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AIM

10/12/17

7−5−4

Potential Flight Hazards

f. 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!

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.