On October 20, 1997, about 1000 Alaska daylight time, a Cessna 208B airplane, N9304F, had a left aileron jam during the landing approach to the Kasigluk Airport, Kasigluk, Alaska. The airplane was being operated as a visual flight rules (VFR) nonscheduled cargo flight under Title 14 CFR Part 135 when the incident occurred. The airplane, operated by Peninsula Airways Inc., Anchorage, Alaska, was not damaged. The certificated airline transport pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed. VFR company flight following procedures were in effect. The flight originated at the Bethel Airport, Bethel, Alaska, at 0930. The pilot reported he was maneuvering to land on runway 35 at the Kasigluk Airport, and began a left turn to enter the downwind portion of the landing approach. During the turn, the pilot utilized full aileron deflection, and noticed the aileron momentarily jammed. He was able to force the control wheel to the right, unjamming the aileron. The pilot continued the landing approach, and landed without encountering further control problems. The pilot indicated that following the landing, he began to taxi to the ramp. Due to a strong crosswind, the pilot deflected the ailerons fully to the left. The aileron once again jammed, and the pilot was unable to force the control wheel to the right. The operator reported the incident flight was the first flight of the day for the airplane. The airplane ailerons utilize balance weights that are attached to the interior surface of the leading edge of the aileron at the inboard, and outboard ends of the aileron. Five screws are inserted through the leading edge skin of the aileron, through the balance weight, and into self-locking nuts attached to a nut strip assembly. Company maintenance personnel conducted an inspection of the aileron, and reported 1 screw was missing. Three of the remaining 4 screws were loose, and backed out sufficient distance to contact the trailing edge of the wing, forward of the aileron. The left aileron was jammed in a full, leading edge down, position. The operator's maintenance personnel replaced the screws, and did not report any obvious failure or damage to the attaching screws. The operator found slightly loose balance weight screws in other Cessna 208B airplanes. In each case, the self-locking nutplates still had proper grip. The operator indicated their maintenance department personnel have since applied loctite to each screw. The Cessna 208B information manual provides a description of a preflight inspection that includes an examination of the ailerons as part of a walk-around inspection of the airplane. In addition, the balance weights should be inspected during every 100 hour inspection. A review of the manufacturer's service condition report data base, and the Federal Aviation Administration malfunction and defect report system did not reveal any other reported incidents of aileron jamming.