What is a flight level (FL)?

A flight level (FL) is defined as a specific altitude at which aircraft operate, referenced to the standard atmosphere. It is particularly significant because it is used when the aircraft altimeters are calibrated to a standard pressure setting of 1013.3 hPa, also known as the International Standard Atmosphere (ISA). This standardization allows for a uniform reference for all aircraft operating at high altitudes, thereby enhancing safety and operational efficiency.

Using flight levels helps in the separation of aircraft in controlled airspace, as pilots and air traffic control both refer to the same baseline altitudes when navigating. Flight levels are typically expressed in feet, but they are written without the word "feet"; for example, FL180 means 18,000 feet.

Regarding the other options, a VFR flight plan pertains to visual flight regulations and does not denote flight levels. The term 'flight level' applies above 18,000 feet in most parts of the world, not limited to below 15,000 feet. Finally, 'flight level' is a formally recognized term in aviation, not an informal one.