Standard Setting: When 1013.25 hPa is set on the subscale, the altimeter reading is called “Pressure Altitude”. If 1013.25 hPa is set, an aircraft would normally fly Flight Levels. A Flight Level is the aircraft’s height, above 1013.25 hPa, expressed in 100s of feet. Flight Levels only occur at 500 ft intervals. (e.g. 4500 ft will be FL45, or FL360 will be 36 000 ft.)

QNH is the equivalent Mean Sea Level (MSL) pressure calculated by Air Traffic Control from the aerodrome level pressure assuming International Standard Atmosphere (ISA) conditions prevail between aerodrome level and MSL. With QNH set on the subscale, the altimeter of an aircraft on the aerodrome indicates aerodrome elevation, that is, the height Above Mean Sea Level (AMSL) (if there is no instrument error). In flight the altimeter will indicate altitude but this will only be the True altitude if the mean temperature in the column of air beneath the aircraft is the same as in ISA conditions (assuming there are no other altimeter errors). If conditions are different from standard, the indicated altitude, sometimes called QNH altitude, may deviate considerably from True altitude. The navigational computer can be used to make an approximate correction for this temperature error.

QFE is aerodrome level pressure, which when set on the subscale, will cause the altimeter of an aircraft on the ground to read zero, assuming there is no instrument error. In flight, with QFE set, the altimeter will indicate height above the aerodrome QFE reference datum, provided ISA conditions exist between aerodrome level and the aircraft and there are no other altimeter errors. In practice, QFE is used mainly for circuit-flying and gives a good indication of height above the aerodrome, any errors involved being only small.

A Flight Level is a Pressure altitude.