American Airlines Virtual

Hope this helps Julio:

Transition Altitude. The altitude at or below which the vertical position of an aircraft is controlled by reference to altitudes.

Transition Level. The lowest flight level available for use above the transition altitude.

Transition Layer. The airspace between the transition altitude and the transition level.


In the US and Canada, the transition altitude is fixed at 18000 feet and the airspace above is known as the Standard Pressure Region

Under conditions of QNH at or above 1013 mb, FL180 becomes the lowest useable FL. If the altimeter setting is lower, the lowest useable FL becomes FL190 or even FL200. This restriction ensures that a minimum of 1000' vertical separation is maintained between the aircraft at 17000' on QNH and the one at the lowest useable level on Standard Pressure Setting (QNE).

Guidance on changing barometric pressure setting:
1.all changes shall be made in the standard pressure region (ie above 18,000 ft) and
2.the change is to take place just after entering or just prior to leaving the standard pressure region. In practice, this will see the pilots changing to QNE (depending upon the altimeter subscale) as they climb through 18000 feet. If the cleared level is FL180, an early change is prudent.

Descending, even when cleared to an altitude at the time cruising level is vacated, the altimeters will remain on standard pressure until just prior to the transition level.

Keep in mind that, in reality, the transition level "floats" based on the QNH - the lower the QNH, the higher the transition level - If the QNH is low and the pilots wait until approaching FL180 to change the altimeter subscale, an Level Bust is possible.

Both primary altimeters are changed at the same time. In most modern aircraft, not doing so will result in some form of an altimeter missmatch error. Depending upon company SOP, the standby altimeter might also be changed at the transition altitude/level or at top of climb/top of descent.

I believe on the SVMI chart with a published Trans Alt: of 12'000, you would set your altimeter to 1013mb/29.92Hg upon reaching 12'000' indicated...........

Here is a quote from the FAA pilot handbooks, which is what I use for the training department on checkride evaluations;

The transition level is the lowest flight level above the transition altitude. 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 the United States and Canada, the transition altitude is 18,000 ft.

The highest allowable VFR altitude is 17,500 for eastbound flight, 16,500 for westbound. Most airliners’ optimal cruise levels range from 28,000 to 41,000 feet depending on weight, and particularly on a longer flight, flying below FL180 would be seriously inefficient with regard to fuel burn and speed.

WLP