Has this ever happened to you? You're flying to the lee (downwind) of a mountain range or ridge line, and you notice that the airplane is making an uncommanded descent. When you try to correct the problem, the airplane ends up climbing past the target altitude. Now you're a thousand feet higher than you were when the whole event began! If you were using an autopilot in the altitude hold mode, you'd have noticed that the trimwheel was running at a healthy rate, trying to keep the airplane at a constant altitude. This, in turn, caused the airspeed to drop (in the descent) or rise (in the climb). What's going on here?
You've probably flown into some mountain wave activity. Unless you know what to expect when flying downwind of mountains — or even small hills or ridges — it can be unnerving to experience these altitude excursions. Especially if you're cruising at a relatively low altitude, the terrain below is rugged, and you're flying in the clouds.
Rough and smooth
Mountain waves are just one type of turbulent air motion that occur downwind of high terrain. Mechanical turbulence caused by air tumbling over peaks and other rough ground is another. Rotor activity caused by winds that rotate violently in compact vortices is another. Turbulence associated with lenticular clouds is yet another. Finally, unstable air kicked up by a front's passage over high terrain can also cause downwind turbulence.
Wave activity is generated by flows of streamlined, stable air becoming displaced as they en-counter elevated terrain. The air flow is displaced upward at first, then it sinks, then rises again in a series of oscillations that finally damp out — at ranges as far as 200 miles downwind. It all depends on the speed of the winds aloft.
Sailplane pilots can use the upward portions of these oscillations to remain aloft for long distances and periods of time. Experienced sailplane pilots can, and do, travel great distances up and down the lee slopes of mountain ranges using the ridge lift that so often occurs very close to the zones downwind of high terrain.
As you might suspect, there can be a great deal of shear and turbulence in mountain waves. The up- and-down motions of the air, along with the changes in wind speed as the air oscillates, can produce a bumpy ride under some circumstances. Other times, the ride can be smooth as silk, with the only sign of atmospheric motion being turbulence-free altitude excursions of the type described at the beginning of this article.
The higher, the better
Wave motions are usually strongest in a vertical slice of the atmosphere that stretches from 1,000 to 2,000 feet above the altitudes of the higher summits to as low as ground level. So the best avoidance strategy is to fly as high as practical when downwind of ridges or mountains.
The stronger the winds at ridge height, the greater the chance of stronger turbulence — and the greater the vertical oscillations of the lee air flow, which means that your altitude excursions may be greater. To help guarantee a smoother ride, some mountain flying experts strongly suggest that pilots of small general aviation airplanes avoid flying near mountains if the winds at summit level are more than 20 knots.
Flying technique
Of course, you've had a preflight briefing that mentioned the chance of mountain waves or other turbulence-inducing factors. And you checked for any pireps that might confirm any turbulence predictions. If you're doing your own sleuthing via Duats or Internet weather providers, you'll want to check airmets for any mention of turbulence. More specifically, you should ask or look for any Airmet Tango listings, which is where all turbulence warnings are posted. (Airmet Sierra is reserved for mention of IFR conditions and mountain obscuration; Airmet Zulu is for icing.)
Should you encounter altitude excursions in VFR conditions, bear in mind the maximum elevation figures for the terrain you're flying over, and stay a safe margin above them. In many cases, the best strategy might be to turn away from the high ground and fly farther downwind, and away from the worst bumps.
If flying under instrument flight rules, advise ATC of your flight conditions and ask for an altitude block that will allow you to ride out the ups and downs.
Slowing to VA (maneuvering speed) is a must if the turbulence becomes aggressive to the point of pushing the "moderate" definition (unsecured objects fly around the cockpit, occupants feel definite strains against seat belts and shoulder harnesses). It can't be overemphasized that VA varies with an airplane's gross weight. The heavier your weight, the faster your VA will be; the lighter, the slower the VA.
More fortunate pilots have airplanes with placards or operating handbooks that publish a range of maneuvering speeds for several different weights. Some airplanes use published turbulence penetration airspeeds. Having these makes it easier to come up with a VA that's correct for your actual weight. Too often, the only VA that we memorize is the only one published — and that's the one for use at maximum gross weight. If you always use this VA in turbulence you could be in for the surprise of your life.
Why? If your actual weight is less than gross and you fly at a max gross weight VA you could experience airframe damage or airframe failure in turbulence. You could reach your airframe's load limits well before the airplane's stall speed. Fly at the correct VA, and the opposite should happen: the airplane will stall, and thereby unload the wings of dangerous G forces, before there's a chance of airframe damage.
