Air Tractor Inc AT 401 N1539U accident near Nekoosa, Wisconsin

Probable cause

The pilot’s inadequate preflight planning, which resulted in his attempt to take off for the agricultural application flight with the airplane over its maximum gross weight in high-density altitude conditions and led to the airplane’s degraded climb performance and subsequent impact with terrain.

NTSB analysis

The commercial pilot loaded the airplane with fuel and a chemical load for the agricultural application flight. During takeoff, the pilot started at the arrival end of the runway, held the brakes, and applied full takeoff power. The pilot reported that the takeoff roll was not noticeably different from any other takeoff he had conducted with a similar load and that the airplane's tail lifted at the usual point on the runway. However, the pilot stated that, as he continued the takeoff roll, the airplane was not "building speed" as it normally did. He attempted to get the airplane airborne but then "he felt that the airplane would not fly," so he dumped some of the chemical load, and the airplane subsequently became airborne. He closed the dump handle because he thought the airplane could clear the corn near the end of the runway; however, the airplane settled back toward the ground. As the pilot attempted to release more of the chemical load, the landing gear and spray booms contacted the corn, and the airplane subsequently impacted terrain, which caused substantial damage.

The airplane manufacturer published a service letter (SL) that contained a procedure allowing the operator to operate the airplane higher than its certificated gross weight for agricultural operations; however, the airplane was calculated to be almost 400 lbs over the maximum gross weight listed in the SL at the time of the attempted takeoff. The SL also stated that, when the airplane is above the certificated gross weight, the operator should use caution when operating the airplane at elevations higher than 1,300 ft; the airport had a calculated density of 2,539 ft. The operation of the airplane in high-density altitude conditions over its maximum gross weight would have degraded the airplane's climb performance and resulted it its inability to remain airborne. The temperature during the accident flight was warmer than earlier in the day. The pilot was not used to flying the airplane in higher temperatures and probably did not account for the high-density altitude at the airport before the flight.

Text from the NTSB. Open the NTSB docket for CEN15LA345 for the full record.