Aero-Tips!

A good pilot is always learning -- how many times have you heard this old standard throughout your flying career? There is no truer statement in all of flying (well, with the possible exception of "there are no old, bold pilots.")

Aero-News has called upon the expertise of Thomas P. Turner, master CFI and all-around-good-guy, to bring our readers -- and us -- daily tips to improve our skills as aviators. Some of them, you may have heard before... but for each of us, there will also be something we might never have considered before, or something that didn't "stick" the way it should have the first time we memorized it for the practical test.

Look for our daily Aero-Tips segments, coming each day to you through the Aero-News Network.

Aero-Tips 11.24.06

As pilots-in-training we usually hear about mountain wave conditions -- wildly rising and falling currents of wind blowing up and over a mountain or ridge, with extremely turbulent rotor clouds, heavy wind shear entering and exiting the wave, and often-smooth wind cores that rise or (dangerously) fall at rates far beyond the descent and climb ability of most airplanes. Most aviation texts warn of mountain wave as a western-US phenomenon... but some of the worst mountain wave conditions, especially in wintertime, can form in the eastern United States.

A Charlotte Story

I was in a Cessna 172 en route from sunny Florida to Charleston, West Virginia for Christmas. My wife and our infant son were in back, snug beneath a thick blanket in the clear winter air. Our route to "Charley West" took us just west of Charlotte, NC hugging the east side of the Great Smoky Mountains for a crossing near Asheville. Winds aloft were from about 320 degrees, and running at about 35 knots at 9000 feet. There were no clouds anywhere along the ridge. Airliners on the Approach frequency began to spark with reports of wind shear and strong turbulence inbound from the north and west. I knew my little 172 was in for a wild ride if I stayed on course. Instead, I asked for a turn to 270 and crossed the lower end of the Smokies before they grew to their maximum height. We crossed the ridge at a 45-degree angle (just like the mountain-flying books say) in smooth air, and were soon on the west side of the ridge for a smooth ride upwind of where the turbulent air formed.

Mountain wave

Mountain wave conditions result when very-stable air hits a mountain or ridge at high velocity. The wind deflects upward, rising in altitude, until it cools and descends under its increase density. The wind-flow accelerates and rebounds off the surface, rising and falling again, sometimes for a great distance. If the wind is strong and the air very stable, areas immediately outside this "wave" are smooth and undisturbed. Flight into or out of the wave entails crossing a big discontinuity that creates strong and dangerous turbulence.

Some of the worst mountain wave conditions happen in the eastern US, downwind of the Smokies, the Blue Ridge, the Appalachians and the mountains of New England, especially New Hampshire's Mount Washington. The turbulence may not actually be greater than what we find in the Rocky Mountain west, but the airspace is much more crowded with airplanes, and pilots are less wary of dangerous mountain waves at relatively low altitudes in the East. Watch for:

• Stable air (positive values for Lifted Index);
• Wind speed at ridge height (6000 and 9000-ft reports in most cases) exceeding 30 knots; and
• Wind direction roughly perpendicular to the ridge.

These three together spell a probable mountain wave on the downwind side of mountains in the East.

Aero-tip of the day: Some of the strongest mountain wave conditions can occur in the eastern US, especially in winter when winds are typically strongest.

FMI: Aero-Tips