Each year a number of airplanes get bent or broken while flying in mountainous terrain because their pilots weren't prepared for the challenges. Mountain flying requires a clear understanding of and a healthy respect for those challenges. AVweb contributor R. Scott Puddy has several years of experience flying light aircraft over some of the most unfriendly terrain in the continental U.S. Here is his list of the "Top Ten" things the well-prepared pilot will consider when flying in the mountains.
July 10, 2002
|About the Author ...
R. Scott Puddy was an ATP, CFI, CFI-I, MEI who taught out of the
Buchanan Field Airport (CCR) in Concord, California. Scott was type-rated
in the Beech/Raytheon King Air 300 series but regularly flew a V35 Bonanza
and practices law in San Francisco.
On the morning of June 18, 2002, Scott perished doing what he
loved: practicing aerobatics in a Yak-52, in the mountains of Brentwood,
He contributed many articles about flying to AVweb in recent
years and also worked as our features editor. His enthusiasm for
aviation and his intensity in pursuing it were simply extraordinary.
Even more extraordinary was his dedication to sharing his passion for
flying with others, by teaching and writing. He touched a lot of lives,
undoubtedly saved many, and his legacy of written words will continue
to do both for many years to come. Scott's warmth, wit, and keen
intelligence will be missed by all who knew him and worked with him.
passions are skiing, flying and flying to skiing destinations as a way to
combine the first two. My fly/ski destination of choice is the Truckee Tahoe
Airport (TRK) in Truckee, California, a GA facility nestled in the southern end of
a valley in the central Sierras, 15 miles from Squaw Valley. Squaw Valley
averages 34 feet of snowfall per year and, in the winter, Truckee frequently
records the low temperature reading for the nation. In the summer months, with
temperatures in the 80s at 5,900 ft. msl, density altitudes over 8,000 feet
are common. In completing more than 175 flights to or from Truckee (and
canceling countless others), I have compiled my own Top Ten list of practical
considerations for mountain flying.
1. Takeoff And Climb Performance
It is elementary that, before takeoff, pilots need to be able to predict
the airplane's performance and ensure it will have he capability to takeoff on
the available runway and clear surrounding terrain during initial climbout.
The simple fact is that, at high altitudes, more is demanded of the plane and
the plane has less to offer. Takeoffs demand more of the plane because the
reference rotation airspeed and climb airspeed are indicated airspeeds. The
actual airspeed necessary to yield a given indicated airspeed increases as
Let's say your rotation speed is 72 KIAS. On a standard day in still air at
6,000 feet, you would have to accelerate from a standstill to 79 knots not
72 in order to reach rotation speed. On a day with an 8,000 foot density
altitude, one would have to accelerate to 81 knots. That is a 12.5 percent
increase over the standard day, sea level value. Similarly, at a density
altitude of 8,000 feet, you have to be moving through the air at 103 kts to
achieve a Vy of 90 KIAS.
Of course, unless you are flying a turbocharged airplane or burning Jet A,
the engine will also have sharply reduced performance available to meet this
increased demand. Therefore, you need to pull out your dusty old pilot's
operating handbook (POH) to see what you can expect to see in terms of takeoff
and climb performance.
With a strong southerly wind, downdrafts
can to be expected when climbing out off of Rwy 18 at TVL.
Unfortunately, the POH's bleak report is only half the story. The POH
values are for still air but the mountain air you will soon be flying in is
often circulating. Unlike in the flatlands, when air circulates in the
mountains, it moves up and down because it follows the terrain. If wind is
blowing down a hill there are downdrafts. Conversely, if wind is blowing up a
hill there are updrafts. If you are looking down the runway into a strong wind
and see sharply rising terrain off the end of the runway, you can expect to
encounter downdrafts during your initial climbout. If the plane is capable of
climbing at 400 fpm and you encounter a 600 fpm downdraft, the math is
relatively easy: You will not climb.
