"Peter Duniho" wrote in message
...
"Chris W" wrote in message
news:gj%ae.640$zv1.481@lakeread07...
Is there an altitude above the ridge line at which the "mountain wave"
effect isn't significant?

Probably would be pretty negligible at 23,000 miles. For example.

Seriously though, yes...you can sometimes fly high enough to avoid the
wave. But that depends on the height of the terrain, the strength of the
wind, and of course the type of aircraft. There is no reliable way to
know ahead of time how high you need to fly to avoid it.

That said, there's being high enough to get all the way out of any
noticeable effects of the wave, and then there's being high enough to
avoid the wave forcing you too close to the terrain. The former may be
impossible, depending on the situation. The latter is usually possible.
I have rarely experienced altitude excursions of greater than 2000-3000
feet as a result of mountain wave, so that's a pretty reliable margin for
crossing ridges.

Of course, there is the question of whether mountain wave can actually
push you into the ground. I've never actually heard of that happening,
and for it to do so, the part of the air mass you're flying in would have
to hit the ground as well. That happens in microbursts, or under virga,
for example, but you'd have to be pretty darn close to the ground in the
first place for a mountain wave to push you into it.

From a practical perspective, a couple of things to consider: higher up
may provide less turbulence (though, don't try to fly through a rotor
cloud). Also, if you want to most efficiently use the mountain wave to
your advantage, pitch up and slow down while it's making you go up, and
pitch down and speed up while it's making you go down. This will increase
the magnitude of your altitude changes, but you'll be spending less time
during the "bad" down areas and more time during the "good" up areas.
Fighting the mountain wave is just that: fighting. And no one wins a
fight with Mother Nature.

Of course, in practice you may have upper and lower limits to acceptable
altitudes, and those need to be taken into account. But inasmuch as you
can allow your altitude to vary with the wave, let it.

Pete


Mountain wave systems extend to and often into the stratosphere so it is
unlikely that you can fly above them unless you have a new airplane that we
haven't been told about :-).

Mike
MU-2