On March 15, 1997, about 1957 Alaska Standard Time, a wheel equipped Piper PA-31-350 airplane, N3542H, received minor damage during a gear up landing at the Ralph Wien Memorial Airport, Kotzebue, Alaska. The airline transport certificated pilot and two passengers aboard reported no injuries. The 14 CFR Part 135 scheduled commuter flight was operated by Cape Smythe Air Service as Flight 270. The airplane last departed Kivalina, Alaska, about 1822, and operated in visual meteorological conditions. A VFR flight plan was filed. According to company personnel, the pilot reported he encountered turbulence while en route from Kivalina. Shortly after the turbulence encounter, the pilot said the main landing gear unsafe light illuminated. He recycled the gear (lowered and retracted), but the light remained illuminated. A subsequent low pass at Kotzebue allowed confirmation from personnel on the ground that the left main landing gear was not completely retracted, and that it would not move from the wheel well when the landing gear handle was selected down. The nose gear and the right main landing gear would extend properly to the down position. The pilot elected to make a gear up landing on the snow and ice covered runway. Postlanding inspection of the left main landing gear disclosed a broken, inboard main gear door hinge. According to company maintenance personnel, the broken hinge occurred prior to the landing, and kept the left main gear from extending. The left, inboard main landing gear door hinge, Piper ID number 46652-2, was sent to the NTSB metallurgical laboratory in Washington, D.C., for inspection. The metallurgist who inspected the hinge noted the hinge material met the manufacturing specifications for composition, hardness, and dimensions. He discovered a fatigue crack which emanated from multiple origins at the tip of a forging flash adjacent to the inside radius of the hinge (see attached metallurgical report). The metallurgical report says, in part: "Most of the fatigue crack region exhibited mud crack features and scale which is characteristic of a severely oxidized surface of aluminum alloy." Piper Service Bulletin (SB) 982, dated July 24, 1980, and Airworthiness Directive (AD)80-25-05, addresses cracking of the main landing gear door hinge. The SB notes three types of hinge assemblies are made for the Piper PA-31 airplane. One type is the hinge mentioned above. The other two hinges are made from 0.25 inch thick aluminum plates or steel. The SB requires inspections of the curved portion of the hinge for cracking at the next 100 hours of operation, or the next scheduled inspection, whichever occurs first, and every 100 hours thereafter, unless/until an acceptable replacement part is installed. No inspection is required for a steel door hinge assembly. A door hinge that has a crack, is to be replaced with the newer, thicker, version (0.44 inch) door hinge, Piper Part number 46652-2. The incident airplane had been retrofitted with the new, thicker, version of the door hinge. This hinge failed in the same curved portion as was noted in the older, thinner version of the hinges, and which was the subject of the above SB/AD. Maintenance information supplied by the Operator disclosed the new type landing gear door hinge was installed on the incident airplane in 1988. The airplane had operated approximately 9,938 hours with the new hinge until the hinge failed. A review of Service Difficulty Reports from 1980 through February 1998, disclosed eight instances of cracking or failures of the new, thicker, hinge, which had been installed to resolve cracking problems with the old version of the hinge.