AIM 

3/21/24 

reference the name of the charted VFR checkpoint, wherever possible, when making position reports on CTAF 

frequencies to reduce the risk of midair collisions. Pilots should evaluate the terrain along the route they intend 

to fly with respect to their aircraft type and performance capabilities, local weather, and their experience level 

to avoid flying into confined areas without adequate room to execute a 180 degree turn, should conditions require. 

Always fly with a planned escape route in mind. 

REFERENCE

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AIM, Para 1

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17, Global Positioning System (GPS). 

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

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A turbo-charged aircraft engine provides a 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. 

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

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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. 

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Potential Flight Hazards