The Altimeter

An altimeter is essentially just a barometer that indicates the pressure it senses in terms of altitude in feet. Atmospheric pressure always decreases with height and the altimeter is calibrated so that for a specific pressure it senses, it indicates a certain altitude. The altimeter, however, rarely indicates the true height.

In order to calibrate altimeters to ensure they all indicate identical readings, they are designed to only indicate the correct altitude when standard atmospheric conditions exist. Actual atmospheric conditions rarely match the ISA value, therefore it is very important to understand how meteorological instrument errors may affect the safety of flight. For example, if the altimeter is being used for vertical separation of aircraft, and all aircraft have the same error, the magnitude of error is not important. If however, the altimeter is being used for landing or for clearing enroute obstacles, the error may be fatal