N1539UAir Tractor AT-401 · 1992

On August 5, 2015, about 1330 central daylight time, an Air Tractor Inc. AT-401 airplane, N1539U, impacted terrain during takeoff at an airstrip near Nekoosa, Wisconsin. The commercial pilot was uninjured. The airplane sustained substantial wing and empennage damage. The airplane was registered to and operated by Kafer-Stone Applicator Service Inc. as a 14 Code of Federal Regulations Part 137 aerial application flight. Visual meteorological conditions prevailed about the time of the accident, and the flight was not operating on a flight plan. The local flight was originating at the time of the accident.According to the pilot's accident report, he loaded the airplane with fuel and its hopper load because the primary load man was not around. He loaded the hopper with approximately 360 gallons of aerial application liquid and serviced the airplane to approximately "3/4 of [its] fuel (about 94 gallons)" capacity. He taxied to the beginning of the runway and performed a run-up. The pilot did not notice anything unusual during the run-up.

Following the run-up the pilot positioned the airplane with its tail section just off the end of the asphalt runway in order to use the full length of the runway. He then proceeded to hold the brakes and apply full takeoff power. He released the breaks and began the takeoff roll. The takeoff roll was not noticeably different from any other takeoff with a 360-gallon load. The tail came up at a point down the runway that he had experienced on other takeoffs so nothing seemed out of the ordinary. The pilot, in part, further stated that, "As my takeoff roll continued I did not feel that the airplane was building speed that this plane normally does. As I reached a point where I normally begin to get airborne I attempted to pull the plane off the ground. After doing so I felt that the airplane would not fly so that is when I began to release some of my load by opening the hopper gate using the dump handle. As some of my load was being dumped I started to become airborne. That is when I closed the dump handle believing I would be able to clear the standing corn without releasing any more of my load. However as I approached the corn field and tried to clear it I felt the airplane settle back toward the ground. That is when I attempted again to release my load. By that time it was too late that is when my main gear and spray booms contacted the corn which proceeded to bring me down."

Air Tractor Service Letter (SL) 304 was reviewed. The SL, in part, stated:

The purpose of this Service Letter is to describe the proper procedure for establishing a special purpose operating weight that is higher than the FAA certificated gross weight of the aircraft. ...

When these aircraft are operating in the RESTRICTED category, the operator may use the guidance provided in CAM 8 and FAA Advisory Circular No. 20-33B to operate at weights higher than the certificated gross weight that is listed in the Flight Manual. ...

The operator is allowed to increase the maximum weight of the aircraft to accommodate agricultural missions. ... AT-401 certificated maximum weight 6,000 lbs ... maximum operating weight 7,860 lbs ...

When operating the aircraft at weights above the certificated maximum weight, the operator is responsible for the following:

1. The maximum hopper load is: 3,250 lbs for all AT-401/402 models ...

2. When operating at weights over the certificated maximum weight, the aircraft should never be flown faster than the design Maneuvering Speed, VA, of 140 mph (CAS). ...

3. CAM 8.10-4(b) (2) states that the aircraft "operator is responsible for adjusting the actual operating weight to provide safe margin of performance for the existing flight conditions. Appendix A [of CAM 8] contains information from which the effects of drag, weight, altitude, and temperature on aircraft performance may be estimated." When planning your next load, be sure to take into account density altitude, runway length/conditions, and terrain. Reduce your aircraft weight as necessary to allow for a safe flight.

4. Air Tractor recommends that the operator use caution when operating the aircraft from unfamiliar runways or at elevations higher than 1,300 ft. When operating the aircraft from any new location, it is recommended that the operator begin operations with a lightly loaded aircraft and work up to a higher weight in increments. Although a flight check is not required by CAM 8, it would be prudent for the operator to test the capabilities of the aircraft from the runway to be used. A record of this flight check can be made in the aircraft logbooks, detailing the elevation and length of the runway used.

At 1258, the recorded weather at the Volk Field Airport (VOK), near Camp Douglas, Wisconsin, about 209 degrees and 26 nautical miles from the accident site, was wind variable at 3 knots; visibility 10 statute miles; sky condition scattered clouds at 5,000 feet, broken clouds at 25,000 feet; temperature 27 degrees C; dew point 12 degrees C; altimeter 30.05 inches. The calculated density altitude at VOK was 2,539.3 feet. Weather reports from nearby airports showed the temperature was cooler earlier in the day.

The operator's accident report indicated that the airplane weighed 7,960 lbs at the time of the accident. The operator was asked to provide a copy of the logbook entry in reference to SL 304 and supplied a copy of the most recent weight and balance.

The calculated weight for the flight using the empty weight of 4,425 lbs listed on the airplane's weight and balance, plus the weight of 94 gallons of fuel at 6.5 lbs per gallon (611 lbs), plus the weight of 360 gallons of aerial application liquid at 8.3 lbs per gallon (2,988 lbs), plus the weight of the pilot at his last annual (218 lbs), equaled 8,242 lbs.