FL180

FL180 is a flight level used in aviation to denote a standard pressure altitude of 18,000 feet. It is defined using the international standard atmosphere reference pressure of 1013.25 hPa (29.92 inHg). A flight level is not an altitude above mean sea level in itself; it is the altitude indicated when an aircraft’s altimeter is set to 1013.25 hPa. Under this setting, the aircraft’s position is described in terms of flight levels for vertical separation.

In practice, FL180 is commonly used as a boundary within which air traffic is managed with a high degree of control. For many jurisdictions, it marks the lower limit of higher-level airspace. In the United States, Class A airspace begins at FL180 and extends upward to FL600, meaning most instrument flight rules traffic above that level operates within controlled airspace with ATC separation.

Other regions may have different transition points, with either different flight levels or altitudes used to delineate lower and upper airspace. The exact boundary between controlled and uncontrolled airspace, as well as the transition altitude or transition level, varies by country and airspace structure. Pilots must use the local ATC clearances and altitude references, which may involve switching between altimeter settings (local QNH versus 1013 hPa) depending on airspace class and phase of flight.

Overall, FL180 serves as a standardized reference point for airway planning and traffic management at mid to high altitudes.