Page 322 - PPL-engelsk 2025

Meteorology

Rim frost can form when an aircraft quickly

descends into warmer, but more humid air.

A deposition occurs (the process where water

vapor turns into ice without first becoming
liquid) on the aircraft’s cold surface.

Rim frost can also form on aircraft left outside

overnight in freezing conditions.

Here’s rim frost on a profile and how

it affects the airflow around the

profile.

At first glance, it might seem harmless, but taking off with rim frost on the

aircraft can be very dangerous.

Just a layer of rim frost with a rough surface, like sandpaper, can reduce lift

by 30%!

Furthermore, if the aircraft somehow manages to take off, the rim layer

promotes further ice formation.

3.9.2.2 Definitions of Icing

Icing Definitions

- Light icing - FBL - No change in heading or altitude required.

- Moderate icing - MOD - Change in heading or altitude desirable.

- Severe icing - SEV - Immediate change in heading/altitude

absolutely necessary

3.9.2.3 Consequences of icing

It can be a real hassle to go out for a flight on a frost-clear day.

Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 322