SEA99LA074

After approximately 900 feet of takeoff roll, the Grumman G-164A 'AgCat' rolled off the left side of the 15 foot wide, soft, sandy airstrip/road and came to rest rotating approximately 200 degrees counter-clockwise from the takeoff heading. There was no evidence of any mechanical malfunction with the aircraft and the left wing spray boom retention arm aft brackets had separated in overload as did the right main landing gear strut. High wheat flanked either side of the landing strip. The operator reported that '[the] left wing and spray boom entered wheat along side of [the] airstrip pulling [the] aircraft off [the] strip and into [the] wheat field. [The] aircraft was unable to recover directional control with rudder.'

On May 27, 1999, approximately 0800 Pacific daylight time, a Grumman-Schweizer G-164A "AgCat," N9970, recently purchased and being operated by Blue Mountain Aviation, and being flown by a commercial pilot, was substantially damaged during a loss of control on takeoff roll at a dirt airstrip approximately eight nautical miles west of Prescott, Washington. The pilot was uninjured. Visual meteorological conditions existed and no flight plan had been filed. The flight, which was departing to dispense roundup and 24D, was to have been operated under 14CFR137 as an aerial application flight.

The operator reported that "on [the] 3rd takeoff of [the] day, as the tail was up and [the] aircraft nearly ready to fly, [the] left wing and spray boom entered wheat along side of [the] airstrip pulling [the] aircraft off [the] strip and into [the] wheat field. [The] aircraft was unable to recover directional control with rudder." (refer to attached NTSB Form 6120.1/2)

An inspector from the Federal Aviation Administration's (FAA) Spokane Flight Standards District Office interviewed the pilot telephonically on two occasions following the accident. On the first occasion (the date of the accident) the pilot reported that "as the aircraft was about to get airborne he heard something break on the right side of the aircraft just before he lost directional control." And, during the second interview (the day after the accident) the pilot reported that "he believed that he heard something break on the left side of the aircraft and thought that the left wing spray nozzle boom had partially separated causing him to loose [sic] directional control."

The airstrip was described in a telephone conversation by the FAA inspector to the investigator-in-charge as a soft, sandy, road flanked on either side by tall wheat. Additionally, the inspector traveled to the accident site on the day of the accident. He reported that the strip being used by the aircraft had an approximate usable width of 15 feet and an approximate usable length of 2,200 feet. He reported that the aircraft departed the left side of the road approximately 300 yards down the usable portion of the strip, and came to rest in a wheat field having rotated about 200 degrees counter-clockwise (refer to photograph 1). The right main landing gear strut had separated and was found beneath the fuselage. The right spray boom longitudinal retaining arms were observed to be still attached to the underside of the right wing assembly. The left spray boom longitudinal retaining arms were observed to be separated at their aft attach points on the underside of the left wing assembly (refer to photograph 2). The spray boom, which is attached at the aft ends of the longitudinal retaining arms, extends aft approximately in line with the wing's trailing edge and beneath the lower wing surface. An examination of the fracture surfaces of these brackets revealed granular, shiny conditions with no evidence of corrosion or pre-existing cracking.

The inspector reported finding no evidence of pre-accident component failure related to the aircraft. He also reported that the left landing gear and brake were examined for evidence of brake drag or binding. He reported that "a small amount [of[ brake drag was noticed between the brake pads and disc, however, the pilot did not state that the aircraft was difficult to keep on [the] runway center as this would be indicative of excessive brake drag." (refer to attachment IN-I).