Altitude is an aviation term referring to an aircraft’s elevation, and is measured in feet. There are several different types of altitude used in the aviation field, so they will all be detailed here. The most common types are True, Absolute, and Indicated Altitudes. All of these measurements can be found using a range of instruments, including aneroid barometers, GPS systems, or radar altimeters, which use radio waves to calculate the distance to the ground.
True Altitude is an aircraft’s elevation above mean Sea Level. This is the most common use of the word Altitude, and it is usually assumed that this is what the speaker is referring to unless he specifies otherwise. Absolute Altitude is the height of the aircraft above the ground over which it is flying, and is also known as Altitude Above Ground Level, or AGL.
Indicated Altitude is the reading on the altimeter, an instrument in the cockpit that lets the pilot know his height above sea level. This instrument is commonly run by an aneroid barometer, which is a device that measures air pressure surrounding the aircraft. Usually, the lower the air pressure, the higher the elevation. Aircraft with this type of altimeter calibrate it every time they land. An airport usually has a posted elevation that the pilot will adjust his altimeter to before he takes off again, and this ensures that the instrument is constantly calibrated.
Pressurization is required in aircraft that travel above altitudes over 10,000ft. Cabin Pressurization is the act of pumping compressed air into the cockpit and cabin of the aircraft to ensure that the crew and passengers do not pass out from lack of oxygen, a problem known as hypoxia. When the body does not get enough oxygen, a person can loose consciousness, which usually spells a bad situation while behind the controls of an aircraft.
Different altitudes are also used as invisible ‘lanes’ on the world’s airspace. This requirement, known as Reduced Vertical Separation Minima, or RVSM, separates aircraft between 29,000ft and 41,000ft, and only allows aircraft flying in the same general direction to carry the same altitude. If an aircraft is traveling a course anywhere from 0 to 179 degrees on the compass dial, then that aircraft is required to be at an elevation beginning with an odd number, like 31,000ft. An aircraft traveling in a direction between 180 and 359 degrees on the compass dial needs to be at an altitude beginning with an even number, like 34,000ft. This helps to keep aircraft safer and avoid mid-air collisions.