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can build up with startling rapidity. Mountain flying
need not be hazardous if you follow the recommendations
below:
6.1.1 File a Flight Plan. Plan your route to avoid
topography which would prevent a safe forced
landing. The route should be over populated areas and
well known mountain passes. Sufficient altitude
should be maintained to permit gliding to a safe
landing in the event of engine failure.
6.1.2 Don’t fly a light aircraft when the winds aloft,
at your proposed altitude, exceed 35 miles per hour.
Expect the winds to be of much greater velocity over
mountain passes than reported a few miles from
them. Approach mountain passes with as much
altitude as possible. Downdrafts of from 1,500 to
2,000 feet per minute are not uncommon on the
leeward side.
6.1.3 Don’t fly near or above abrupt changes in
terrain. Severe turbulence can be expected, especially
in high wind conditions.
6.1.4 Understand Mountain Obscuration. The
term Mountain Obscuration (MTOS) is used to
describe a visibility condition that is distinguished
from IFR because ceilings, by definition, are
described as “above ground level” (AGL). In
mountainous terrain clouds can form at altitudes
significantly higher than the weather reporting
station and at the same time nearby mountaintops
may be obscured by low visibility. In these areas the
ground level can also vary greatly over a small area.
Beware if operating VFR−on−top. You could be
operating closer to the terrain than you think because
31 JULY 08
ENR 5.7−4 AIP
15 MAR 07 United States of America
Nineteenth Edition Federal Aviation Administration
the tops of mountains are hidden in a cloud deck
below. MTOS areas are identified daily on The
Aviation Weather Center located at:
http://www.aviationweather.gov.
6.2 Some canyons run into a dead end. Don’t fly so
far up a canyon that you get trapped. ALWAYS BE
ABLE TO MAKE A 180 DEGREE TURN.
6.3 VFR flight operations may be conducted at night
in mountainous terrain with the application of sound
judgment and common sense. Proper pre−flight
planning, giving ample consideration to winds and
weather, knowledge of the terrain and pilot
experience in mountain flying are prerequisites for
safety of flight. Continuous visual contact with the
surface and obstructions is a major concern and flight
operations under an overcast or in the vicinity of
clouds should be approached with extreme caution.
6.4 When landing at a high altitude field, the same
indicated airspeed should be used as at low elevation
fields. Remember: that due to the less dense air at
altitude, this same indicated airspeed actually results
in a higher true airspeed, a faster landing speed, and
more important, a longer landing distance. During
gusty wind conditions which often prevail at high
altitude fields, a power approach and power landing
is recommended. Additionally, due to the faster
groundspeed, your takeoff distance will increase
considerably over that required at low altitudes.
6.5 Effects of Density Altitude. Performance
figures in the aircraft owner’s handbook for length of
takeoff run, horsepower, rate of climb, etc., are
generally based on standard atmosphere conditions
(59°F, pressure 29.92 inches of mercury) at sea level.
However, inexperienced pilots as well as experienced
pilots may run into trouble when they encounter an
altogether different set of conditions. This is
particularly true in hot weather and at higher
elevations. Aircraft operations at altitudes above sea
level and at higher than standard temperatures are
commonplace in mountainous area. Such operations
quite often result in a drastic reduction of aircraft
performance capabilities because of the changing air
density. Density altitude is a measure of air density.
It is not to be confused with pressure altitude − true
altitude or absolute altitude. It is not to be used as a
height reference, but as a determining criteria in the
performance capability of an aircraft. Air density
decreases with altitude. As air density decreases,
density altitude increases. The further effects of high
temperature and high humidity are cumulative,
resulting in an increasing high density altitude
condition. High density altitude reduces all aircraft
performance parameters. To the pilot, this means that
the normal horsepower output is reduced, propeller
efficiency is reduced and a higher true airspeed is
required to sustain the aircraft throughout its
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