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Strictly speaking a flight level is an indication of pressure, not of altitude. Only above the transition level (which depends on the local QNH but is typically 4000 feet above sea level) are flight levels used to indicate altitude; below the transition level feet are used.
25 sty 2024 · A flight level (FL) is a standard pressure altitude, expressed as three numbers. Aircraft flying on flight levels all measure their altitude from the same pressure setting, 1013.2 Hpa (or 29.92 InHg).
Flight level and standard pressure. A Flight Level (FL) is the vertical distance of an aircraft above the ISOBARIC SURFACE of 1013.25hPa (hecto Pascal) or 29.92 in Hg (inches of Mercury). An ISOBARIC SURFACE is the invisible landscape that connects all points with the same atmospheric pressure.
6 paź 2023 · Practically speaking, pressure altitude is the altitude you read on your altimeter when it is set to 29.92 Hg. More specifically, pressure altitude is the height above a Standard Datum Plane (SDP), which is a theoretical level where the air pressure equals 29.92 inches of mercury (Hg).
Flight levels are usually designated in writing as FLxxx, where xxx is a two- or three-digit number indicating the pressure altitude in units of 100 feet (30 m). In radio communications, FL290 would be pronounced as "flight level two nine (r) zero".
The flight levels correspond to the altitude in 100-ft increments (hectofeet) above the pressure surface of 1,013.25 hPa calculated according to the standard atmosphere. For example, FL050 is the pressure surface 5,000 ft above the pressure surface 1,013.25 hPa.
With Standard Pressure (1013.2 mb) set, an aircraft altimeter indicates Pressure Altitude (Flight Level), and is used by all aircraft operating above the transition altitude to provide a common datum for vertical measurement.