Neither the POH nor the local Automated Weather Observation System can
quantify the downdrafts you can expect. Nevertheless, it is your obligation as
pilot in command to determine in advance of the takeoff roll that the plane is
capable of achieving a positive rate of climb on climbout. That is where
experience comes into play. An early turn to a heading away from the down
drafts is usually a good idea. Sometimes, waiting until the winds subside is
an even better idea.
Of course, you can also use the presence of mountain updrafts and
downdrafts to your advantage. The updrafts generated by air encountering
rising terrain can extend thousands of feet above the surface. If you are
circling in a valley to gain altitude to clear surrounding terrain, you might
as well camp out in a column of rising air. On a windy day, you can generally
find one on the downwind side of a valley or the upwind side of a hill. Adding
a 500 fpm updraft to your plane's meager 300 fpm rate of climb is like adding
a turbocharger, only it's cheaper, lighter and easier to maintain.
2. Landing Performance
If it is harder to get an airplane into the air at altitude then shouldn't
it follow that it is easier to get an airplane onto the ground? Nope.
Increased ground effect, increased touchdown speeds, runway ice and (once
again) downdrafts make mountain landings a challenge. The point is that you
can float a long way if you fail to control your airspeed when landing in
rarified mountain air.
Control thy airspeed on approach!
Ground effect is an often misunderstood phenomenon. It results because the
ground interferes with the circulation of wingtip vortices, causing an abrupt
reduction in induced drag and since thrust remains the same a
corresponding excess of thrust over drag. When thrust exceeds drag, the
airplane will accelerate, or climb, or both. That, coupled with an increase in
the wing's angle of attack, is the sensation you feel in ground effect.
Wingtip vortices result from the circulation of air from the high pressure
area under the wing to the low pressure air over the wing. A by-product of the
vortices is wing downwash: The air flowing from the portion of the wing
influenced by the wingtip vortices is pushed down. At that portion of the
wing, the relative wind is from a more upward direction and lift, which is
perpendicular to the relative wind, is tilted rearward. The reward-tilted lift
force has a vertical component, which is lifting the plane, and a horizontal
component, which is holding the plane back. That induced horizontal component
of lift is what we refer to as "induced drag." When ground
interference impedes the circulation of the wingtip vortices, induced drag is
diminished. That is ground effect. Induced drag increases at higher altitudes
as air density decreases and so does ground effect. It follows then, that the
greater the induced drag, the greater the effect of eliminating induced drag.
When you finally make it through ground effect and contact the runway, your
ground speed will be higher than normal. As discussed above, if your airspeed
is 72 KIAS at touchdown on day with an 8,000 foot density altitude, your
ground speed will be 81 knots or 93 mph. If you land at, say, 80 KIAS, your
groundspeed will be 90 knots or 104 mph. Try that in your car; that's moving.
It's best to keep your airspeed under control during the approach to landing
and to keep the airplane pointed straight after touchdown.
If it happens to be a winter touchdown, you have the potential of ice on
the runway to contend with. If you encounter ice on the runway while braking
there are two possibilities: One, the entire airplane may not stop or, two,
half the airplane may stop while the other half keeps going, an undesirable
maneuver also known as a groundloop. You need to plan on having less than
optimal braking and, to the fullest extent possible, minimize use of the
brakes on rollout if there is a possibility of encountering runway ice.
...And Downdrafts On Approach
Finally, there are issues with downdrafts. Beware the perched atop a
mountain plateau. If the ground drops off sharply in front of the approach end
of the runway and there is a strong wind along the runway, you can expect to
encounter a strong downdraft on short final if you use the runway numbers as
your landing target. You will be low and slow and will need to overcome both
the airplane's established sink rate and the down draft before you can even
think about climbing all this with an airplane that is already hampered by
the effects of high density altitude.
