Are you prepared to encounter a mountain wave?

By MARGARET W. LAMB

On Nov. 8, 2020, a professional pilot was flying a Beechcraft King Air C90A from Colorado Springs southwest to Alamosa, Colorado. The route crossed two parallel mountain ranges: First the Wet Mountains, rising above 12,000 feet msl, and then the lofty Sangre de Cristos, with several peaks above 14,000 feet. Between these ranges lies a deep valley with a floor of around 8,000 feet. Mountain wave conditions existed, with winds aloft blasting from 220º directly at the ranges.

It was a dark night. Based in Alamosa, the pilot had safely flown two Colorado Springs trips across the same mountains earlier that day, when southwesterly winds aloft at his altitude were 80 to 90 knots. On this sixth leg back to base in Alamosa, heading into the wave, he filed for FL240, anticipating turbulence. He had briefed weather carefully and knew that the terrain underneath bristled with turbulence.  

A Beechcraft C-90A.

The pilot was hand-flying. As he approached the mountains, his true airspeed and wind speed page reported a headwind of 107 knots. Soon afterwards, the headwind increased to 134 knots and his ground speed shrank to 80 knots. Turbulence was continuous moderate, occasionally severe. Then the airplane began to porpoise. Altitude wandered ±100 feet, airspeed ±10 knots. The airplane started sinking. Suddenly the King Air stalled in a full buffet. Indicated air speed was far above stall speed. The stall warning horn blared.  

The pilot simultaneously banked left and let the nose drop. By the time his 180 was completed, the airplane was 4,000 feet lower and the ground speed  was 360-380 knots.

On the turn back, a series of bright white flashes came from the right side of the aircraft, followed by a bang. He thought the right engine compressor stalled, believing that the sudden tailwind involved with the left turn momentarily starved the engine of air. The right engine kept running, however, and he headed back to Colorado Springs for the night.

Winter time, when the jet stream has moved south, is the season for mountain

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