Aero-Tips!
A good pilot is always learning -- how many times have you heard
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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.
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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.