On August 20, 1994, at 0210 mountain daylight time, a Beech 99, N14MV, collapsed the left main landing gear after turning off the runway at Helena, Montana. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed. The airplane was substantially damaged and the airline transport pilot and commercial pilot were not injured. The flight had departed from Billings, Montana, on August 20, 1994, at 0115, as a cargo flight conducted under 14 CFR 135. It was reported that after departing runway 27, via the high speed taxi way, the left main landing gear collapsed. The airplane skidded another 52.5 feet before coming to rest on the taxi way. The maintenance inspection and subsequent repairs were to the outboard rear spar, ribs and upper and lower skins on the left wing. Other repairs or replacements were to the left main landing gear assembly, fuselage belly and left flap. During the post crash inspection of the left main landing gear, it was found that the down lock hook, link, fitting and pin were broken. A metallurgical examination of the broken parts revealed fractures typical of an overstress separation. According to the metallurgist's discussions with the manager of the Air Safety group at Beech, the failure was most likely a result of the down lock hook not properly engaging the lock plate on the drag link, either from inadequate rigging or from excessive binding of the fitting. The Beech manager reported that this condition could occur repeatedly and may not be easily detected unless this portion of the gear was inspected directly. The collapse of the gear could then occur under certain loading conditions.