Flight levels, transition and the semicircular rule

An altitude above mean sea level and a flight level are not the same thing. The difference is not in the air but in the reference pressure – and in Germany, when to change is expressly regulated for VFR.

Applicability: Germany (airspace of the Federal Republic of Germany)
Rulemaking authority: Aeronautical Information Publication Germany – AIP VFR (DFS Deutsche Flugsicherung GmbH), effective 20 AUG 2026, ENR 1-14 and ENR 1-15
Reviewed by NorthTrack on: 2026-09-13

Altitude or flight level

Altitude
Reference: QNH · Written as: e.g. 4,500 ft MSL
Height
Reference: terrain · Written as: e.g. 1,200 ft AGL
Flight level
Reference: 1013.2 hPa · Written as: e.g. FL 065

A flight level is therefore not an altitude but a pressure surface. Two aircraft at the same flight level are the same distance apart – but not necessarily the same distance above the ground.

The altimetry abbreviations

ALT — Altitude
The reference is mean sea level; the setting is QNH.
HGT — Height or height above
The reference is the terrain or the aerodrome.
FL — Flight level
A pressure surface referenced to 1013.2 hPa — not a height above the ground.
AMSL — Above mean sea level
Used on the aeronautical chart and in the AIP.
MSL — Mean sea level
The reference surface for heights above the sea.
AGL — Above ground level
The reference is the terrain directly below.
AAL — Above aerodrome level
Cloud heights in a METAR are referenced to the aerodrome elevation.
ELEV — Elevation
The elevation of the aerodrome above mean sea level.
QNH — Atmospheric pressure reduced to MSL by ICAO Standard Atmosphere and altimeter sub-scale setting to obtain aerodrome elevation when on the ground
The altimeter then shows altitude above mean sea level; on the ground at an aerodrome, that aerodrome's elevation.
STD — Standard (altimeter setting)
1013.2 hPa — to be set above the applicable limit.
hPa — Hectopascal
The unit of atmospheric pressure; QNH is reported as four digits in whole hPa.

When the change is made in Germany (VFR)

AIP VFR Germany ENR 1-14
On flights under visual flight rules up to an altitude of 5,000 ft MSL, or up to a height of 2,000 ft AGL where that flight altitude exceeds 5,000 ft MSL, the altimeter is to be set to the QNH of the nearest aerodrome with an air traffic control unit along the route. Above those altitudes the altimeter is to be set to 1013.2 hPa.

The second condition is the interesting one: it stops you flying on standard pressure at low height above the ground in mountainous country. Where the terrain is high enough that 2,000 ft above ground lies above 5,000 ft MSL, the QNH stays set until then.

This page describes the VFR rule
ENR 1-14 of the AIP VFR governs the altimeter setting on VFR flights. The transition altitudes and levels for instrument flight operations are in the IFR part of the AIP (AIP Germany ENR 1.7) and are not reproduced here.

Semicircular cruising levels

Within the FIR/UIR boundaries of the Federal Republic of Germany the semicircular cruising levels of ICAO Annex 2, Appendix 3 apply (ENR 1-15). The rule follows the magnetic track:

000° to 179°
IFR: odd levels – FL 030, 050, 070, 090, 110 … · VFR: odd plus 500 ft – FL 035, 055, 075, 095, 115 …
180° to 359°
IFR: even levels – FL 020, 040, 060, 080, 100 … · VFR: even plus 500 ft – FL 045, 065, 085, 105 …

Extract from the table in AIP VFR ENR 1-15. That table additionally gives the corresponding altitudes in feet and metres.

Between FL 290 and FL 410 inclusive the minimum vertical separation between available flight levels is reduced to 1,000 ft (RVSM airspace). That is rarely relevant to general aviation, but it explains the unfamiliar spacing in the table above FL 290.

The semicircular rule follows the track, not the direction of the destination
What counts is the magnetic track over the ground. On a return flight along the same route the semicircle therefore normally changes – and with it the appropriate flight level.

How this ties into the airspace structure

The numbers interlock: in Germany Class C begins at FL 100, and at FL 130 in the Alpine area. Choosing a flight level under the semicircular rule therefore means checking at the same time which airspace class you are entering, and whether a clearance and a listening watch are required there. More on that under airspace classes in Germany.

Pressure altitude, density altitude and ISA deviation can be worked out directly in the NorthTrack calculators. Open Aviation Tools

Official sources

DFS Deutsche Flugsicherung GmbH: Aeronautical Information Publication Germany – AIP VFR – Effective 20 AUG 2026. Open source (retrieved 2026-09-13)
DFS Deutsche Flugsicherung GmbH: Aeronautical Information Publication Germany – AIP VFR, GEN 2-1 to GEN 2-7 "Abkürzungen / Abbreviations" (bilingual German/English) – Effective 20 AUG 2026. Open source (retrieved 2026-09-13)