These procedures describe the method for providing adequate vertical separation between aircraft and for providing adequate terrain clearance during all phases of a flight. This method is based on the following basic principles:
• States may specify a fixed altitude known as the transition altitude. In flight, when an aircraft is at or below the transition altitude, its vertical position is expressed in terms of altitude, which is determined from an altimeter set to sea level pressure (QNH).
• In flight above the transition altitude, the vertical position of an aircraft is expressed in terms of flight levels, which are surfaces of constant atmospheric pressure based on an altimeter setting of 1013.2 hPa.
• The change in reference from altitude to flight levels, and vice versa, is made:
– at the transition altitude, when climbing; and
– at the transition level, when descending.
• The transition level may be nearly coincident with the transition altitude to maximize the number of flight levels available. Alternatively, the transition level may be located 300m (1000ft) above the transition altitude to permit the transition altitude and the transition level to be used concurrently in cruising flight, with vertical separation ensured. The airspace between the transition level and the transition altitude is called the transition layer.
• Where no transition altitude has been established for the area, aircraft in the en-route phase shall be flown at a flight level.
• The adequacy of terrain clearance during any phase of a flight may be maintained in any of several ways, depending upon the facilities available in a particular area.
The recommended methods in the order of preference are:
• the use of current QNH reports from an adequate network of QNH reporting stations;
• the use of such QNH reports as available, combined with other meteorological information such as forecast lowest mean sea level pressure for the route or portions thereof; and
• where relevant current information is not available, the use of values of the lowest altitudes or flight levels, derived from climatological data
• During the approach to land, terrain clearance may be determined by using:
– the QNH altimeter setting (given altitude); or
– under specified circumstances a QFE setting (given height above the QFE datum).
This method provides flexibility to accommodate variations in local procedures without compromising the fundamental principles. These procedures apply to all IFR flights and to other flights which are operating at specific cruising levels in accordance with the rules of the air.
System of Flight Levels
Semicircular Cruising Level System (ICAO)
Cruising Levels Applicable in RVSM Airspace