OBJECTIVES:
Unlike a car or a boat, operating an aircraft takes place in the 3 dimensions. In addition to the horizontal plane, an aircraft also requires height to operate between its point of departure and point of arrival.
Vertical navigation is important for 3 reasons:
The Altimeter is the instrument used to measure the vertical distance of an aircraft above a specified reference point or level. In its simplest form, an altimeter can be compared to a barometer measuring atmospheric pressure.
An altimeter is an active instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry.
Air pressure decreases with an increase of altitude — about one millibar (0.03 inches of mercury) per 27 feet (8.23 m) near sea level.
A pressure altimeter (also called barometric altimeter) is the traditionnal altimeter found in most aircraft. Inside, an aneroid capsule measures the air pressure from a static port (a small hole) outside the aircraft and makes the needles in your altimeter move.
Explained in a simple way, the altimeter contains a little metallic vacuum box called an aneroid barometer (aneroid means NO AIR) that reacts on pressure differences. Since the higher you go, the lower the pressure will be, there is an easy reference between pressure and altitude. There is a small needle connected to the metallic box and that needle is put in front of an altitude scale.
To correct the altitude for the pressure level, there is an adjustable knob to insert the correction.
That correction is either to the standard pressure (1013 Hpa or 29.92 inches) or to the local pressure.
(Source: Doc 9426 - Air Traffic Services Planning Manual / Doc 8168 - Aircraft Operations - Vol. 1)
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