On November 27, 1999, about 0134 mountain standard time, a Dassault Falcon DA-20 twin turbo fan jet, N216SA, registered to RBS Aviation Group, Incorporated, and operated by Smith Air, Incorporated, as a 14CFR135 non-scheduled domestic cargo flight, was substantially damaged while landing at Boise Air Terminal, Boise, Idaho. Visual meteorological conditions prevailed and an instrument flight plan was filed. The captain, an airline transport pilot, and commercially-rated first officer were uninjured. The flight originated from Eppley Field, Omaha, Nebraska, approximately two hours and 45 minutes prior to the accident. In a written statement prepared by both the captain and first officer, the pilots reported that while on final approach to runway 10L, the captain instructed the first officer to lower the landing gear. After the gear handle was placed in the down position, the pilots noted that the down-and-locked indication (green light) for the left main gear was not illuminated. The pilots completed the abnormal gear extension procedures (A through C) as outlined by the checklist, (see attached Landing Gear checklist) but were unable to extend the left main gear. After confirming that the left main gear was not extended, the pilot was cleared to land on runway 10L. During the landing roll-out, the aircraft skidded off the runway, and came to rest on the north side of the runway. The aircraft received substantial damage to the left aileron and leading edge devices. After the accident, a maintenance crew lifted the left wing and then opened the emergency gear panel and pulled the left hand cable for the left-hand main gear uplock. The door lock released, and the left main gear door opened and the gear extended. The maintenance crew then pushed the gear into the down locks. Further inspection of the landing gear revealed that the pin, (part number MY20248-001) which is part of the forward gear door lock, (part number VTS4424) was corroded (rusted) and cracked at the point of rotation (see attached Illustrated Parts Catalog and Figures). The mechanic performing the inspection reported that a more detailed inspection of the landing gear door locks might have prevented this accident.