On April 18, 2001, at 1330 hours Pacific daylight time, a Cessna 172B, N4672L, was substantially damaged during an off-airport emergency landing, 28 miles west of Lancaster, California. The commercial certificated pilot and one passenger received serious injuries and the second passenger received minor injuries. The personal flight was operated by the owner under 14 CFR Part 91. The flight departed from Camarillo, California, at 1300, and was destined for Rosamond, California. Visual meteorological conditions prevailed and no flight plan was filed. In a hospital emergency room interview, the pilot reported that after flying 20 - 25 minutes the aircraft's engine experienced a partial loss of power. The pilot reported having an adequate fuel supply and thought he may have experienced carburetor icing. In his accident report to the Safety Board, the pilot stated that he "experienced involuntary descent. Applied full power, adjusted pitch to 80 mph (IAS) for max. perf. climb. Unable to gain/maintain altitude resulting in forced landing." In a telephone interview with the Safety Board investigator on June 5, 2001, the pilot said he had good recollection of the events leading up to the accident. He recalled that in the short time period before the accident the airplane "wouldn't climb." He was wearing a noise attenuating headset but thinks that the engine "sounded fine" and was running smoothly but the airplane "wouldn't climb" at 80 knots indicated airspeed. Despite having full throttle applied he recalled the engine tachometer being at the lower limit of the green arc. In later discussions about the accident, the male passenger agreed the engine sounded normal. When the Safety Board investigator told him that the accident site was on the east side of a small mountain ridge and that surface wind at Fox Airport in Lancaster was reported from the southwest at 18-knots with gusts to 24-knots, the pilot said that both he and the male passenger thought a downdraft may have contributed to their accident. When the Safety Board investigator told him about the effect of the sloping terrain on his perception of aircraft climb performance, he did not recall the rising terrain. He said that he "put in a little flap to keep from settling" as he was having difficulty climbing. When the Safety Board investigator told him the flaps were full down at the accident site, he couldn't recall when or if he put them down. He thought he put down only about 10 degrees of flap extension. The Teledyne Continental party representative traveled to the accident site with the Federal Aviation Administration representative the morning following the accident. He reported the aircraft wreckage was located in a north-south oriented valley/box canyon with terrain rising to the south. At the site, there was one fresh scrape area in the dirt, then a second 150-feet further south. The wreckage was resting in a nose-low, left-wing-low attitude adjacent to the second scrape mark with the nose pointed approximately southeast. The wing flaps were extended. There was sharply rising mountainous terrain within 1/4-mile to the east, south, and west. Fuel was still draining from the vent mast on the left wing. The Teledyne representative said that the terrain was sloped sufficiently that, while leaving the area (downhill) in off-road driving conditions, it was necessary to apply braking to control the speed of his automobile. The location was at latitude 34 degrees 45.69 minutes north and longitude 118 degrees 38.51 minutes west (GPS). The elevation was 3,443 feet (msl, GPS). The Safety Board investigator examined the airplane on June 1, 2001, at the facilities of Aircraft Retrieval Service in Pearblossum, California. The wings and empennage surfaces had been removed to facilitate recovery of the aircraft. There was crushing damage to the lower nose area of the aircraft, the nose wheel was bent aft, and there was a dented area on the lower fuselage approximately beneath the pilot's seat and about the size of the nose wheel. One blade of the propeller was bent smoothly and evenly aft approximately 90 degrees along its span with modest leading edge damage near the tip that was typically 1/8-inch deep. The other propeller blade was undamaged. The engine tachometer hour meter read 293.9 hours. The fuel vent in the left wing and the vented fuel cap on the right wing were open (vented) when suction was applied. The fuel selector valve was in the "off" position. The carburetor was broken from the engine sump at its base and the fracture surfaces exhibited a uniform grainy appearance. The throttle butterfly valve was closed and the throttle control in the cockpit was bent to the side at the mid-travel position. The mixture control was in the "2/3-rich" position and the carburetor heat was in the "off" position. The air filter gasket was seated. The fuel inlet screen held a small amount of debris, however, the screen area was about 95-percent unobstructed. The float bowl was clean and dry and the composition floats and needle valve assembly were visibly undamaged. Both magnetos sparked at the spark plug leads when the engine was turned by hand. The spark plug electrodes were round and unfouled and exhibited a light gray/brown appearance. The left magneto was timed at 26 degrees BTDC and the right magneto was timed at 28 degrees BTDC. According to maintenance records provided by the aircraft owner, the magneto switch function was checked during the annual inspection about 2 flight hours prior to the accident. When rotated by hand the engine exhibited mechanical continuity and valve function. The combustion chambers, viewed with a borescope, exhibited uniform whitish-brown deposits on the piston face and cylinder head with shiny, unscored, cylinder walls. The differential compression of the 6 cylinders, using an 80-psi reference, were respectively: 76, 60, 74, 62, 68, and 53 psi. There was approximately 4 quarts of clear, light-brown oil in the engine, and there was no visible oil on the firewall or underside of the fuselage. The oil screen contained several small pieces of nonmetallic matter resembling gasket residue and three small flecks (about 1/32-inch) of nonmagnetic metal. The exhaust muffler outlets were free of obstructions. A generic carburetor icing probability chart used by the Safety Board indicated that the ambient conditions of temperature and dew point were conducive to "light icing-glide or cruise power." The density altitude at the accident site was about 5,000 feet.