A Travel Air's travails
An April 12, 1999, crash of a Beech Travel Air shows how the mountain-related turbulence caused by even the relatively diminutive Appalachians can do you in. The previous day, a cold front had passed through the eastern seaboard. Behind it, strong, gusty winds pushed in unstable air from the northwest. Warnings were out for moderate to occasionally severe turbulence below 12,000 feet because of northwesterly winds over rough terrain. According to a preliminary report from the National Transportation Safety Board (NTSB), the pilot and his three passengers boarded the Travel Air at Roanoke, Virginia's Roanoke Regional/Woodrum Field and took off with full fuel tanks at 1:57 p.m., bound for Claxton, Georgia, on an IFR flight plan. The pilot was cleared to climb to 8,000 feet, but a controller stopped the airplane's climb for traffic at 6,000 feet. The pilot said he had received a HIWAS (hazardous in-flight weather advisory system) broadcast warning of the chance of turbulence. After the Travel Air was re-cleared to 8,000 feet, there was no response from the pilot. Based on radar tapes and witness statements, the airplane crashed at about 2:12 p.m., killing all aboard.
Virtually all witnesses and other pilots in the area at the time confirmed that the turbulence and winds were strong. One witness said he saw the Travel Air, "and the wings were rocking side to side...then the wing dropped off to the right and...it did a complete doughnut and then it went straight down."
A Virginia State Police sergeant happened to be flying in a Cessna 182 at 9,000 feet near Roanoke at the time of the crash. He said that he was unable to control either his altitude or airspeed. In an attempt to maintain his assigned altitude, the sergeant said he slowed to 52 knots with full power, but that he still descended at 700 feet per minute. Then came an uncommanded climb and an acceleration that he could not control. "With the power all the way out and the nose pushed over, I couldn't stop the climb," he said. "Next thing I know, the airspeed indicator is in the yellow arc and my GPS is showing 190 knots over the ground. In my 25 years of flying, I never experienced anything like it."
Other pilots in the Roanoke area, including those flying a Piper Cherokee and a Beech King Air B200, also confirmed wild rides. "It was a terrible day," the King Air pilot said. "That was a day I would rather not have flown in. I strapped in really tight and we were coming up out of our seats on arrival and departure."
The final report on the Travel Air crash will be more than a year in coming. But from first-hand experience I can tell you that April 12 was indeed a rowdy day in the lee of the Blue Ridge. That was the day Associate Editor Pete Bedell and I flew from AOPA's home field at Frederick, Maryland, to the Sun 'n Fun Fly-In at Lakeland, Florida. Our route passed almost directly over the Travel Air's accident site — but three hours before the time of the crash.
Initially, we flew at 8,000 feet, and beneath an overcast in turbulent air. A climb to 10,000 feet took us up through an icy, bumpy layer of cumulus clouds, and the remainder of the first leg of the flight was flown at 12,000 feet — well above the cloud tops. Up there, the air was a welcome change from the beating that we took down low. It was surpisingly smooth. Up high worked for us. But just a few thousand feet below, hearts were pumping fast.
You don't need the Rockies
This all goes to prove that you don't have to be flying in or around tall mountain ranges like the Rockies to experience hazardous mountain waves and winds. Even a small hill can have a great impact on the air's stability. I'm looking out my window now, and about five miles west of AOPA's offices is Catoctin Mountain. Actually, it's more of a small ridge than a mountain. At its highest, the Catoctin ridge is about 2,300 feet msl. But the ridge lift created by westerly winds flowing over that small bump of land once helped a local sailplane pilot set a record by climbing to 22,000 feet. That kind of lifting power from such a small rise in terrain is something to think about.
Wx Briefs
- Vaisala Inc., a manufacturer of automated weather observing systems (AWOSs), has received FAA certification of its new SA20 StrikeAlert thunderstorm sensor. The sensor can detect and report lightning strikes within 30 nm of an airport and provides azimuth and distance information to pilots who tune into SA20-equipped AWOSs. The first 10 sensors will be installed at airports in Minnesota.
- Scientists from the National Center for Atmospheric Research (NCAR) and the U.S. Army's Cold Regions Research and Engineering Laboratory (CRREL) spent this past April studying the freezing drizzle and freezing rain conditions on New Hampshire's Mt. Washington. The purpose of the program is to im-prove in-flight ice detection using remote sensors and computer models of small-scale areas. The summit of Mt. Washington is famous for its consistently high winds and nearly omnipresent icing conditions in clouds and precipitation. — TAH
E-mail the author at [email protected].
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