Accident Details
On August 13, 2018, at approximately 0908 mountain daylight time, a Cessna 172N airplane, registration N5660J, sustained substantial damage during an accident near Afton, Wyoming. The airline transport pilot and two passengers received serious injuries. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a local aerial observation mission.
Pilot Account
According to the pilot, while performing aerial observation of markers for a local mine in a valley adjacent to mountainous terrain, the airplane encountered downdrafts and lost altitude. In an attempt to escape the high terrain, the pilot executed a 180° left turn with approximately 45° of bank. After completing the turn, the airplane's stall horn activated. The airplane was unable to climb above the terrain and struck several trees before impacting the ground, causing substantial damage to the wings and fuselage. The pilot reported no preexisting mechanical anomalies that would have prevented normal operation.
Wreckage and Performance
The wreckage was recovered at an elevation of approximately 6,750 feet mean sea level (msl) in heavily forested terrain. Based on environmental conditions at the accident site, the density altitude was calculated to be about 8,765 feet msl. Aircraft performance charts indicated that, under the prevailing conditions, the airplane would have had a climb rate of approximately 300 feet per minute.
Weight and Balance
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, which was below the maximum gross takeoff weight of 2,550 pounds. The center of gravity calculation was within the specified limits.
Weather Conditions
A weather study of station models surrounding the accident area indicated light winds with variable winds of 10 knots or less and clear to scattered clouds. A National Oceanic and Atmospheric Administration numerical model reported a temperature of 18.2°C and a dew point of 1.8°C. The model also indicated a surface-based inversion extending to about 500 feet above ground level (agl). No low-level wind shear or significant turbulence was indicated, and winds from 500 feet agl to the surface were calm. A search of pilot reports over the region found no reports within three hours before or after the accident. No SIGMETs or AIRMETs were active at the time for turbulence, low-level wind shear, or high winds. However, the National Weather Service had a current AIRMET Sierra for mountain obscuration conditions.
Witness Report
A commercial pilot flying approximately 15 miles north of the accident area reported moderate turbulence over the mountain ridge lines. That pilot had landed around the time the accident airplane took off.
Mountain Flying Guidance
The Federal Aviation Administration's Aviation Safety Program publication "Tips on Mountain Flying," in its 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 further advised pilots to "plan to be at that altitude at least three miles before reaching the ridge" and to "cross a ridge at a 45° angle to the ridge" to allow a quicker turn away in the event of a severe downdraft or turbulence.