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g. 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.
h. 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 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.
i. 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 degrees Fahrenheit (15 degrees Celsius), 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 areas. 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
operating parameters. It means an increase in runway
length requirements for takeoff and landings, and
decreased rate of climb. An average small airplane,
for example, requiring 1,000 feet for takeoff at sea
level under standard atmospheric conditions will
require a takeoff run of approximately 2,000 feet at an
operational altitude of 5,000 feet.
NOTE−
A turbo-charged aircraft engine provides some slight
advantage in that it provides sea level horsepower up to a
specified altitude above sea level.
1. Density Altitude Advisories. At airports
with elevations of 2,000 feet and higher, control
towers and FSSs will broadcast the advisory “Check
Density Altitude” when the temperature reaches a
predetermined level. These advisories will be
broadcast on appropriate tower frequencies or, where
available, ATIS. FSSs will broadcast these advisories
as a part of Local Airport Advisory, and on TWEB.
2. These advisories are provided by air traffic
facilities, as a reminder to pilots that high
temperatures and high field elevations will cause
significant changes in aircraft characteristics. The
pilot retains the responsibility to compute density
altitude, when appropriate, as a part of preflight
duties.
NOTE−
All FSSs will compute the current density altitude upon
request.
j. Mountain Wave. Many pilots go all their lives
without understanding what a mountain wave is.
Quite a few have lost their lives because of this lack
of understanding. One need not be a licensed
meteorologist to understand the mountain wave
phenomenon.
A71I1M0.65R CHG 2 37//1315//078
2/14/Potential Flight Hazards 7−5−5
1. Mountain waves occur when air is being
blown over a mountain range or even the ridge of a
sharp bluff area. As the air hits the upwind side of the
range, it starts to climb, thus creating what is
generally a smooth updraft which turns into a
turbulent downdraft as the air passes the crest of the
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