On July 17, 1998, about 2100 Alaska daylight time, a Piper PA-28-140 airplane, N1793J, was destroyed when it collided with trees during takeoff from an off airport landing site approximately 72 miles east-southeast of Bethel, Alaska, at position 60 degrees 01 minutes North latitude, 160 degrees 11 minutes West longitude. The private pilot and the three occupants sustained minor injuries. The flight was conducted under 14 CFR Part 91 as a personal flight. Visual meteorological conditions prevailed at the time of the accident, and a flight plan was filed with a relative. The flight was departing the landing site on a return leg to Bethel. The pilot told the NTSB investigator on July 19, 1998, that during his landing at the site, the mechanical flap handle jammed in the flaps extended position, and could not be retracted. After landing, the pilot was able to retract the flaps and could not find a discrepancy. During the subsequent takeoff, while climbing through 25 feet agl, the flap handle fell to the retracted position, and the airplane began to descend. The pilot said that he again extended the flaps, but they would not lock in place. The airplane did not have room remaining to land, and collided with trees located in the takeoff path. The four occupants escaped from the airplane, which was then consumed by a postcrash fire. Subsequent inspection of the flap handle revealed the flap locking pin, part number 480-715 (59-040-187-1500), was not located in the flap handle assembly. Loss of this locking pin will result in air loads driving the flaps to the retracted position. The retaining roll pin, part number 480-733, was visible in the hole where the locking pin should have been. This roll pin installs through a drilled pilot hole in the locking pin. In order to remove the locking pin, either the roll pin must be removed, or the locking pin must fracture into two pieces. The airplane was cut into sections during recovery, and the missing locking pin was not found. Examination of photographs taken before recovery show what appears to be a metal pin of approximately the same dimensions as the missing locking pin lying beneath the flap handle assembly. Review of SDR records show three other cases of the locking pin fracturing and falling free of the flap handle. Early model PA-28 series airplanes used a 3/16 inch diameter pin; later model PA-28 series airplanes used a 1/4 inch pin. The accident airplane was manufactured with the smaller pin. No Service Bulletins or Airworthiness Directives apply to the locking pin, part number 480-715.