flight departed Grygla, Minnesota, at 1800 cdt. According to the pilot's written statement on NTSB Form 6120.1/2, the airplane was descending to make a fourth spray pass. The pilot said he was,"...beginning to flare the aircraft [and it] settled into the wet field... ." The statement continues, "...as the aircraft came over the wet field I experienced a sense of partial loss of power." During a telephone interview the pilot said the airplane seemed as though it did not want to fly when it got near the ground. He said the airplane engine was producing power. According to the pilot, the engine was not missing. He stated there may have been a power interruption, but he was not sure. The pilot said he sets the airplane's power to a particular setting after takeoff and leaves it there until the airplane gets light. On the accident date he did the same thing, the power was set and remained so throughout his spray runs. The field being sprayed was planted with sunflowers that were between 2 and 3 feet high. The pilot said the field was bordered on the west, north and east borders by a line of 30 foot high trees. The spray run when the accident occurred was being made from east to west. The airplane made its approach to the field over the 30 foot high trees on its east boundary. The fields surrounding the sunflower field were pastures and mowed hay fields. The pilot said there was a light wind crossing the sunflower field from the northwest. He said the speed was about 5 miles per hour. During a subsequent interview the pilot said he was not sure what his airspeed was as he passed over the trees upon conclusion of the third spray run. He said he usually flies over the field at 130 knots indicated airspeed. At the top of the reversing turn he said he wasn't certain what the airplane's airspeed was. According to the pilot, the maximum altitude he attained during the climbing reversal turn was about 300 to 400 feet above the ground. He stated he noticed no loss of power during the descent toward the fourth spray run. The pilot said as he started to flare the airplane for the next spray run the airplane settled into the wet field. The on-site examination of the airframe and engine revealed no mechanical anomalies that would prevent normal flight. The magnetos were examined and were capable of producing spark. The left magneto's "P" lead had a broken tension spring. Its "P" lead ground spring inside the magneto was clean and free of electrical arcing residue. Evidence of the magneto's "P" lead grounding out in its mount was not observed. The examination of the magneto switch revealed no anomalies that would prevent it from correctly operating. According to the FAA publication Aviation Weather for Pilots and Flight Operations Personnel, "Wet soil....[and] thick vegetation .... insulates against heat transfer between the ground and the atmosphere." The text continues, "....stubble fields.... heat most rapidly." The book's chapter on turbulence states, in part, "...low level convective turbulence.... on approach can cause abrupt changes in airspeed and may even result in a stall... ."
THE PILOT'S FAILURE TO MAINTAIN ALTITUDE AND/OR CLEARANCE AT THE BEGINNING OF THE SPRAY RUN. A FACTOR WAS TURBULENCE DUE TO THE DIFFERENT AIR TEMPERATURES OF ADJOINING TERRAIN.
Source: NTSB Aviation Accident Database
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