N2761QCessna 182K · 1967

On May 29, 2021, about 1312 Alaska daylight time (AKDT), a Cessna 182K, N2761Q, sustained substantial damage when it was involved in an accident about 77 miles east of Cordova, Alaska. The commercial pilot and passenger sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot stated that, after completing a preflight inspection and reviewing weather conditions for the departure and proposed destination locations, which included winds aloft and METARs, they departed on the cross-country flight. About 2 hours and 15 minutes into the flight, while at an altitude of about 12,500 ft mean sea level (msl), the airplane encountered severe turbulence and windshear, and shortly thereafter, a downdraft. Unable to escape the downdraft, the pilot attempted to level the wings and pitched up as the airplane impacted terrain. The pilot reported no preaccident mechanical failures or malfunctions that would have precluded normal operations.

The airplane impacted snow- and ice-covered terrain on the north side of Mount Hawkins in the Wrangell St Elias National Park at an elevation of about 6,570 ft msl. During the impact, the airplane sustained substantial damage to the fuselage, wings, left horizontal stabilizer and elevator.

A review of flight track data revealed that, after departure, the flight proceeded to the south and climbed to an altitude of about 11,000 ft msl before initiating a left turn to the east. The flight continued on an easterly heading at an altitude between about 11,000 and 12,900 ft msl. The last in-flight data point was at 1311:19, when the airplane was at an altitude of 11,800 ft msl with a groundspeed of 108 kts and on a track of 117°. (see figures 1 and 2.)

Figure 1 - Flight track.

Figure 2 - Flight track, end of flight.

No frontal boundaries existed near the accident site; however, a relatively strong pressure gradient was present along the northern Gulf of Alaska. The National Weather Service, Anchorage, Alaska, upper air soundings from 0400 and 1600 both indicated favorable conditions for severe mountain wave conditions between 9,000 and 14,000 ft mean sea level with updraft and downdraft speeds between 2,000 and 3,000 ft per minute. These conditions would be prevalent near any mountains with tops between 5,000 and 13,000 ft msl; the mountains to the south of the accident site were between this height. The Polar Operational Environmental Satellites imagery from 1337 also showed transverse wave banding at the accident site, which is indicative of mountain waves.

FAA Advisory Circular (AC) 00-6B, Aviation Weather, describes mountain wave conditions and the associated aviation hazards therein:

A mountain wave is an atmospheric wave disturbance formed when stable air flow passes over a mountain or mountain ridge. Mountain waves are a form of mechanical turbulence which develop above and downwind of mountains. The waves remain nearly stationary while the wind blows rapidly through them. The waves may extend 600 miles (1,000 kilometers) or more downwind from the mountain range. Mountain waves frequently produce severe to extreme turbulence. Location and intensity varies with wave characteristics.

Figure 3 - AC 00-06B mountain wave conditions.

Mountain waves often produce violent downdrafts on the immediate leeward side of the mountain barrier. Sometimes the downward speed exceeds the maximum climb rate of an aircraft and may drive the aircraft into the mountainside.