Do you know, that there are 5 types of altitudes used in aviation? They are as follows:
Absolute Altitude: In simple terms, it is the altitude above the ground level over which the plane is flying. It changes constantly as the terrain height over a geographical area changes continuously.
Pressure Altitude: It is the altitude of the aircraft above the standard mean sea level. It shows the airplanes altitude assuming that the sea level pressure is 1013.25 hpa (29.92 inHg).
Indicated Altitude: It is the altitude shown by the aircraft altimeter. It changes as the altimeter is updated with the actual sea level pressure (say 1004 hpa) at lower altitudes and 1013.25 hpa at higher altitudes.
Density Altitude: It is the altitude that the aircraft feels it is flying at eg., at higher altitudes there is lower air density, now, if the density gets lower even at low altitudes, the aircraft ‘feels’ that is it flying higher.
True Altitude: It is the actual altitude of the aircraft above the sea level that it is flying at. It calculates the aircraft altitude based on the current sea level pressure instead of the standard 1013.25 at higher altitudes.
Question – Do Airplanes fly at Altitude settings or FL (Flight Level) or both ??