Riding the Mountain Waves

Initially drive south, then east, finally south again for 530 miles—the crow only endures 390 or so miles. I could either zigzag on summer lava-hot or winter snow-covered, icy highways for eight hours or literally leap over them for three or so hours in a small plane.

That difference between ground-pounding on a highway versus traveling GA was one of the reasons I learned to fly decades ago.

Flying to Denver was usually fast, pushed by prevailing northwest winds. Returning to Jackson Hole was typically much slower. I recall one trip when my ground speed was no greater than the traffic under me on Interstate 80.

Heading back to Jackson one day, I flew my usual route. North to Fort Collins, Colorado, then slip through the Medicine Bow Mountains gap west of Centennial, Wyoming. In hindsight, that shortcut probably saved only 10 miles or so. That day was different. As a relatively new pilot, I had failed to see the bigger picture.  

Meteorologists called this area the Great Basin High. Prevailing winds for much of the northwest are from the west and northwest.

The Wind River Range lies north of the Medicine Bow Range of the Rockies. Together they form a gap of a hundred miles or so, an hourglass shape that creates a venturi. Think of the Great Basin High as a set of bellows pointed at southeastern Wyoming. I flew into that and the venturi effect that day.  

Often, turbulence is the harbinger of mountain waves, not the ideal ceiling—and visibility unlimited—day.

Our mighty Cessna 172, my passengers, and I pointed our collective noses northwest to “shoot the gap.” A little nose-up trim—and then just a bit more—as I wanted to cross the ridge, not meet it.

A little warning bell in my head began ringing, quietly at first. I was essentially swimming up a smooth waterfall of air with airspeed steadily decreasing.

Moments before the stall warning was about to shock me into awareness, “Let’s fly over Medicine Bow instead.” Terrain thousands of feet lower, and over relatively flat terrain. There would be no short cut today. I made a lazy turn away, lowered the nose slightly, airspeed thankfully increasing.

Mountain waves often reveal their presence with visual clues like rotor or roll clouds. FAA weather charts can help for higher altitudes but when just a few thousand feet agl, they may be less useful.

Mountain flying 101: Cross passes at an adequate altitude and angle to permit retreat. Maintain maneuvering airspeed at all times. Maintain vigilance when close to terrain, especially mountainous terrain. Look for visual clues.

Fast-forward a few years, a few ratings, and 600 miles north. Lessons learned, some forgotten.

On a flight from Kalispell to Great Falls, Montana, as captain I said, “Ladies and gentlemen, this will be a scenic, pleasant 45-minute flight. Great Falls weather is fair and warm. Enjoy your flight.” I couldn’t have been more wrong.  

We launched into beautiful Montana summer skies. Midway between Kalispell and Great Falls is the picturesque, 8,000-foot-high, banana-shaped Sawtooth Range in Montana.

As a mountain-bred pilot, I’ve always enjoyed observing the ridges and valleys from on high and low. Snow-capped peaks nourishing verdant valleys.  

That day’s flight would be memorable in another way.

As we climbed, my copilot and I noticed a cloud layer east of the mountains. It transformed as we continued our climb. Roll clouds, also known as rotors or standing mountain waves. Big Sky guardians.

I said to my first officer, “Ask for lower. We don’t want to be caught in those.” 

“Horizon 253 descend, maintain one two thousand.”

 Too late.

Flight idle and nose down, which normally produced a 2,000-3,000-feet descent rate, resulted in a 2,000-feet-per-minute climb. Rats. We had just been caught by Mother Nature’s roller coaster.

Airspeed was approaching VMO. I slowly brought the nose up and announced, “Uh, Salt Lake Center, unable to maintain assigned altitude. We had been lifted above our assigned altitude.” 

“That’s OK, Horizon 253. It’s been happening all day.”

All day? Thanks for the heads up, ATC.

As the adage says, “What goes up must come down.” And we did.

We flew through the apex of the wave, then descended at several thousand feet per minute while slowing to maneuvering airspeed. Nose up but descending fast. I was reminded of a space shuttle slicing through the atmosphere during reentry. Anything in the airplane not tied down floated, even dust from the flight deck carpet, then slammed to the nearest surface moments later as we were caught by the next wave.

“Geez, we gotta get out of this.” I needed an anchor even at 18,000 feet. Already at gear extension speed, I called “Gear down.” It did the trick, and the wave spit us out, apparently through toying with us. We were rattled but safe. We hadn’t exceeded any G-load limit on the airframe.

We continued our descent into Great Falls, leaving “the wave” behind and above. Winds howled as I wrestled the plane to the ground and landed.

We experienced moderate turbulence even at our gate. I requested chocks for all three landing gear. We breathed a sigh of relief as our passengers applauded, not because of our airmanship. We were all just happy to be on terra firma again.

So why was that flight so rotten? I had flown that route many times. What had I missed? 

It was all about the interaction of weather and terrain. Eighty-knot tailwinds aloft and higher-than-normal temperature difference between the surface and aloft.

An analogy: Place a rock in a smooth-flowing stream. The flow becomes disrupted as water flows around the rock. If the velocity of water increases over the rock, so does the turbulence. The Sawtooth Range was that rock in the river. 

Thermodynamics was another factor. Warm surface temperatures out on the prairie caused the air to rise rapidly. Combine the rapidly rising air mass with the high winds aloft, and you have a good recipe for turbulence. Throw in an 8,000-foot rocky mountain range and you’ve got standing mountain waves. I had every factor that day but failed to recognize all of them.

As the rock doesn’t move in the stream, the standing waves it produces don’t either. I like the term “stationary” better. The stationary trait of a mountain wave may produce more turbulence flying downwind than upwind.

Using my 80-knot tailwind and a stationary wave, my groundspeed and therefore penetration speed through the wave would be 160 knots faster than an upwind approach and theoretically more turbulent. We didn’t test it. Our return flight was by a safer, much smoother route.  

Thanks to the National Weather Service, National Oceanic and Atmospheric Administration and FAA, pilots have a multitude of information available to them for flight planning and in flight. I had consulted them but missed a few critical clues.

The winds aloft compared to lower altitudes were extreme. Ground temperatures compared to higher altitudes were high as well.

Use your resources. Unless contact with the ground is imminent, slow to maneuvering speed and let the airplane ride the wave. Lower flaps and land only within the aircraft’s manual limitations.

More than one airplane has been lost due to excessive flight control inputs.