Definition
A level of constant atmospheric pressure expressed in hundreds of feet, referenced to the standard altimeter setting of 29.92 inches of mercury (1013.2 hPa). For example, FL250 represents the pressure level corresponding to 25,000 feet on an altimeter set to 29.92.
Plain English
An altitude described in hundreds of feet, measured using a standard pressure setting that every aircraft uses at higher altitudes. FL250 means 25,000 feet on that standard setting.
Context Anchor
You will see flight levels in high-altitude airspace, instrument clearances, and air traffic control instructions such as climbs, descents, and assigned cruising levels.
Derivation
Called a 'flight level' rather than an 'altitude' because it is not a true height above the ground or sea — it is a layer of air at a specific pressure. All aircraft using the same standard setting share the same reference, so they stay safely separated even if the actual altitude varies slightly with weather.
Why Pilots Care
Correct use of flight levels prevents mid-air collisions by guaranteeing vertical separation between aircraft.
Analogy
It is like everyone agreeing to measure from the same starting line. The actual ground below may rise or fall, but all aircraft using flight levels are using the same reference setting.
Intuition Check
Do not read “flight level” as simply “the aircraft’s exact height above the ground” or “level flight.” Here it means a standard pressure-based altitude, written in hundreds of feet.
Example Sentence 1
Example Sentence 2
All aircraft above 18,000 feet must maintain flight level assignments with altimeters set to standard pressure.