Have you ever wondered why sometimes our reporting expresses altitude as FL (flight level) rather than meters or feet? While it’s a different way to measure altitude, it’s a little more complicated since FL is more about a reference to air pressure. There’s an altitude at which this changes- a transition altitude. Let’s try to explain what all of this means and why it matters.
What’s the difference between feet and flight level?
First, we need to define some terms with some precision as they might often be used a little more loosely. The graphic below should help with visualization. Most of these definitions come with help from the fantastic resource that is SKYbrary.
Altitude: Some may think that altitude is simply how high an object is- or how high an aircraft is flying. But more precisely, altitude is measured from mean sea level and is very much a measure of distance in terms of height. Depending on where in the world you are, this is often expressed in feet or meters.
Flight Level (FL): Yes, flight level in a way is used to indicate altitude. But while this is often used to indicate how high an aircraft is flying, it’s not so simple. FL is actually defined as a “surface of constant atmosphere pressure which is related to a specific pressure datum, 1013.2hPa, and is separated from other such surfaces by specific pressure intervals.”
If we try to make this more understandable, we can say that FL is a measure of air pressure. It is based on a standard atmosphere pressure at sea level of 1013.25 hectopascals.
FL is measured in increments of 100 feet. So FL60 is 6,000 feet (above mean sea level when the pressure at sea level is 1013.2hpa). FL61 is 6,100 feet according to a standard atmosphere.
But why is it important to have these two methods of expressing height? According to BoldMethod, this is especially important for aircraft flying long routes over hundreds or thousands of miles. This is because atmospheric pressure changes in new regions. Pilots don’t need to worry about updating to local altimeter settings. Instead, all aircraft above the transition altitude fly the same constant pressure altitude.
The transition level: Moving from feet to flight level
It’s only after a certain height above sea level that flight level is used instead of feet. This height is known as the transition level- which varies depending on location (regional or airfield pressure setting also known as QNH.
According to SKYbrary, transitional altitudes are “local, regional or national and vary considerably between about 3,000ft and 18,000ft. The USA and Canada have a common one of 18,000ft…In Europe and much of the rest of the world, the transition altitude varies from airport to airport.”
So for North America, under conditions of QNH at or above 1013 hpa, FL180 becomes the lowest useable FL. If the pressure is lower, the lowest useable FL becomes FL190 or even FL200.
Transition altitude, where pilots are required to change from a local altimeter setting to a common standard is thus important to ensure aircraft are flying at specified altitudes or flight levels, maintaining proper vertical distance from other aircraft.
Did you find this interesting? Did you know what exactly flight level represented before reading this article? Let us know in the comments.