If you plan to fly at high altitudes or internationally, you need to understand how transition altitudes work. Here's what you need to know...

Most of our articles focus on the United States, so here's a little taste of how things work around the world, as well as how the rules apply right here in the USA.

Transition Altitudes, Defined

A transition altitude is the altitude where pilots are required to change from a local altimeter setting, to a common standard of 29.92 inches of mercury (or 1013.2 hectopascals).

All aircraft flying above a transition altitude will have the same altimeter setting for any given flight level, regardless of local altimeter settings. This is one way to ensure that aircraft flying from areas of high pressure to areas of low pressure (or vice versa) will not come into conflict with each other.

This is especially important for aircraft flying long routes over hundreds or thousands of miles. As atmospheric pressure changes around you in new regions, you don't need to worry about updating to a local altimeter settings. Instead, all aircraft above the transition altitude fly the same constant pressure altitude (29.92 Hg / 1013.2 hPa).

Here's an example what it might looks like for an aircraft flying through various pressure regions:

Above the transition altitude, altimeter readings are communicated as "flight levels," not as hundreds or thousands of feet. When you set your altimeter to 29.92, you're flying at standard pressure altitude.

When you're flying in the flight levels with a standard altimeter setting of 29.92, your altimeter isn't displaying your height above sea level (true altitude). You'd only come close to reading true altitude from your altimeter under standard atmospheric conditions.

Flying In Foreign Airspace? You'll Need To Know The Terms "Transition Level" And "Transition Layer"

A transition altitude is where you set the altimeter to standard when climbing. A transition LEVEL, on the other hand, is the lowest altitude you may fly in cruise using the STD altimeter setting. ATC determines this altitude (usually broadcast over ATIS), and you must revert your STD altimeter setting to the local altimeter setting on descent while passing through the transition level. You can see this note published on the following Jeppesen chart for Paris Charles-de-Gaulle Airport (LFPG):

In the United States, when the surface-based atmospheric pressure in the local area drops below 29.92 in Hg, then the lowest usable flight level is raised from FL180 to FL185, or higher.

Otherwise, someone flying IFR at 17,000 feet MSL could be closer than 1,000 feet to aircraft flying in the lower flight levels.

A transition LAYER is simply the space in-between, and varies in depth from country to country per national regulations. Up to 1,500 feet of altitude can be marked as a transition layer in many nations. As you climb into the transition layer (through the transition altitude), you set a STD altimeter setting. As you descend through the transition level, you revert to the local altimeter setting and thus enter the transition layer.

We don't have published transition layers in the USA. In many international locations, transition layers exist solely for climbs and descents. You won't see an aircraft flying there in cruise. Here's how it all fits together:

Transition Altitudes Aren't Always At High Altitudes

In the USA and Canada, the transition altitude is always an easy-to-remember 18,000 feet MSL, but this isn't true everywhere else in the world. In Europe, transition altitudes can reach as low as 3,000 feet MSL.

Do you have any experience flying through publish transition altitudes, layers, or levels? Tell us in the comments below!