background image

AIM

4/20/23

7

6

5

Potential Flight Hazards

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.

d.

Don’t fly near or above abrupt changes in terrain. Severe turbulence can be expected, especially in high

wind conditions.

e. 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 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.

f.

Navigating in confined terrain when flying through mountain passes can be challenging. For high

traffic

mountain passes, VFR checkpoints may be provided on VFR navigation charts to increase situational awareness
by indicating key landmarks inside confined terrain. A collocated VFR waypoint and checkpoint may be
provided to assist with identifying natural entry points for commonly flown mountain passes. Pilots should
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

AIM, Para 1

1

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.