When controlling his airspace, an air traffic controller can give instruction in order to change aircraft altitude.
A pilot shall configure his aircraft altimeter instrument with only three possible altimeter settings:
- Altimeter set on local QNH
- Altimeter set on standard pressure 1013hPa (29.92inHg)
- Altimeter set on local QFE (forbidden in some areas and airport but still used in some regions)
|QNH||Atmospheric pressure at mean sea level (may be either a local, measured pressure or a regional forecast pressure (RPS)).|
|QFE||Atmospheric pressure at a specified datum such as airfield runway threshold.|
The standard pressure 1013hPa altimeter setting is mainly used and adapted for the highest cruise altitude in order to maintain separation of all aircraft whatever their origin and destination airfields. The altitude change at high flight level in function of local atmospheric pressure will be very slow and there will be very few influences.
The local QNH altimeter setting is mainly used and adapted for the landing and take-off procedure, low altitude routes close to the landmark and the approach phase of the flight. With taking the same local reference, all aircraft altitudes are constant with a constant air pressure value (QNH).
In their airspace, air traffic controllers shall give clearances including altitude and flight level. The rule is:
- ATC gives clearance using altitude based upon local QNH at and below sector's transition altitude.
- ATC gives clearance using flight level based upon standard pressure at and above sector's transition level.
When in a climbing flight, passing through the transition altitude the altimeter setting will be changed from QNH to STD. The transition altitude (TA) is the highest altitude that will be assigned to an aircraft.
When in a descending flight, passing through the transition level the altimeter setting will be changed from STD to QNH. The transition level (TRL) is the lowest available FL, depending on the local pressure, that will be assigned to an aircraft.
QNH shall be given with the first descend clearance to an altitude below the transition level by the air traffic controller.
The transition layer is the airspace located between the transition altitude and the transition level. The transition layer is defined inside the associated TMA (terminal area) where the transition altitude is published.
The altitude of the transition level shall always be greater or equal than the transition altitude.
The first assignable flight level for IFR aircraft by ATC should be in that case the next IFR flight level above transition level which ensure a minimum of 1000ft separation.
Note that in some countries, the difference between TA and TRL or the transition layer thickness is less than 1000ft, and the dual assignation in clearance of TA and TRL may create vertical separation loss.