crossing ridges

Without experience the visual aspects of mountainous country can be very deceptive. It is difficult to be able to look out the windshield and say with any certainty whether or not you are higher than the ridge you are approaching.

In approaching and crossing ridges, the novice pilot is well advised to start out by maintaining a 2,000-foot clearance over both mountains and valleys.

When a mountain ridge is approached from the upwind side there is usually a cushion of air to help you up and over the ridge, providing the wind is blowing somewhat perpendicular to the mountain. Once the wind hits about 20 knots or more at mountaintop level there will be turbulence associated with any downdrafts on the lee side, but this depends a great extent on the stability of the air. Under stable conditions, there may only be a laminar flow with smooth down air on the downwind side of the mountain. And, too, the updraft generated on the upside of the mountain may extend beyond the mountaintop to form updrafts on the downwind side (above ridge level).

If a mountain ridge is approached from the downwind or lee side, the pilot runs the risk of encountering a downdraft and turbulence. When the airplane is flown perpendicular to the mountain there is also a possibility of encountering a downdraft that could cause the airplane to impact the mountain.

I am not implying that there will not be sufficient room to turn away from the ridge if it is approached "head on, but when you approach the mountain at an angel, it will permit a safer retreat with less stress on the aircraft should severe turbulence or downdrafts be experienced.

Remember that in addition to the load factor induced by the turbulence, the load factor also increases during a banked turn. These are not separate forces, but add together for a total force on the airplane. Also, the stall speed increases as the square root of the wing load factor. In a 60-degree bank, the load factor is 2 Gs (gravity units). The square root of 2 is 1.41 or a 41 percent increase in stall speed.

For this reason, ridges are always approached at an angle, a 45-degree angle is recommended by most instructors. Even when the stability of the air is such that the 45-degree angle approach isn't necessary, do it anyway (or at least remain in a position to turn to lowering terrain). The reason for this recommendation is because Mother Nature can provide some surprises.

As an example of such a surprise, consider the pilot who departs Aspen, Colorado with a right downwind from runway 33. He follows the Roaring Fork River to Independence Pass. Knowing he is in an area of prevailing westerly winds aloft, the pilot assumes there will be a cushion of air to help him up and over the pass. But, Mother Nature may have stalled a high pressure area over the Upper Arkansas River Valley near Leadville.

This high-pressure area is a mountain of air that is creating an instability in the atmosphere. In regaining stability, it subsides, that is, the air flows down the mountain toward areas of lesser pressure. This might occur in the vicinity of Independence Pass and the subsidence can overpower the westerly winds aloft, presenting a downdraft where an updraft is expected. So, for safety sake, remain in a position to turn to lower terrain!

#### exiting downdraughts

Whenever you are caught in a downdraft, it is wise to immediately turn toward lowering terrain. Compute the rate of climb for the density altitude that you are flying. Perhaps the POH gives a value of 400 fpm rate of climb at 8,000-feet density altitude. In a strong or sustained downdraft, if the descent rate, after transitioning to the best rate-of-climb airspeed, is greater than your computed best rate of climb, transition to cruise speed to escape the downdraft. If turbulence is a concern, accelerate to the manoeuvring speed.

Perhaps you are descending at 500 feet per minute and transition to cruise airspeed. The airplane may now be descending at 1,100 feet per minute. Accept this temporary increase in descent rate. Although the airplane is descending faster, it will exit the area of the downdraft in lesser time, providing an altitude loss that will be less than fighting the downdraft at the best rate of climb speed. (Steve Philipson, a co-worker in the development of the Civil Air Patrol Mountain Fury program, has run mathematical profiles to prove this point).

Although we (mountain instructors) advocate that it is best to approach mountain ridges at a 45-degree angle, it is not necessary to do so when you are four or five miles away from the mountains. Wait until you are within about 1/2 mile to 1/4 mile from the ridge, then manoeuvre to approach at the 45-degree angle. If you are crossing a series of ridges, you might consider crossing one ridge to the right and the next to the left and so on, to stay somewhat on course.

If you elect to make a flight without maintaining the 2,000-foot clearance altitude above the ridges, you can determine if you have sufficient altitude to cross the ridge by picking two spots. The first spot is whatever you can see over the ridgeline. The other spot is an arbitrary point. In the example to the right, the first point is the bottom of the yellow arc and the arbitrary point is the top of the yellow arc.

As you get closer to the ridge the spacing between the two spots will increase if you are higher than the ridge (the pick arc). If the distance decreases, there is not sufficient altitude to cross the ridge.

don't like this method of determining sufficient ridge clearance. As an instructor I found my students developing "tunnel vision," where they concentrated only on the points and became unaware of other things going on in and around the airplane.

It is easier (and better) to just be aware of the terrain. If you can see more and more of the terrain on the other side of the ridge, you are higher than the ridge and can probably continue. If the terrain on the other side of the ridge is disappearing, get out. Turn around, gain more and try again.

If this technique causes you worry or concern rather than challenging your ability, don't do it. Fly over the ridge with 2,000-foot terrain clearance.

In the photo to the left the airplane has been following I-70 (Colorado) east from the Dillon Reservoir area along Straight Creek. The Eisenhower Tunnel is below the nose to the left. Denver is ahead of the airplane. The ridgeline ahead is the Continental Divide. Loveland ski area is visible on the right side.

The wind is from the west-northwest (left rear of the airplane). The airplane is being flown up the right (south) side of the canyon to be in an area of updraft. Can the pilot make a commitment to cross the ridgeline at this point? NO.

Once the pilot manoeuvres to a position (maintain the 45-degree angle approach) where the throttle can be reduced to idle and the airplane has sufficient altitude to dive and hit the top of the ridgeline, the pilot can make the commitment to cross the ridge. I'm not suggesting that it is proper procedure to reduce the throttle to idle -- this is how you just the proper position for making a commitment.

If you have gotten to this position without encountering a downdraft, any downdraft experienced can be overcome by lowering the nose slightly to maintain airspeed while crossing the ridge. Once you have made the commitment it is a good idea to fly toward lowering terrain. This "safe ridge crossing" technique may be used whether approaching the ridge from upwind or downwind.

A word of caution. If you are trying to cross an extended plateau as opposed to the ridgeline, this rule will not work. In this case you will need additional altitude and you must remain in a position to turn to lower terrain.

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