Transition Altitude, Transition Level and Transition Layer – What is the difference?
This information is for South Africa, the definitions may be the same, but please keep in mind that the numbers are based on South African law.
Let’s have a look at the definitions:
Transition Altitude is the altitude when flying where you are required to change from a local QNH to the standard of 1013.25 hectopascals or 29.92 inches of mercury.
Transition Level is the altitude when flying where you are required to change from standard of 1013.25 back to the local QNH. This is above the Transition Altitude.
The Transition Layer is the layer in between Transition Altitude and Transition Level.
We will now take a look at a few more rules that apply to each of the above.
First we must know if we are flying within 25 nm of a airport with ATSU (Air Traffic Service Unit). If we are the information for the Transition altitude will be published on the chart or in the AIP’s. For example we will take a look at FACT which has a transition altitude of 7500 ft, this is the point where we would change from local QNH to 1013 (Flight Levels).
If we are flying in VMC (Visual Meteorological Conditions) and are more than 25 nm from a runway with *ATSU your transition altitude will be 2000 ft AGL. So passing 2000 ft is when you would change from local QNH to 1013.
However if you are flying IFR and more than 25 nm from a runway with *ATSU your transition altitude will be the lowest safe cruising altitude.
*I will insert a link to a PDF of the registered airfields below, you will need to sign up to the mailing list to access it.
Transition levels can change and they do, but they will not be less than 1000 ft above the transition altitude within 25 nm of a runway with an Air Traffic Service Unit.
Transition levels are normally given to you by ATC as you are approaching the airfield or starting your descent. They will be included in the approach and landing instructions.
However if you are more than 25 nm away from a airport with Air Traffic Service Unit, and flying in VMC conditions, the transition level will be 3000 ft AGL (1000 ft above the transition altitude). For IFR flights the transition level will be the flight level 500 ft above the lowest safe cruising altitude.
The above picture would represent the area outside the 25 nm from a runway with ATSU. This would be for most of the area in South Africa. Remember it is always good to follow the rules as you can quite easily find yourself in conflict with other traffic if you haven’t changed to or from the 1013.
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