Flight Level - Transition Altitude

Transition Altitude

While use of a standardised pressure setting facilitates separation of aircraft from each other, it does not provide the aircraft's actual height above ground. At low altitudes the true height of an aircraft relative to an object on the ground needs to be known. The pressure setting to achieve this is called QNH or "altimeter setting" and is available from various sources, including air traffic control and the local METAR-issuing station.

The transition altitude (TA) is the altitude above sea level at which aircraft change from the use of altitude to the use of flight levels. When operating at or below the TA, aircraft altimeters are usually set to show the altitude above sea level. Above the TA, the aircraft altimeter pressure setting is normally adjusted to the standard pressure setting of 1013 hectopascals (millibars) or 29.92 inches of mercury and aircraft altitude will be expressed as a flight level.

In the United States and Canada, the transition altitude is 18,000 ft. In Europe, the transition altitude varies and can be as low as 3,000 ft. There are discussions to standardise the transition altitude within the Eurocontrol area.

On 25 November 2004 the Civil Aviation Authority of New Zealand raised New Zealand's transition altitude from 11,000 feet to 13,000 feet and changed the transition level from FL130 to FL150.

The transition level is the lowest flight level above the transition altitude. The table on the right shows the transition level according to transition altitude and QNH. When descending below the transition level, the pilot starts to refer to altitude of the aircraft by setting the altimeter to the QNH for the region or airfield. Note that the transition level is, by definition, less than 500 ft above the transition altitude. Aircraft are not normally assigned to fly at the transition level as this does not guarantee separation from other traffic flying (on QNH) at the transition altitude; the lowest usable flight level is the transition level plus 500 ft.

The transition layer is the airspace between the transition altitude and the transition level.

In some countries, e.g., Norway, the transition level is determined including a buffer of minimum 1000 ft (depending on QNH) to the transition altitude. Therefore aircraft may be flying at both transition level and transition altitude, and still be vertically separated by at least 1000 ft. In those areas the transition layer will be a given vertical distance between 1000 ft and 1500 ft, depending on QNH.