Any time you are concerned about encountering down drafts, altitude above
the ground is your best friend. When landing on a mountain plateau runway in
windy conditions, you should select an aim point about a quarter-length down
the runway. Given the wind, your touchdown groundspeed will be lower so you
shouldn't need the entire runway to stop the plane. Your extra altitude when
you approach the threshold will keep you up out of the downdraft or on the
runway if you do encounter it.
3. Cold Starts
Winter overnight temperatures in the minus 20s give a whole new meaning to
the phrase "cold start." The best options, in order, are: Leave the
plane in a heated hangar; transfer the plane to a heated hangar the night
prior to departure; preheat the engine before the departure; pull the prop
through before attempting to start the engine.
If a heated hangar is not available, the airframe requires special
attention as well. If the plane has been parked outside for several days in a
freeze/thaw cycle, there is a possibility of ice buildup anywhere that water
could penetrate. The empennage, ailerons, and flaps deserve special attention
along with all vent lines (fuel tanks, battery, and crankcase). An overlooked
icicle could ruin your whole day.
If a heated hangar is not available you may need to deice the plane. You
can purchase covers for the wings and empennage and they can save you hours of
time in deicing an airplane at below-freezing temperatures. Other useful
equipment includes a nylon-bristle push broom (to brush off loose snow), a
good snow shovel, deicing fluid, scrapers, plus warm waterproof gloves and
apparel. Allow plenty of time. Airplanes have a lot of surface area.
4. Hot Starts
At least starting the plane for a warm weather mountain departures should
not be a problem, right? Wrong again. If you have cooked in the mountains you
know that boiling or steaming food at high altitudes takes longer because
water vaporizes at a lower temperature. The same holds true for avgas. It is
difficult to hot start some fuel injected engines at sea level because the
heat from the engine causes vapor lock in the injection lines. That problem is
magnified in the lower ambient air pressure at high altitude airports as the
fuel vaporizes at a lower temperature. Be sure that you have your hot-start
procedures down cold.
5. Winter Weather
The Sierras have winter weather systems that blow in from across the
Pacific and storms or low visibility conditions that seem to arise from
nowhere. You can plan for the storms that blow in from the Pacific because
they will be highlighted on the morning reports of the Weather Channel. You
have to anticipate the possibility of the storms that arise from nowhere by
understanding their cause.
The three essential ingredients to storm cloud development are moisture, an
unstable airmass and a lifting mechanism. If a moist, unstable airmass is
blown up the face of a mountain range, the terrain provides the lifting
mechanism. Therefore, you can have clear weather in the valley but sudden
storms in the mountains. Similarly, you can encounter a sudden low overcast in
the mountains if a moist, stable airmass is blown up the face of a mountain
range. Winter weather in the mountains is changeable and unpredictable. Again,
the well-prepared pilot will anticipate these potential changes based on a
thorough understanding of weather theory and the surrounding conditions and
will be ready to execute the standard 180-degree turn when necessary.
6. Summer Weather
Summer weather in the mountains can be even less predictable and more
severe. The summer sun heats the surface and generate rising air that only
exacerbates the inherent topographical lifting mechanisms. Also, summer
mountain storms tend to occur in the afternoons and then subside in the early
evening as the sun's heat dissipates. It's best not to plan on a mid-afternoon
summer departure. If you are looking at level three thunderstorms at 5:00 in
the afternoon, wait a couple of hours. Conditions could be CAVU by 8:00 p.m.
7. IFR Operations
Your IFR capabilities will be of limited use to you in flying to and from
mountain airports. Generally, your use of instrument approach procedures is
limited to descending through a high overcast layer or to climbing through
thin ground fog. Conditions more severe than this usually require that
light-plane operators take risks that many pilots would consider to be
Instrument arrivals are of limited use because most of the available
approaches have very high minimum descent altitudes. In most cases, the
problem is that, given the terrain surrounding the airport, there is no way to
fashion a missed approach procedure beginning at a low altitude over the
How do you shoot a missed approach from
Truckee has an RNAV approach and a GPS approach, but the MDAs are 2,300 ft.
