I don’t know about you, but I fly an Experimental airplane because it outperforms anything I could afford to buy in the certified aircraft market. Period. My RV-10 may not be faster than an SR-22 or Cessna Corvalis, but it can carry more, and more economically. And sure, it can’t carry everything my Cessna 210 used to be able to carry (we would have had to build a Murphy Moose for that), but what we can get in there is impressive, all the while saving 3-4 gallons per hour of pricey avgas. In my world that means when I get where I’m going in the RV-10 and refuel, I can still afford cab fare to take me away from the airport. As my husband and I approach our “fixed income” years, that factor alone has become increasingly important.
A simple diagram of a mountain lee wave, showing where lenticular clouds or cloud streets may form.
We’ve proven the affordability/performance equation of the airplane repeatedly in the nearly six years it has been flying by taking it on at least one transcontinental cross-country a year. Anything shorter really isn’t a good test of a true cross-country machine, and our -10 is most definitely all that.
During any grand U.S. transcontinental journey in a light airplane there are mountains somewhere along the way. And where there are mountains and wind, particularly in the clear air after a good, cleansing cold front, there is a behemoth phenomenon that plagues even high-performing power planes: mountain wave.
Unable to Maintain
My first encounter with mountain wave was in our Cessna 182 over the relatively benign looking Appalachians. Flying along without an autopilot I noticed that I was fighting to maintain altitude. Odd—the air was smooth as glass.
In the course of just a couple minutes I’d lost 40 knots indicated airspeed in an attempt to stay at my assigned altitude of 9000 feet msl. Carburetor ice? Not impossible, as there were clouds (we were not IMC, however). I checked. No difference, and now I was definitely outpowered, despite having the throttle pushed up and the rpm cranked, too. As we descended through 8500 msl, I ‘fessed up to ATC that maintaining altitude wasn’t on the agenda. We agreed on a block to 7,000 msl, but after that, she said, I’d be on my own. At 7100 msl, just above a misty stratus layer, things looked up and the airplane began to gain airspeed, then climb. We were heading southeast, and after one more cycle, the phenomenon died off and we were able to continue without issue, but the creepiness of it all stuck with me.
If you think you can’t ride mountain wave safely or use it to your advantage, then you haven’t spent time studying how sailplanes do it. This is the FlightAware track of N55LK, a Schemp-Hirth Duo Discus two-place glider, last spring as it worked its way, nonstop from Minden, Nevada, to Gunnison, Colorado, in one day of wonderful wave soaring. Imagine what you could do with an engine!
A little research on the subject turned up gnarly pictures of military aircraft missing large sections of empennage, wingtips, and in the case of one Boeing 747, an entire engine pod after encounters with mountain wave at altitude. FAA teaching materials on the subject, and the archives of ASRS Callback make mountain wave encounters appear treacherous to the extreme. Read it all and you’d never take off with a threat of winds over pointy rocks ever again.
Well, that’s one way to deal with the problem, but if you fly long cross-countries as I do, you’d soon tire of spending perfectly good days on the ground and discovering that you need twice as long to fly across the country as you would to drive the same trip! There had to be a happy medium.
A year later we stopped in at Sugarbush, Vermont, on the way to Canada in our Cessna, figuring we could play for a day or two in the gliders that hang out there. Unfortunately, the weather didn’t want to cooperate and we took advantage of the first break in the clouds to bail out of the tight little valley and continue our trip north and east. A week later we were clawing our way west against a 40-knot headwind at 12,000 msl on top of a relatively thick stratus layer when the hair on the back of my head stood straight up. There I was again, rhythmically pulling the nose up to hold altitude (or close to it) and then pushing the nose over to yellow line, oscillating through 50 knots of indicated airspeed. I noted the “wave” to ATC, who chuckled and said, “Yeah, we get that on days like today.” And sure enough, as we approached the ridge east of Sugarbush, I caught a sparkle, then a glint of sunlight bouncing off the polished fiberglass of one, then two slender sailplanes. A closer look and I counted five, then the mother lode. Coming up through a small hole in the overcast was a Piper Pawnee towing a leggy Schweitzer into the mountain wave. Instead of struggling against the phenomenon these savvy pilots were playing in it, like surfers off a Hawaiian beach. I needed to know more. A little more time with sailplanes and mountains is what was required.
Now the RV-10 is unquestionably a higher performing machine than the Cessna, but even it can’t outpower mountain wave. It does, however, have a marvelous wing for gliding, and a decent L/D ratio for a power plane. What I realized I needed to learn to do was understand the wave and make it work for me. So I took a little time to pick the brains of some experts in Minden, Nevada, where glider pilots make 1000-mile treks in wave every spring, to learn more about the phenomenon. My logic? If Steve Fossett could get to 50,671 feet unpowered using mountain wave, then there’s got to be a way to make it work for power pilots. And there is, but the key is in being able to identify wave and understand how it works, then how to work with it.
Seen from space: mountain waves create cloud street patterns that can be read easily on infrared satellite images.
