To illustrate, if an aircraft is parked at Sherman Field with the local altimeter setting in the
Kollsman window, the indicated altitude should be the same as the airfield elevation, and the
indicated altitude will be an MSL altitude. Therefore, the altimeter should indicate approximately
30 feet MSL since Sherman Field is 30 feet above mean sea level.
Altimeters are subject to mechanical errors caused by installation, misalignment, and positioning
of the static ports that measure the pressure. Collectively, these errors are referred to as
instrument error. Instrument error is determined prior to takeoff by noting the difference between
field elevation and indicated altitude. For example, an aircraft taking off from Sherman Field
(elevation +30 feet MSL) with an indicated altitude of 70 feet would have an instrument error of
+40 feet. If the instrument error is in excess of 75 feet, the aircraft is considered unsafe for
instrument flight. Calibrated altitude is indicated altitude corrected for instrument error.
True altitude is the actual height above mean sea level. It is found by correcting calibrated
altitude for temperature deviations from the standard atmosphere. On a standard day, MSL/true
altitude is equal to calibrated altitude. If there is no instrument error, true altitude would also be
equal to indicated altitude. MSL altitude is very important since airfields, hazards, and terrain
elevations are stated in feet above mean sea level.
Above ground level (AGL) or absolute altitude is the aircraft's height above the terrain directly
beneath the aircraft and is measured in feet AGL. Absolute altitude is not normally displayed on
a barometric altimeter, but it can be calculated by subtracting the terrain elevation from the true
altitude. Additionally, it can be displayed directly on a radar altimeter.
Pressure altitude is the height above the standard datum plane. The standard datum plane is the
actual elevation above or below the Earth's surface at which the barometric pressure is 29.92 in-
Hg. Federal Aviation Rules (FAR) require all aircraft operating above 18,000 feet MSL set 29.92
into the altimeter to ensure consistent altitude separation. Since most mountains in the U.S. are
well below 18,000 feet MSL, there is less concern with terrain avoidance than with aircraft
separation above that altitude. Thus, a pilot flying a pressure altitude will have an altimeter
setting of 29.92 instead of the local altimeter setting. In short, a pressure altitude is the height
above the place in the atmosphere where the pressure is 29.92 in-Hg. Whether this place is
above, below, or coincides with sea level is of little concern.
When aircraft fly pressure altitudes, they are assigned a flight level (FL) of three digits,
representing hundreds of feet above 29.92. As an example, an aircraft assigned FL250
(pronounced "flight level two five zero") would be flying a pressure altitude, and the pilot would
fly the aircraft so that the altimeter reads 25,000 feet with 29.92 in the Kollsman window. These
above altitude definitions are illustrated in Figure 1-8.
General Structure of the Atmosphere, and Atmospheric Temperature and Pressure