flightlevel

Flight level refers to a specific altitude in aviation. It is used when the altimeter is set to a standard pressure setting of 1013.25 hectopascals (hPa) or 29.92 inches of mercury (inHg). This standard setting is crucial for maintaining vertical separation between aircraft in controlled airspace, especially at higher altitudes where atmospheric pressure can vary significantly.

When an aircraft is operating at or above a certain transition altitude, typically around 18,000 feet in the United States, the altimeter is set to this standard pressure. The indicated altitude then becomes the flight level. For example, if an aircraft's altimeter is set to 1013.25 hPa and it reads 35,000 feet, it is said to be flying at Flight Level 350.

The use of flight levels ensures that all aircraft in a given area are using the same reference for altitude, regardless of local atmospheric pressure variations. This standardized system is fundamental to air traffic control and collision avoidance, allowing for predictable and safe separation between all participating aircraft. Below the transition altitude, aircraft altimeters are set to the local QNH (the actual atmospheric pressure at sea level in that area) to provide accurate altitude above ground level.