Evidence in Cloud
In general, wave sets up in the lee of mountain ranges, regardless of height, whenever wind speed gradually increases with altitude and wind direction is roughly 30 degrees or more perpendicular to the ridgeline. Strong low-level winds in a stable atmosphere can trigger wave when ridgetop winds build to 20 knots or more, as well. SR-71 pilots have reported mountain waves at 60,000 feet. As far as the ride up there? It can be quick. The vertical airflow of a standing wave may exceed 8000 feet per minute.
If upper-level cloud exists, wave is easy enough to pick out. Even in flat-as-a-board south Florida, I’ve seen telltale signs of wave after a strong cold front rips through. Just look up for rippling streets of high cirrostratus…that’s your downstream wave. In the mountains, wave can be identified by looking for standing lenticular cloud parked on the ridges or peaks (that’s the wave crest), or by rotor cloud that seems to churn and reform in twisting shreds of windshear and downdrafts just downwind and under the standing wave.
Windshear can also create waves when an atmospheric inversion separates two layers with a marked difference in wind direction. If the wind encounters distortions caused by thermals coming up from below and protruding in the inversion layer, it will create significant shear waves.
Beyond that, hydraulic-jump-induced waves form when a thin lower layer of dense air flows over mountains, generating a shock wave of sorts at the trough of the flow. Hydraulic jump can be several times higher than the mountain that triggers it, and kick-butt turbulent, too. Look for towering roll clouds and avoid them. They cause structural teardowns in flight. Need I say more?
When you see this lens-shaped cloud parked just above a mountaintop, you are looking at the signature for mountain wave. Expect invisible turbulence on both the upstream and downstream sides of that cloud.
Working It
With evidence of wave well in hand, glider pilots regularly suit up and head out, willing to tolerate the bucking bronco ride through typical low-level turbulence associated with wavy days to get to the rising air and ride it, sometimes all day long.
We power pilots can do that, too. The trick is to understand that in wave, you’ve got to keep your airspeed under control and well below structural cruising speed, even below maneuvering speed, at all times. Let the tailwind do the heavy pushing, and the wave do the heavy lifting. As you climb and if you are in ATC control ask for block altitudes, which will allow you to safely ride the wave up and down along its cycle. And even if you are VFR, be ready to talk to ATC as you approach 17,000 msl (remember, positive control Class A airspace starts at 18,000 msl, and strong wave can get you 1000 feet in the blink of an eye). You can do this downwind or upwind (although you need to understand the limitations of upwind travel, a simple math equation of my speed minus wind speed needs to equal a positive number—seriously, some pilots forget!). Yes, you can tack into the wind, as sailboats do, too. And you definitely want to have oxygen on board and be ready to use it. You won’t have time to fumble with it in wave.
Oh, and avoid all those nasty clouds we talked about earlier. Ever fly under a mammatocumulus cloud or through a rotor? I once landed with passengers’ Thanksgiving dinner all over the ceiling of a light twin after making that mistake as a novice charter pilot coming into Scott Valley airport in the Trinity Alps of northern California. You can be assured, after cleaning that aircraft, I never, ever made that rookie mountain pilot error again.
As seen from above: cloud streets such as these, parallel to the ridgeline and perpendicular to the wind direction, are distinctive markers for mountain wave.
This one is pretty self-explanatory. It clearly diagrams the best area for lift, and one can immediately see where to navigate in order to climb like an elevator. Oh, yeah, and those turbulent zones are pretty well diagrammed, too.
Don’t Try This At Home… Or Without Training
Laurie Harden, proprietress of SoaringNV, in Minden, Nevada, holds a wave camp training program each April and notes that a significant number of participants are power plane pilots. “Lets face it, the more you know, the better you can cope with our mountain conditions,” she said. SoaringNV keeps several dual- and a couple single-place ships on hand for training during the five-day course. Harden’s gone out of her way to employ some of the most experienced sailplane CFIs for the project, some of whom hold U.S. height and distance records for glider flights.
Last summer I had the opportunity to fly with one of those pilots, Stuart Gardner, and I learned more from one flight with him than I had in a year’s worth of FAA safety seminars on mountain flying and air currents. I’m not alone. The venerable Sean Tucker took a bit of training at SoaringNV and came away spouting testimonials. “He told me that after a couple flights with us, he was able to safely fly his airplane through the mountains on a day where he would have formerly sat on the ground to wait out the wave. Instead he had one of the most high-performing, economical flights ever in his airplane back to the west coast—all because he understood how to use the wave to his advantage and avoid its dangers,” said Harden.
Not heading west anytime soon? Check out Chilhowee Gliderport in Benton, Tennessee, or Sugarbush Soaring in Warren, Vermont. Find a soaring operation near or downwind of the mountains and use this windy transitional season to learn what you can about mountain wave phenomenon. You’ll find it a great way to maximize your efficiency and your fun, along with your safety quotient, for a long-flying future.
