On March 7, 1994, at 1935 mountain standard time, a Rockwell International 690C, N5938K, operating as Casper Air Service Flight 81, sustained substantial damage during descent near Yampa Valley Regional Airport, Hayden, Colorado. The pilot was not injured; he was the only occupant. Visual meteorological conditions prevailed for this on demand, commercial cargo flight. An IFR flight plan was filed and cancelled by the pilot approximately 5 minutes prior to the accident. During a visual descent for landing the airplane collided with trees in mountainous terrain approximately 9,600 feet mean sea level. The pilot switched from the left to the right seat due to wind screen damage and flew the aircraft to Yampa Valley Regional Airport, a distance of approximately 15 miles. He landed at Yampa Valley incurring no further damage. Airport elevation is 6,604 feet above mean sea level. The nose section, windscreen, both wings, the number one engine, and both horizontal stabilizers sustained damage. (See attached photographs). Light conditions were a dark night with the lights of two towns and the airport visible in front of the aircraft. According to the pilot, he had the airport in sight and was proceeding visually to a base leg when he collided with the trees. He stated that he did not know what he had hit until he inspected the aircraft following landing. The pilot also stated that he had previous night experience at Hayden.