FL

A flight level is a standard altitude reference used in aviation that removes dependence on actual barometric pressure at ground level. Instead of reporting height above sea level (which varies with weather), pilots and air traffic control use flight levels, which are pressure altitudes calibrated to a standard datum of 29.92 inches of mercury (or 1013.25 hectopascals). Flight level 350, written as FL350, means 35,000 feet above this reference pressure, regardless of what the real atmospheric pressure is on the day.

In practice, every aircraft flying at or above 18,000 feet is required to set its altimeter to the standard datum and report position using flight levels instead of altitude above mean sea level. Below 18,000 feet, pilots use local altimeter settings. This transition altitude varies by country and region; in the US it is exactly 18,000 feet, but Europe uses different values. The convention eliminates the constant corrections needed if every aircraft tried to measure height from their local ground level.

Flight levels typically increment in steps of 500 feet in most countries (FL300, FL305, FL310), though some regions allow 1,000-foot increments for less congested airspace. Long-range cruise planning often uses flight levels between FL250 and FL450, with the chosen level depending on wind patterns, aircraft weight, and fuel efficiency. Maintenance records and flight plans reference FL extensively when documenting service ceilings and operational envelopes.

The name itself is straightforward: it is the level at which an aircraft flies, expressed as a shorthand for the pressure altitude. The FL prefix is universally recognized in commercial aviation and is part of both ICAO and FAA standard operating procedures. Mechanics and engineers encounter flight levels primarily when reviewing maintenance limitations, performance data sheets, and pressurization system specifications that define how high and how safely an aircraft can operate.