On March 19, 2003, at 1930 mountain standard time, a Beech E-90, N711TZ, operated by Mountain Flight Service, Incorporated, and piloted by an airline transport pilot, was substantially damaged when the airplane impacted mountainous terrain and subsequently nosed over, approximately 1-1/2 miles southeast of Kremmling-McElroy Field (20V), Kremmling, Colorado. Night visual meteorological conditions prevailed at the time of the accident. The non-scheduled, on demand, flight-for-life flight was being conducted under the provisions of 14 CFR Part 135. The cross-country flight originated at Grand Junction, Colorado, on an instrument flight rules (IFR) flight plan, and was en route to 20V. However, at 1928, the pilot cancelled the IFR flight plan and continued under night visual flight rules. The pilot, a paramedic, and a flight nurse on board all reported sustaining minor injuries in the accident. The pilot reported that he maneuvered for a left hand downwind leg for landing from the east to west at 20V. The pilot set up his downwind leg at 8,400 feet mean sea level putting him at what would have been 1,000 feet above the airport elevation of 7,411 feet. The pilot reported it was very dark and he could see the airport, but could not see the terrain. The pilot reported that suddenly he saw the ground. The airplane impacted the terrain and came to rest. The pilot reported that the airplane was experiencing no malfunctions prior to the accident. An examination of the airplane at the accident site showed the airplane resting inverted on the snow-covered edge of a mountain ridge at an elevation of 8,489 feet. The airplane's nose cone, forward fuselage, and nose gear doors were crushed aft. The nose gear was broken out. The airplane's right wing, outboard of the engine nacelle was bent downward approximately 20-degrees near mid-span. The airplane's left wing and left engine nacelle were twisted upward and bent aft. The airplane's vertical stabilizer and rudder were broken aft at the base. The right horizontal stabilizer and elevator were bent and twisted aft. Both propellers' blades showed torsional bending, chordwise scratches and tip curling. Flight control continuity was confirmed. An examination of the engines, engines' controls, and other airplane systems revealed no anomalies. Published terminal procedures for runway 27 at 20V indicates high terrain of 8,739 feet south-southeast of the airport. The published airport diagram for 20V directs right traffic for the pattern to runway 27.