Tutorial

2(e). Atmosphere

To account for the effects of pressure, density and temperature on an aircraft's altitude, pilots look at more than one kind of altitude reading. They actually keep track of 5 different altitudes.

Pilots need to bear in mind 5 different types of altitude:

• true altitude: actual aircraft's height above sea level usually expressed as MSL (mean sea level); all elevations on aeronautical charts are expressed in terms of true altitude
• absolute altitude: the height of an aircraft above the terrain over which it is flying
• indicated altitude: after setting the altimeter at the current altimeter setting this is the uncorrected altitude read directly from that altimeter
• pressure altitude: after adjusting the altimeter's setting to 29.92 inches, this is the altimeter reading that corresponds to the altitude in the standard atmosphere where the pressure is the same as you are; a change of one inch in the pressure setting will change the altimeter reading by 1,000 feet
• density altitude: the pressure altitude corrected for temperature variations (this is most important to takeoff and climb performance of an aircraft)

Prior to takeoff, each pilot sets the aircraft's altimeter with the correct altimeter setting of the airport from which the pilot will depart. This setting is given by a controller in the local control tower who also gives the indicated altitude. Along the route the pilot continues to adjust the barometric altimeter setting according to radio reports or controllers contacted en route. From this setting and other data the 5 different altitude readings are derived.