The pilot completed two successful takeoffs and landings in the airport traffic pattern and was taking off for a third time when the accident occurred. After the pilot applied takeoff power and the airplane had accelerated forward for less than 75 feet before the pilot’s seat slid back to the aftmost position. The pilot let go of the control yoke to prevent the airplane from pitching upward, but as the airplane continued to accelerate, he was unable to reach the rudder pedals and brakes. The pilot was eventually able to reach the throttle and reduced it to the idle position. The airplane subsequently departed the left side of the runway and came to rest in a ditch, resulting in substantial damage to the horizontal stabilizer and elevator. Postaccident examination of the airplane revealed that the pilot’s seat was not correctly installed and that the front left seat rail guide was not engaged with the rail. This resulted in the seat being position too high and prevented complete engagement of the seat adjustment rod. When the seat was adjusted, its position would lock, but when sufficient aft force was applied to the back of the seat, the adjustment rod would disengage and allow the seat to move. The pilot stated that he previously removed and reinstalled the seat to perform preventative maintenance.