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
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
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
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
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
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
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
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
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?
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.