Lesson 7 · 6 min
Heights, altitudes and QNH
Why controllers pass a pressure setting with almost every clearance.
Underlined terms below have a definition — click one to read it, or open the full glossary.
An altimeter is a barometer
An aircraft measures air pressure and converts it to a height. Because the pressure of the day changes, the instrument has to be told what pressure to treat as the reference.
The three settings
— set this and the reads altitude above mean sea level. This is what the passes to and arriving traffic.
— set this and the reads height above the aerodrome, so it shows zero on the . Still used at some UK airfields for flying.
, 1013 hPa — used above the . Everyone sets the same reference, so aircraft are by rather than altitudes. FL80 means 8000 feet on the standard setting.
Why it matters to you
Two aircraft using different references are not comparing like with like. Passing the correct , and getting it , is how everyone in the ends up on the same vertical picture.
In this simulator the is passed with and instructions, and must be .
Try it
With QNH set, what does the altimeter read?
What is the standard pressure setting?