AGL and 1,440 ft. AGL, respectively. The localizer approach into South Lake
Tahoe Airport, 27 miles to the South, is directly over Lake Tahoe which is
notably flat. However, the approach terminates into a box canyon and the MDA
is 1,026 ft. AGL in order to meet TERPS missed approach obstacle clearance
There are at least three factors that limit the use of instrument departure
procedures. First, at the higher altitudes, many GA aircraft would be unable
to meeting the climb gradient requirements imposed by the terrain surrounding
the airport. For example, the departure procedure for TRK requires a climb
gradient of 425 ft. per nautical mile (635 fpm at 90 KIAS). Second, given the
mechanical lifting generated by the mountainous terrain, IFR mountain weather
is almost always accompanied by forecasts for at least occasional moderate
rime ice in clouds and precipitation. Third, if the weather is anything lower
than a high overcast, the IAP for the departure airport would probably not
have minimums low enough to facilitate a return to the airport. Therefore, if
you were to encounter problems (such as icing) during an instrument departure,
your closest available landing airport could be many, many miles away.
8. Night Operations
Nighttime VMC under a high overcast in
Your instrument flying capabilities will come in handy for nighttime VFR
mountain operations. Particularly if there is a new moon and a high overcast,
the mountains can be pitch black at night. Flight under those conditions is
IFR for all practical purposes, despite the absence of any measurable
restriction to visibility. There may be no visible horizon, no lights on the
ground and no means of differentiating between the sky and shear granite
walls. It is up to you to devise the blind departure procedure. The published
instrument departure procedure (if you can meet climb gradient requirements)
is one alternative. Another, if you know you can safely climb to pattern
altitude, is to climb in the pattern to a safe en route altitude and then turn
on course. That would allow you to keep track of your position on climbout,
make it easy for you to report your accurate position to other departing and
arriving aircraft and keep you close to a lighted runway in case a problem
arose early during the flight.
9. En Route Operations
Where should we set up camp?
Even if you can land and take off from your mountain destination, you still
need to be able to get there and make it back home. Mountains can be desolate
and inhospitable terrain, as the namesakes for Donner Lake (8 nm West of TRK)
would attest. IFR is a good idea for en route operations, but this time it
stands for "I follow roads." Keep civilization close at hand if
possible. I overfly Interstate 80 to Truckee and carry minimum survival gear,
including a down sleeping bag, just in case.
Some GA aircraft may be unsuitable for operations in actual IMC because of
the high MEAs. The MEAs en route to Truckee, for example, are between 11,000
and 13,000 feet MSL. The risk of icing is ever-present in IMC over mountains
and, if your plane is not equipped for flight in icing conditions, you always
need to have a plan in mind to escape any icing conditions you might
Remember the downdrafts. If you are flying at or near the MEA you will
probably be low enough to be within the influence of the mountain waves over
the highest terrain. You are most at risk when you are flying into the wind
toward rising terrain.
10. Plan B
In light of all the considerations discussed above, it is more imperative
than ever that you have a Plan B in mind at all times when flying GA aircraft
in the mountains. You should have another means of completing the trip, an
alternate route to get you out of trouble or the option of delaying your
return to the flatlands. Any time you feel that you are required to complete a
flight you run the risk of feeling pressured into making a bad decision. If
bad weather is a risk, whether forecast or not, you should have a Plan B in
mind for completing the flight to a safe alternate or via a return to your
departure airport. If icing is a risk, you should have in mind a plan for
escaping icing conditions (preferably by being able to descend to an altitude
where the temperature is above freezing).
A Final Thought
Even if you do everything I recommend in this article, it is not an
adequate substitute for training and experience. However, if you give
consideration to these ten issues before your next venture into mountainous
terrain, you will be a step ahead.