Altimetry

Flight Level, Altitude, Height, Elevation, Level - All the Same? Not at All...

Terminology

Let's clear up any confusion you might have regarding these terms. We'll go through the terms related to altitude measurement / altimetry step by step:

When Does the Controller Use Which Term?

Important for the application of Air Traffic Control Service are primarily Altitude and Flight Level, as these are the two level terms used in aviation to establish vertical separation. But when do we use Flight Level, and when Altitudes?

In Germany, the so-called Transition Altitude (TA) - the altitude at which an aircraft in a climb switches its altimeter from QNH to standard pressure - is always at 5000 feet. Consequently, below the Transition Altitude, Altitudes (with QNH reference) are used, and above it, Flight Levels (FL).

Additionally, there is the Transition Level (TL). This is the lowest usable flight level above the Transition Altitude. In practice, this means that during descent, you fly using Flight Levels with the standard pressure reference until the Transition Level, and below that, with Altitudes.

The Transition Level is chosen based on air pressure so that there is always at least 1000 feet between the Transition Level (TL) and the Transition Altitude (TA). The TL is usually at FL60 or FL70. However, in cases of extreme air pressure, it can also be at FL50 or FL80. The following table shows the respective Transition Levels based on air pressure:


QNH ab QNH bis Transition Level
943 977 FL 80
978 1013 FL 70
1014 1050 FL 60
1051
FL 50
If you've understood the difference between Altitude and Flight Level and that, for example, at a QNH of 1051, 5000 feet and FL50 are not the same height but are actually 1000 feet apart, then you're becoming an altimetry expert. However, this topic is anything but trivial.

Vertical separation for approach controllers

As mentioned earlier, a Transition Level does not necessarily correspond to the true altitude of an aircraft. Instead, the actual altitude depends on the current air pressure. If the air pressure is higher than 1013.25 hPa, an aircraft at FL60 is actually higher than 6000 feet. If the pressure is lower than 1013.25 hPa, an aircraft at FL60 is actually lower than 6000 feet. The following image illustrates this:

Flight_levels.pngPhoto: Kreuzschnabel/Wikimedia Commons, License: artlibre

For separation, this means:

Therefore, aircraft in descent should only be allowed to fly at Flight Levels until the Transition Level, and ideally cleared only to 5000 feet on Altitudes, to ensure separation is always maintained.

If an aircraft is cleared, for example, to 6000 feet, another aircraft at FL70 is only vertically separated if the pressure is ≥ 1014 hPa; otherwise, there is no vertical separation.

For further reading: Altimeter Pressure Settings.


Revision #2
Created 17 September 2024 21:53:18 by 1583954
Updated 26 September 2024 17:12:35 by 1583954