Some years back while flying over the Appalachians, I heard a pilot on the frequency commenting to air traffic control that she thought her airspeed indicator was malfunctioning since it was wandering up and down by 20 knots.
Rather than suggest mountain waves as a culprit, the controller only wanted to know whether the pilot intended to declare an emergency and/or land. It was a day when the winds were modest, but there were some mild mountain waves. I was getting them too, with the airspeed indicator in my Beechcraft Baron fluctuating about 10 knots above and below my normal cruise speed.
“Mountain waves,” I said anonymously on the frequency, hoping to prompt a realization out of the pilot or ATC.
I presumed this pilot was using an autopilot and failed to notice the automatic trim moving up and down to maintain altitude. Were she not using the autopilot, it would have been clear to her that it was the atmosphere causing the excursions, not the airspeed needle moving up and down. But there are other cues such as angle of attack changes and wind noise. Perhaps the use of noise-canceling headsets made the increase/decrease in airspeed inaudible. But the angle of attack would change significantly with 20-knot airspeed excursions.
Soon after, the pilot—apparently not getting the hints—diverted and presumably spent some money to have a mechanic chase down a problem that didn’t exist.
Every pilot’s first encounter with a mountain wave can be a little unnerving, especially if the waves are strong enough that you can’t maintain altitude and the terrain is not far enough below your wings. Airplanes can and have been pushed into the ground during mountain wave encounters. Record-setting aviator/billionaire Steve Fossett was a high-profile victim when the Bellanca Decathlon he was flying couldn’t overcome a mountain wave in eastern California.
As winds aloft flow perpendicular to the undulating mountains, the wind starts conforming to the terrain, creating its own invisible undulations in the sky—often to very high altitudes. Generally speaking, the bigger the mountains, the bigger the waves. Pilots in the Rockies (especially sailplane pilots) are well-versed in the effects of mountain waves, as well as their relative, the rotor. Pilots in the middle of the country and eastward may never experience serious wave action thanks to smaller terrain features.
What often catches pilots off guard is the fact that the ride can be perfectly smooth during wave encounters. When it’s turbulent, pilots are less prone to notice minor waves. In addition, lighter, lower-powered airplanes are more adversely affected by waves than heavier airplanes with more power. While the Baron I was flying only had plus or minus 10 knots of fluctuation, the Piper Arrow that diverted was reporting 20-knot excursions.
One of the most exhausting flights I ever had was in a Cessna P210 with former AOPA Pilot editor Richard Collins. He let me fly left seat from Maryland to Asheville, North Carolina, one windy day to visit his dad. Over Virginia and North Carolina we cruised at FL180 in a stiff crosswind. In perfectly smooth air, we started getting waves so strong that the airspeed fluctuated between best-rate-of-climb speed and max structural cruise (yellow arc). With the extreme range of airspeed and no automatic electric trim to let the autopilot fly the profile, the exercise of constant trimming and managing cowl flaps to keep the engine temps under control were testing my mettle, to say the least.
If you can imagine, it got worse. Eventually, the waves got so strong that we couldn’t maintain altitude and needed a block altitude between 16,000 feet and FL180. In the downside of the wave, with climb power and max-rate-of-climb airspeed, the P210 would descend at 500 feet per minute. On the upside, the airplane would climb 500 fpm with descent power and the airspeed nudging the yellow. And through all this was the constant motion of trim, cowl flaps, power, and—since we were climbing and descending through the turbo system’s critical altitude—the mixture and prop controls, too.
Needless to say, I think Collins lucked out letting me do all of that monkey motion. As a longtime CFI, I think he enjoyed watching a young pilot have his rear handed to him in some unusual weather in a very complex airplane. Had we been in a lower-powered airplane, conditions like we encountered that day could have been downright dangerous. Peaks in the Appalachians can reach 6,000 feet and a diminutive airplane like a Piper Cherokee or Cessna 172 simply doesn’t make enough power at that altitude to overcome such downdrafts.
Airliners, too, are affected by waves. Mountain waves coming off the Rocky Mountains can exist at altitudes higher than most jets can fly. They also can exist hundreds of miles east of the mountains over featureless terrain.
On windy days, mountain waves can occur 100 miles east of the Appalachians as well. At cruise altitude, some jets operate within a narrow airspeed band, known as “coffin corner.” This is where the difference between VNO and stall speed may be only 10 or 20 knots. In older jets, the band can be smaller. Throw in a mountain wave with excursions of 10 knots and you can see where trouble lies. The only solution in such a case is to descend to a lower altitude to widen the airspeed band and ride it out.
Mountain wave activity is often noted in your preflight weather brief. If it’s early in the day and there are no reports, simply check the winds aloft for direction and strength. If the winds are blowing strong perpendicular to the hills, you can bank on at least some wave activity—and plenty of low-level turbulence.
As always, leave yourself an out if you start encountering waves. Although it may add some length to your trip, you could plan a route over lower terrain to give yourself a little more room between you and the ground. Ask ATC if there have been any reports of wave action and give pireps of any you encounter, for others to plan on.