Flight Training - September 2014 - 51

The radio altimeter calculates height above
the ground based on the time radio waves
take to reflect back to the airplane. Radio
altimeters work well only at relatively low
altitudes, and they are expensive.
Airliners use radio altimeters as a
part of autoland systems that allow safe
landings in very poor visibility. The radio
altimeter tells the autopilot when to begin
flaring for a landing and controls the autothrottles. Radio altimeters also are the
core of ground proximity warning systems
that alert pilots that they are too close to
the ground or a mountain peak.
MEASURING THE HEIGHT ABOVE SEA
LEVEL. Ordinary airplane altimeters are

called pressure altimeters because they
use atmospheric pressure to measure
height above sea level. In simple terms, an
altimeter is a barometer-an instrument
that measures atmospheric pressure-that
has been calibrated to read in feet above
sea level instead of inches or mercury
or metric system millibars, as ordinary
barometers do.
A barometer makes a good altimeter
because atmospheric pressure decreases
with altitude, as shown on this page,
which shows the standard atmosphere for
temperature and atmospheric pressure up
to 10,000 feet above mean sea level. The
numbers show that atmospheric pressure
doesn't decrease at a regular rate with
altitude. Nevertheless, for relatively low
altitudes such as the ones at which most
general aviation airplanes fly, a common
rule of thumb says atmospheric pressure
decreases by one inch of mercury for each
1,000 feet in altitude gained.
You should use this figure for altimeter
questions on FAA knowledge tests that
require calculations. Knowledge tests probably are the only time you'll need to use the
one inch of mercury per 1,000 feet figure.
ALTIMETER DEFINITIONS. A good way
to begin learning about altimeters is by
learning the terms used in relation to
measuring aircraft height above sea level.
Absolute altitude: The vertical distance
of an aircraft above the terrain. Pilots usually use agl for "above ground level."

STANDARD ATMOSPHERE
Altitude
(feet)

Pressure
(inches Hg)

Temperature
(F)

Surface
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000

29.92
28.86
27.82
26.82
25.84
24.89
23.98
23.09
22.22
21.38
20.57

59.0
55.4
51.9
48.3
44.7
41.2
37.6
34.0
30.5
26.9
23.3

Source: Aerodynamics for Naval Aviators

Elevation: The height above mean sea
level of an object or location that's fixed to
the ground, such as mountains or the tops
of towers.
Indicated altitude: The altitude read
directly from an altimeter after it is set to
the current altimeter setting.
Pressure altitude: The altimeter reading when the pressure-setting scale is set
to 29.92 inches of mercury, which is sea
level pressure in the standard atmosphere.
True altitude: The true vertical distance of the aircraft above sea level-the
actual altitude. Elevations of airports, terrain, and obstacles found on aeronautical
charts correspond to true altitudes.

HOW ALTIMETERS WORK. Altimeters in
most training aircraft are sensitive aneroid barometers. An aneroid is a flexible
metal bellows from which some air was
removed before it was sealed. Pressures
lower than the inside pressure allow
the bellows to expand; higher pressures
squeeze it, making it thinner. A lever
attached to one end of the bellows moves
as the pressure changes to point at a number on a dial, indicating the surrounding
atmospheric pressure.
Newer altimeters replace mechanical aneroids with pressure transducers,
which convert pressures into electrical
signals that can be used to show altitude
in digital form instead of with the pointer
of an aneroid altimeter.
SETTING AND READING AN ALTIMETER.

You don't have to learn much about
weather to realize that the weather affects
altimeters, because atmospheric pressure changes are associated with weather
changes. Atmospheric pressures lower
than the surrounding pressure are associated with foul weather because air is
rising from surface areas of low pressure.
As the rising air cools, its water vapor
condenses to form clouds and maybe
precipitation.
Air is sinking from higher altitudes to
form surface areas of high pressure, which
generally have clear skies because sink-

THREE-POINTER ALTIMETER

10,000 feet pointer
Altimeter setting
window
100 feet pointer

1,000 feet pointer

Dial setting knob

SEPTEMBER 2014 FLIGHT TRAINING

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