On September 23, 2011, about 1005 mountain daylight time, a 1968 Cessna 177 airplane, N3152T, was substantially damaged following a forced landing after a partial loss of engine power near Rock Springs, Wyoming. The certified airline transport pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed at the time of the accident. The cross-country flight was being operated in accordance with 14 Code of Federal Regulations Part 91, and a flight plan was not filed. The flight departed the Rock Springs-Sweetwater County Airport (RKS), Rock Springs, Wyoming, about 0955, with a planned destination of Alva, Oklahoma. In a written statement submitted to the National Transportation Safety Board investigator-in-charge (IIC), as well as in a telephone conversation with the IIC, the pilot reported that he had to lean the mixture to start the engine, but did not remember readjusting the mixture control prior to takeoff, nor did he remember using the airplane’s Before Takeoff checklist prior to takeoff. The pilot further reported that during the engine runup prior to departure he observed that both magnetos dropped 50 RPM, and that he cycled the propeller three times, and each time the propeller recovered in a normal time span of between 3 to 5 seconds. He stated that the engine was strong and smooth on takeoff and climb, but about 500 feet above ground level when he began a right hand turn, the engine began to vibrate and the airplane started to lose altitude. The pilot reported that as he was unable to maintain altitude, he elected to make a forced landing to a draw. During the landing roll the airplane impacted either a hole or a berm with the left main landing gear, which resulted in the left landing gear to collapse. The airplane sustained substantial damage to the outboard section of both wings. At the request of the IIC, a National Transportation Safety Board senior air safety investigator conducted a post-accident run of the airplane’s engine at the facilities of Beegles Aircraft Services, Greeley, Colorado on October 18, 2011. The investigator reported that the engine started immediately, ran smoothly for about 5 minutes, and responded to all throttle movements and cycling of the propeller. Both magnetos were checked, with barely a detectable drop on each. The investigator added that on engine shutdown there was a slight RPM increase as the mixture was leaned, which was to be expected. The investigator reported no anomalies with the engine that would have precluded normal operation. At 0954, the weather reporting facility located at RKS reported wind 290 degrees at 5 knots, visibility 10 miles, temperature 16 degrees Celsius (C), dew point -2 degrees C, and an altimeter setting of 30.45 inches of Mercury. The density altitude at the time of the accident was calculated to be 7,907 feet. The 1968 Cessna 177 Owner’s Manual, which addresses Maximum Rate-of-Climb Data in the performance section, states under Note #1 - Flaps up, full throttle and mixture leaned for smooth operation above 5,000 ft. The elevation of the Rock Springs-Sweetwater County Airport is 6,764 feet mean sea level.