Page 232 - PPL-engelsk 2025

Meteorology

3.3.6 Altimeter

Since the altimeter is used to indicate the aircraft's altitude above terrain and

to ensure vertical separation between aircraft, it is necessary that all altimeters

are set according to the same relationship between pressure and altitude.

This has led to the introduction of an artificial atmosphere called the
International Standard Atmosphere (ISA).

The ISA conditions are:

Pressure at MSL 1013.25 hPa

o
Temperature at MSL + 15 C
o
Temperature gradient -2 C per 1000 FT altitude
The air is Dry (100%)

From this, it can be deduced that the isobaric surfaces and isotherms are

mutually parallel and parallel to MSL. This implies that there must be calm

winds.

These conditions do not exist in the real atmosphere, so it must always be

remembered that the altitude reading must always be corrected to find the true

altitude.

The altimeter only shows correctly in the

standard atmosphere!!

The height of the tropopause in the standard atmosphere is 36,000 feet and is
the same globally.

3.3.6.1 Altimeters

Altimeters work on the same principle as barometers (see the beginning of the

section). When flying at a certain altitude, you are actually following an isobaric

surface.

Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 232