# When crossing a mountain ridge at low height above terrain what consideration is given to turbulence?

Some background: I am not a pilot, but a researcher studying air-flow across mountain terrain using Computational Fluid Dynamics (CFD).

During my studies, I have been particularly interested by high-altitude ridges (2 to 3000m a.s.l., say 10000 ft) that are exposed to regional winds that are generally across the ridge. I have been seeing very interesting turbulence patterns above, and in the lee of the ridges (rotors).

Here is the question: what instructions are given to pilots needing to cross such a ridge at low heights above the terrain, in either direction relative to the wind? What does common sense dictate?

I am naturally thinking mostly of helicopter pilots doing mountain work, but any other aircraft are very welcome (fire-fighters, ULMs ...).

• What instructions? - "Remember Steve Fosset"? Commented Jul 20, 2015 at 8:41
• @RedGrittyBrick Yep, I can imagine that is not the desired outcome of a flight. :-) Basically, I am looking for anything ranging from instructions given by official bodies, down to the more direct language a flying instructor could use inside the cabin. Anything more precise than just "stay the h*** away from it!" The reasoning behind such instructions would be most interesting. Commented Jul 20, 2015 at 8:48
• I've done a little helicopter mountain flying, as number 2, and the general rule is "don't fly on the leeside". If you need to cross a ridge, and the wind is behind you, don't cross the ridge. There is a theoretical line called the "demarcation line" where the wind changes from rising, on the windward side, to falling on the leeward side. The falling air, coupled with the possibility of rotors as you mention, combine to "don't fly there". Don't cross the ridge, fly along it and look for an alternative - plan ahead! Commented Jul 20, 2015 at 9:56
• I found this which might be useful Commented Jul 20, 2015 at 10:10
• @Simon, thanks for the link, there is a clear figure that shows the demarcation line you mention. Commented Jul 20, 2015 at 10:15

From an older FAA Publication on mountain flying:

Mountain Wave

When the wind speed is above about 25 knots and flowing perpendicular to the ridge lines, the air flow can form waves, much like water flowing over rocks in a stream bed. The waves form down wind from the ridge line and will be composed of very strong up and down drafts, plus dangerous rotor action under the crests of the waves. If enough moisture is present, lenticular clouds can form to give a visual indication of the wave action. These clouds are reported in the remarks section of hourly sequence reports as ACSL (altocumulus standing lenticular) or CCSL (cirrocumulus standing lenticular)

Pretty much sums up what you're seeing I guess. As for action:

Ridge and Pass Crossing

On most mountain flights, you will need to cross at least one ridge or pass. Experienced pilots recommend crossing a ridge or pass at the ridge elevation plus at least 1,000 feet. If the winds at mountain top level are above 20 knots, increase that to 2,000 feet. Plan to be at that altitude at least three miles before reaching the ridge and stay at that altitude until at least three miles past it. This clearance zone will give you a reasonable safety zone to avoid the most severe turbulence and down drafts in windy conditions.

If conditions or airplane performance dictate, you may need to fly along the windward side of a ridge to find updrafts for gaining altitude before crossing a ridge. You may also need to circle before reaching the ridge if climbing out of a valley airport. When you actually cross a ridge, you should do so at a 45• angle to the ridge. This allows you to turn away from the ridge quicker if you encounter a severe downdraft or turbulence. Once you have crossed the ridge, turn directly away from it at a go• angle to get away from the most likely area of turbulence quickly. Plan your crossing to give yourself the ability to turn toward lower terrain quickly if necessary.

As for helicopter mountain flying specifically, I'm afraid you are unlikely to find somebody on here with that experience. You might have better chance on searching for it online and phoning up flight schools that offer such courses (I found a few in the states) for information.

• This is the type of instructions I was looking for, thank you. "you should do so at a 45• angle to the ridge. This allows you to turn away from the ridge quicker" The rationale behind this is not clear to me: would going straight across not be a better way of getting out the downdrafts encountered? Commented Jul 20, 2015 at 10:19
• @ALANWARD If you're flying towards a ridge and you encounter very strong downdrafts, then you may not be able to get over the ridge at all. That means you need to turn back and being at a 45° angle means you can turn away with a 90° turn or even less. If you were flying straight at the ridge you would have to make at least a 135° turn (or whatever) to do it, requiring more space to turn. Mountain flying can mean limited room for maneuvering (including vertically; a light aircraft could be close to its service ceiling), so always having a way out is really important. Commented Jul 20, 2015 at 13:15
• @Pondlife OK, so that would be flying towards the ridge from the leeward side. Understood, nice comment. Commented Jul 20, 2015 at 13:19
• @ALANWARD: It is easier to always approach ridges at angle than think about whether you are approaching from leeward side and need it or windward side and don't. So it's needed when you are approaching from leeward side, but it's recommended always to keep the procedure simple. Commented Jul 20, 2015 at 13:58
• @JanHudec Makes perfect sense, to keep it simple. Commented Jul 20, 2015 at 21:20

Power pilots (which I am also) are the wrong pilots to ask about this topic. We are trained to stay away from this kind of topography for all the reasons listed

Glider pilots fly ridges and mountain wave all the time, very close to the ridge line. Ridges and mountains provide all sorts of lift, which is the key to great soaring, and we are taught to fly IN it, not away from it.

My recommendation is to read the first part of Helmut Reichmann's Cross-country Soaring called "Flight in Lift" which details the answers to the questions you're asking.

As far as helis are concerned, which I also fly, the the big considerations for mountain flying is how much excess power you have. When you encounter a downdraft in a heli, you have to add power to arrest a decent, so it's important to know your power, weight, and density altitude limits. There are a few helis that are known as good mountain machines, like the AS350.

In any aircraft, if you're going to fly in the mountains, you have to be able to read the signs: surface winds and winds aloft, cloud patterns, areas of airmass convergence, etc

• I was wondering what glider pilots are told, thanks for the reference. Commented Jul 21, 2015 at 5:45
• Interestingly, AS350 around where I live are used mostly for sanitary transport between hospitals (small regional to large), just flying down large valleys. But they also get to do some primary intervention from time to time, which makes for more "interesting" flying. Commented Jul 21, 2015 at 5:50

I regularly watch a couple of YouTube pilot video channels and this particular one had a recent series on mountain flying. There too much there to re-state here but I think this particular video would be very interesting to you.

There are additional videos from this series that you might also find helpful.

Yeah, the instructions are DON'T GO NEAR ROTORS OR YOU MIGHT DIE.

You never want to pass close over a ridge, even if does not have lenticular cloud or other visible sign of downdraft. Once you get caught in the down draft it can be almost mathematically impossible to escape. My instructor's instructor was killed in Maine by trying to cross a mountain too close.

The procedure is to navigate to a valley or pass where you can clear the mountain by a healthy margin.

• You seem to confirm the impression I had - that most pilots consider low-level mountain flying a bit like a death trap. I wonder what a "healthy margin" could be - and how to figure it out. Commented Jul 20, 2015 at 21:19
• Eh, you might die from lots of stuff. Every glider pilot who has flown wave has encountered rotor. Rarely kills 'em. Oh and You're thinking of the down wave, not the rotor, which, as the name implies rotates, not just down drafts.
– rbp
Commented Jul 21, 2015 at 2:15