Page 782 - PPL-engelsk 2025

Aircraft general knowledge

The magnetic compass is often called an “alcohol

compass”. However, it is not alcohol that is in the

aircraft compass, but acid-free paraffin.

The compass needles align with the earth’s magnetic

lines of force.

These lines of force are relatively weak and do not run in straight lines, but wind

like rivers over the landscape (see the isogons on the flight map). This can be due
to the presence of, for example, iron and other magnetic substances in the

subsoil.

8.12.9.1 Sources of error

When we work with maps, they are referenced in relation to true north

(geographic north). The angular difference between true north and what our

compass should point to (magnetic north) is called variation.

You may have wondered why the phrase “should” is used.

There may be things about our compass or things in the plane that distract the

Earth’s magnetic field, causing an error.

Around every wire there is a magnetic field that can disrupt the compass’
reading. The headset’s small built-in magnets also cause our compass to display

incorrectly.

The aircraft mechanic can check the compass and adjust (deviate) it so that it

shows approximately correctly.

The residual deviation is stated in a so-called

deviation chart, so that you can see how many

degrees you need to correct for different

courses.

Flyveteori PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 782