On August 13, 2018, about 0908 mountain daylight time, a Cessna 172N airplane, N5660J, was substantially damaged when it was involved in an accident near Afton, Wyoming. The airline transport pilot and two passengers sustained serious injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 local aerial observation flight. According to the pilot, while conducting aerial observation of markers for a local mine in a valley next to mountainous terrain, he experienced downdrafts and the airplane lost altitude. In an attempt to exit from the high terrain, the pilot performed a 180° left turn with about 45° of bank. However, after completing the turn, the airplane's stall horn began to sound. The airplane was unable to climb over terrain and struck several trees and subsequently impacted terrain, which resulted in substantial damage to the wings and fuselage. The pilot reported no preaccident anomalies with the airplane that would have precluded normal operation. The airplane wreckage was recovered at an elevation of about 6,750 ft msl in heavily forested terrain. Using the environmental factors at the accident site, the density altitude was calculated to be about 8,765 ft msl. Additionally, aircraft performance charts indicated the airplane would have had a climb rate of about 300 ft per minute under the conditions at the accident site. Review of the weight and balance form provided by the pilot revealed that the airplane was operated at a gross weight of 2,294 pounds at takeoff, under the maximum gross takeoff weight of 2,550 pounds. The center of gravity calculation was within the specified limits. A weather study of the station models surrounding the accident area identified only light winds with variable winds of 10 knots or less and clear to scattered clouds. A National Oceanic and Atmospheric Administration numerical model indicated that the temperature was 18.2°C and the dew point was 1.8°C. According to the model, a surface-based inversion existed to about 500 ft agl. No low-level wind shear or significant turbulence was indicated, and the winds from 500 ft agl to the surface were calm. A search of pilot reports over the region indicated that there were no reports 3 hours before or after the accident. No SIGMETS or AIRMETS were current at the time of the accident for any turbulence, low-level wind shear, or high winds. The National Weather Service had a current AIRMET Sierra for mountain obscuration conditions. According to a commercial pilot who was flying about 15 miles north of the accident area, there was moderate turbulence over the mountain ridge lines. The pilot had landed about the time that the accident airplane had taken off. The Federal Aviation Administration's Aviation Safety Program publication "Tips on Mountain Flying," section on ridge and pass crossing, stated that "experienced pilots recommend crossing a ridge line or pass, at the ridge elevation plus at least 1,000 ft.” The publication also stated, "plan to be at that altitude at least three miles before reaching the ridge …This clearance zone will give you a reasonable safety zone to avoid the most severe turbulence and down drafts in windy conditions." The publication further stated "when you actually cross a ridge, you should do so at a 45° angle to the ridge. This allows you to turn away from the ridge quicker is you encounter a severe downdraft or turbulence.”