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.
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 waves. The classic diagram of a mountain wave depicts winds approaching a range from the left (or west), rising over the peaks with a lenticular cloud at the crest of the wave, then descending on the lee side with a rotating air current somewhere underneath, then rising again into a lesser crest, then down again.
Just because you are flying a single-engine piston aircraft at a lower altitude, don’t think for a minute that you will be less exposed to severe turbulence and downdrafts under mountain wave conditions.
One time I was trying to cross the Sangre de Cristo range from east to west, against wave conditions, in a Cessna 182. Infamous La Veta Pass offered the lowest route, about 10,000 feet. I was over the Great Plains, swinging back and forth, nosing the headwind, hunting for a decent way over.
Oddly, the air was smooth. When I realized that I had traveled 10 miles in 20 minutes, I turned away to the south and ended up flanking the whole range 100 miles down to Santa Fe.
And, indeed, flying south of these mountains is the only safe way west, under high westerly wind conditions. Many pilots flying from the East to California attempt the crossing, when better conditions await to the south.
How can you make your own mountain wave forecast?
If the forecast wind at mountaintop height is greater than 20 knots and more or less perpendicular to the axis of the mountain range, a wave will exist. Compare surface barometric pressures at airports upwind and downwind of the range. If the pressure at the upwind airport is more than .12 inch higher than the pressure at the downwind airport, lee side downdrafts may exist. Also look at winds/temps aloft for windshear, and for a discrepancy in the temperature lapse rate that may indicate a trapped lee wave.
When you’re planning a flight across mountain barriers, take a close look at the sectional chart. What terrain underlies your route? Evaluate details: Heights posted for the major peaks, the elevations of mountain passes, widths and altitudes of the intervening valleys. River drainages. Features that may block or channel wind. Mark your route on a paper chart. Imagine where downdrafts may propel you into an unexpected camping trip. You do carry survival equipment, don’t you?
At your departure airport and in flight, search for signs of a mountain wave. Some visible signs are rotor clouds, lenticulars, and cap clouds.
In very dry climates, like the American southwest, the atmosphere isn’t moist enough to form a whole rotating roll cloud, like we see along the eastern face of the Rockies near Fort Collins, Colorado.
In the central Colorado mountains, a rotor cloud can be a simple little whiff, just a fragmentary rotating strand. You must gaze at a rotor cloud to see its whirl. Rotor clouds are a sign of turbulence in the mountain wave and a pilot must be ready to be tossed about. In my Navion, many times in severe turbulence near rotor clouds, I’ve had to hold on to my seat with both hands and lean away from the canopy post.
Another wave cloud is the lenticular. Every pilot knows what one looks like: A lentil-shaped, smooth, elongated cloud poised at the top of the wave, often just slightly downwind of the crest of a mountain range. Typically, lenticular clouds are stacked as their various layers of humidity are lifted over the peaks.
However, when there is little moisture in the atmosphere, lenticular clouds can be tiny and be influenced by convective currents. The particular small formations shown began as standard lenticular clouds and, due to heating of the land below, quickly turned into this rollicking formation. Meteorologists call them unsteady lenticulars. They speak of invisible mountain wave, and severe turbulence.
You have to search to find a cap cloud.
One moonlit winter night years ago I was flying my sturdy Navion from Santa Fe to Denver, across the Sangre de Cristo Mountains. Skies were clear with no signs of a wave. At 13,500 feet, approaching 14,345-foot Blanca Peak in the moonlight, I noticed a tiny tufted white cloud capping the peak. I called Flight Service and asked if they had reports of a forecast of a mountain wave. “Nope,” they said.
Just past the peak, wham! The vertical speed indicator started unwinding. The airplane was tossed about and going down fast. I banked right, dropped the nose, and fled east down the drainage of the Huerfano River out on to the Great Plains. In a minute I lost more than 3,000 feet.
So here’s the beta: Be aware of winds aloft. Understand underlying terrain and its influences on wind. Analyze the weather. Read clouds. Practice unusual attitudes — and always have an escape route.
Margaret Lamb, Commercial, Instrument, ASEL, CFII, A+IGI, is the author of Flying Colorado Mountain Weather.
Long time Rockies flyer and advisor to the unwary.
Bottom line is very simple…..Timing is everything!..
Scott, thanks for the well written article. It struck home in many ways. Prior to becoming a pilot I was a Deputy Coroner (Las Animas county south of La Veta Pass & from the Sangre de Cristos east about 100 miles for the width of the country), and we received a call one day that about 30 miles south of the pass, that there was a wreckage spotted which could have been one missing for about ten years. I treked up to nearly the top of the ridge and recovered the remains. The 172 was only about 100′ short of clearing the ridge on a westbound heading. On a good day it would have had difficulty in clearing the Sangre de Cristos.
Years later, I nearly lost it in a Piper Archer III in the Sierra Madre with a downdraft that fortunately was between two ridges, otherwise I would have hit the peaks. That put me into IMC and spatial disorientation. We were saved by requesting vectors from Hermosillo approach and they got me out of a bad predicament. I began working on my instrument rating as soon as I returned to the USA.
Long story short, due to my repetitive flying in the mountains of northern Mexico (a couple of thousand hours) I went to a high performance turbo 540 Lycoming in a T206H, upon the advice of a seasoned pilot. That increased my odds of surviving those unexpected deadly winds…but still no guarantee.
My own experience with mountain wave turbulence:
https://youtu.be/T20H_JbJN90
Very good article from Margaret Lamb, I will read her book. During my aviation career I have encountered numerous problems filming in and around mountains. On my second solo xcountry in my T6, I was caught in a mountain wave flying into San Diego. I was nose down 70+* power off, above red line and climbing over 4000 FPM.
Flying to the AOPA convention in San Jose to give a talk, I suspected I would encounter a mountain wave. Fox Field surface winds were 50 KTs. Reports from the Tehachapi Pass were in excess of 50KTs gusting to 70Kts.To clear the mountains required 11,000 FT. I climbed to 14,500 clear above a cloud layer tops of 12,000 FT. Scanning my panel I noticed the IVSI flexing down up down settling on 4000FTM. down. There was no feeling of descending just the IVSI indication and increasing loss of altitude on the altimeter. I had enough experience to make a heading change to the direction I just came from where I knew it was safe and no rocks in the clouds. Go to a mountain stream and watch the water flow through rocks, the winds do the same in canyons and mountains. I have had film crews take wind speed readings along a film flight path before I made the film run. Speeds have varied from 5 KTS to 25, 30 plus as the wind is squeezes through the canyon rock passes. I am convinced that most accidents of people supposedly flying into mountains are in fact planes pushed into mountains. I’m writing my book about my career in the warbird, airshow and film industry, explaining many stories like these. Many safety tips which I hope make your flying safer.