On February 26, 2001, at 1144 mountain standard time, a Cessna 172F, N8389U, operated by Flatirons Aviation Corporation of Boulder, Colorado, was substantially damaged when it collided with a barbed wire fence and terrain during a forced landing 3 miles north of Larkspur, Colorado. The commercial certificated flight instructor and the private pilot receiving instruction were not injured. Visual meteorological conditions prevailed, and a VFR flight plan had been filed for the instructional flight being conducted under Title 14 CFR part 91. The flight originated at Boulder, Colorado, approximately 1010, and departed Colorado Springs, Colorado, approximately 1120. In a telephone conversation with the flight instructor, he said the fuel tanks had been filled the night before the accident, and the airplane flew approximately 1.1 hours the next morning just prior to their flight. In his written accident report, the instructor said that shortly after departing Boulder, they became cold (the outside air temperature was 20 degrees F.), and that the heater controls were not in the usual position on the instrument panel (both pilots were accustomed to flying later model Cessna 172s). The private pilot located a "white knob" but could not read the nomenclature plate. Believing it was the heater control, he pulled the knob. The flight proceeded to Colorado Springs, where they made one touch-and-go landing, and headed back towards Boulder at 9,000 feet. At 1135, the instructor noticed the left fuel gauge was reading EMPTY and the right fuel gauge was reading nearly FULL. The fuel selector was switched to the right tank. A few minutes later, the engine lost power. The instructor assumed control of the airplane and turned towards a private airstrip about 2 miles away. The rpivate pilot was instructed to make emergency radio calls and try restarting the engine. As the airplane descended, the instructor realized that strong headwinds were forcing the airplane to land short of the gravel runway. The instructor attempted to "stretch the glide," but the airplane stalled and dropped 15 feet, striking the ground on all three landing gears. The airplane also struck a barbed wire fence, damaging the right wing and shearing off the nose landing gear. The instructor said they had been aloft for 1.7 hours when the engine lost power. Postaccident inspection disclosed the private pilot receiving instruction had inadvertently pulled the fuel strainer control knob instead of the cabin heat control knob. There were no fuel stains on the bottom of the airplane. The salvage crew that retrieved the wreckage reported draining a total of 2 ounces of fuel from the wing tanks. Early model Cessna 172s placed the fuel strainer control on the instrument panel (later models placed it in the engine compartment with access by opening a small door on the engine cowling). The valve is spring-loaded to the CLOSED position. The control knob is pulled to drain fuel, and released to shut off the flow. The salvage crew examined the fuel drain and reported that the spring was missing. According to the Cessna 172F SERVICING INTERVALS CHECK LIST, the fuel strainer should be disassembled and cleaned every 100 hours. The last 100 hour inspection performed on N8389U was on December 14, 2000, 85.67 hours before the accident.