Page 233 - PPL-engelsk 2025

Meteorology

The requirement for an altimeter is that when you are at airport X, it should

show the airport's altitude above MSL. After flying to airport Y, it should also

show airport Y's altitude above MSL when you land.

This requirement must be met regardless of altitude differences, pressure
differences, and temperature differences between the two airports.

How do we do this?

We send a trusted person out to the airport's reference point with a highly

accurate altimeter.

When the person arrives at the reference point, the altimeter is placed at the

correct altitude (not just to read the numbers). The setting is adjusted until the

needle shows the reference altitude above MSL. The air pressure is then read
in the "window" on the altimeter. This is called the airport's QNH.

When a plane arrives from a different airport, the pilot must set the airport’s

QNH on the aircraft’s altimeter before landing.

QNH is the atmospheric pressure reduced to MSL in
accordance with the standard atmosphere.

So far so good. We can now land at an airport with certainty of accurate altitude

measurement.

But let’s assume
that we need to

fly from Airport

X to Airport Y or

vice versa.

Between Airport X and Airport Y is Mount Heaven, a solid granite peak at 4000

feet.

Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 233