Mountain Waves and airliners


New Member
What is the technique taught for airliners that encounter a mountain wave at cruise altitude? Are you required to try to maintain altitude? If so, to what extent?

Eeek. No one “touched” this question. Because I am new to this board, was it a silly or ridiculous question?

From what little I recall from the few sailplane flights I made (actually found a moderate wave and “went up” quickly – pretty cool without an engine), the wave is glassy smooth, but with a lot of vertical component – either way depending on where you are in the wave. It was the rotor and shear areas that one wanted to watch out for. So I was just wandering, if you are cruising in an airliner and encounter wave action, is it of any concern? (I hope it is not too silly a question!)

Avoid avoid avoid!

Usually, the dispatchers will route our flight clear of any known mountain wave activity. I can't remember the exact procedure, but we'll usually slow down to turbulence penetration speed, climb or turn off course to avoid the worst of it.
I've been in mountain wave turbulence before. It doesn't happen that often, but the true danger is that it can happen in clear air, hundreds of miles from any terrain features.
Things brings up an interesting topic: Mountains and mountain flying. I'm sure that there have been pilots like myself who do all their training over flat terrain, then go fly one day for an airline requiring flight over mountainous terrain.......guess my question do you train for that being in a flat state?? Do the airlines take that into consideration?
Mountain waves are considered seriously. Depending on the severity, you may or may not be able to hold altitude. The autopilot will try to hold altitude in the most basic designs. On the more sophisticated FCC type systems (such as on the MD-11, 777, A-340s, etc), the system will try to hold altitude until you near a high or low speed buffet margin, then may leave the altitude to maintain speed with the margins. Actually, I don't know that the 777 has that model in it (yet, Boeing is going to be including flight envelope protection in the future), but the MD-11 and the Airbus's do.

The problem is that autopilot algorithms are really a built up version of the original altitude hold function, with tons of "patches" to make them work. There hasn't been much change to the basic logic, but back to the topic at hand.

The main issue is usually keeping it on speed. The autothrottles move too slowly to keep it out of the buffet margins in all but the real mild waves, so you have to either disengage them or override them to keep it on speed. This will generally prevent getting into the high or low speed protections where the system will use pitch to maintain the speed envelope (at least on the MD-11).

I have been fortunate in that I have never had a problem maintaining altitude in any wave action. Our charts (domestically) are marked with areas of known severe wave action, so we can avoid them if the winds are blowing across the Rockies or the Sierra (no other ranges in the lower 48 produce significant waves). When we know of them or encounter them we slow to our turbulence penatration speed (this is NOT Va) and fly the slower of the IAS or Mach range, and within the range we use, we fly to the upper portion of that when up high (gust factor at altitude is less due to lower q for the same velocity of gust, however, loss of control due to stall is a real issue. Overspeed is not likely to result in loss of control, but stall is fairly probable). So, what it works out to is the mid-range for the IAS range or the upper end of the Mach range, for practical purposes, as the IAS range is only applicable below about FL280, where the q is higher.

There have been cases of aircraft experiencing structural damage during wave action. An Emery DC-8 had an engine separate over the southern Rockies a few years ago, for example.

Incidentally, the U.S. airline that pays the most attention to forecasting and avoiding areas of known turbulence is NWA. For some reason the other carriers have never gotten on that bandwagon, although there is some talk of that happening in the future. I don't expect my carrier to do it, though, boxes don't care!
Hey seagull-

Thanks for the info – very interesting. It would be interesting to know what the various accelerations were on that DC-8 to make the engine separate.

BTW, as a pax, I sometimes wonder if I wouldn’t be treated better as a box, turbulence or no turbulence.
Hiya Zone,

You can notice a little mountain wave activity by your airspeed increasing and decreasing as the AP maintains altitude. Also be looking for the lenticular clouds.

At NWA, we have a very sophisticated system for predicting mountain wave activity. Our Met people have data going back something like 60 years and they have very precise data on forcasting the location and intensity of mountain wave activity. Our Jepp high charts have special mountain wave routes and mountain wave avoidance routes printed on them.

If we get a Turbulence Plot (TP, for short) over ACARS or in the WX designating a mountain wave area, we usually are flight planned on a deviation route to avoid the area, and we have different codes for the expected intensity. Sometimes we avoid the route, sometimes the route AND altitude.

We also get TPs for clear air, upper front, trough line and PIREP turbulence, among other things. You get a MSG with the lat/longs and you plot them on a chart. Works great for TRWs too.

On March 5, 1966 a BOAC 707 was at 16,000' downwind of Mt. Fuji in Japan. It was caught in violent turbulence and the vertical fin was torn off. When it departed the aircraft to the left it took the left horizontal stabilizer with it. The Tail section came off next and all 4 engines were ripped from their pylons. A US navy A-4 was in the area and was told to look for the wreckage of the 707. The pilot was caught in severe turbulence and lost control of his aircraft. After he landed the A-4's G-meter was checked and regestered +9 and -4 G's. The A-4 was grounded for inspection. There were winds blowing from the west at 60 to 70 knots and of the 100 other aircraft that flew within 80 NM of the mountain most reported moderate to severe turbulence. Four had experienced severe turbulence within 27 NM on the eastern side.

The above summary was from Air Disaster 1 by Macarthur Job.

It should be noted that Transport category aircraft are only stressed to 2.5 G's
That's unbelievable...

I don't think our little puddle jumping Cessna's would fair too well...
This has nothing to do with airliners but my Dad and a friend experienced a mountain wave in a Bonanza A36 Thurs. night the week before last. They were flying from Roanoke, VA (ROA) to Nashville, TN (BNA) to visit me and experienced a mild one while climbing out of ROA. Somewhere over the Pulaski, VA (PSK) VOR they were getting in excess of 3000 fpm vertical speed (a good bit in an A36). They said it was very smooth and they never had the corresponding sink. Lucky for them I guess. According to an AOPA article I read, mild waves in this area of southwest VA/western NC/east TN are fairly common.
Ooopss, just realized how old this thread was! Oh well, someone may enjoy it!