Altitude Measurement

AVIATION WEATHER

CHAPTER ONE

Figure 1-7 Barometric Altimeter (aka Pressure Altimeter)

112.

ALTITUDE MEASUREMENT

Altitude is defined as the height above a given reference. The instrument displaying altitude in the

cockpit is called an altimeter. The barometric altimeter is an aneroid barometer calibrated to display

altitude in feet, as opposed to pressure in inches of mercury (Figure 1-7). Since an altitude includes

not only the height number, but also the reference, altimeters have a Kollsman window that shows

the reference pressure, known as the altimeter setting. The altimeter setting is the value to which the

scale of the pressure altimeter is set so the altimeter indicates true altitude at field elevation. It is

very nearly equal to the station pressure corrected to MSL pressure (not exact, but close enough for

instructional purposes). An adjustment knob allows the altimeter setting to be changed. If the local

altimeter setting is dialed in to the Kollsman window, the altimeter will indicate the altitude in feet

above MSL. If 29.92 is set, the altimeter will indicate the altitude above the standard datum plane.

MSL and pressure altitudes are the two altitudes most often displayed on the altimeter and are

discussed in the next section.

Altitudes

Indicated altitude is the altitude read directly from the altimeter. Since altimeters need no power

(except for lighting, they operate by measuring the outside pressure), they will always indicate

some value. Figure 1-7 shows an indicated altitude of 5635 feet. For an indicated altitude to be

useful, however, the altimeter needs to have the correct reference for the situation by dialing

either the local altimeter setting or 29.92 in to the Kollsman window. This way, the indicated

altitude will be equal to either the MSL or pressure altitude (still to be discussed).

General Structure of the Atmosphere, and Atmospheric Temperature and Pressure 1-9