As you may already know, aircraft calculate which altitude they're at by using pressure. As air pressure is constantly changing, pilots have to adjust the altimeter so it shows the correct altitude. To do this, pilots adjust it to the field's QNH or altimeter, depending on where you're flying at. They know what the current pressure is by having a look at the METAR, as shown on the chapter before. As an aircraft rises higher and higher in the sky the air becomes thinner, thus leading to a lower pressure and the altimeter showing a higher altitude.
When an aircraft is climbing, the pilot must switch from the local QNH to the standard one (1013 mb) when crossing the transition altitude. When an aircraft is DESCENDING, the pilot must switch from standard QNH to the local one whever it's crossing the transition level.
Let's have a look at some definitions now!
- Altitude: Vertical distance between mean sea level (MSL) and an aircraft, when it is BELOW the transition layer (and therefore, has the local QNH/altimeter set). When speaking about a vertical distance below the transition layer, pilots and controllers talk about "feet"; for example, "6000ft".
- Flight Level: Vertical distance between mean sea level (MSL) and an aircraft, when it is ABOVE the transition layer. When speaking about a vertical distance above the transition layer, pilots and controllers talk about "Flight Levels"; for example, "FL250".
- Height: Vertical distance between the ground and an aircraft (AGL - Above Ground Level).
- Transition Altitude (TA): Altitude above which climbing aircraft shall have standard QNH set.
- Transition Level (TL): Flight Level under which descending aircraft shall have the local QNH set.
- Transition Layer: No-flight zone between the TA and the TL.
Content updated: 5. July 2018.