Page 244 - PPL-engelsk 2025

Meteorology

In the last case, we see that a drop in temperature, with unchanged QNH, will

also result in a decrease in true altitude. The altimeter setting will, however,

not change. The altimeter is only correct at 15ºC.

If flying to a place with lower temperature, the altimeter will show incorrect

values. Here, the error will be 4% for every 10 degrees of deviation. In this
example, there is a 25-degree difference in temperature (-12º to +13º).

This results in an error of 2.5 x 4% = 10%, and as shown in the diagram, the error

is 10% of 1000 feet = 100 feet, so in reality, we are only at 900 feet.

In practice, as we fly VFR (Visual Flight Rules), differences in temperature

and/or pressure between the departure and destination points have no effect,

since we fly and land visually.

When specifying an aircraft's performance, it is based on density altitude in

standard atmospheric conditions.

In practice, density altitude can be calculated as follows:

Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 244