On November 15, 2001, approximately 1530 Pacific standard time, a Cessna 402B, N910WT, experienced a collapse of the left main landing gear during the landing roll at Tri-Cities Airport, Pasco, Washington. The airline transport pilot and his passenger were not injured, but the aircraft, which is owned and operated by Aeroflite Executive Services, sustained substantial damage. The 14 CFR Part 91 business flight, which departed Boeing Field, Seattle, Washington, about 50 minutes prior to the accident, was being operated in visual meteorological conditions. The aircraft was on an IFR flight plan. According to the pilot, he inadvertently let the aircraft touch down on the front lip of the displaced threshold. When the left main gear impacted the lip, which was about two inches high, it started rapidly loosing air and going flat. As the landing roll continued, the aircraft began veering to the left, and the pilot was unable to keep it from departing the side of the runway surface. After it departed the runway, the left main gear sank into the soft dirt surface and experienced an overload collapse.