Information Sheet No. 4.4.1I
A clear understanding of the airspeed of an aircraft and how it relates to pressure and
altitude is essential in order to effectively navigate. This chapter will explain the theory,
principles, and techniques required to accurately calculate required airspeed.
Altitude is defined as height above a given reference. Altitude relates to the navigation
problem because of the corresponding density changes with changes in altitude. These
pressure and temperature changes at different altitudes affect True Airspeed, thereby
influencing the DR plot.
All aircraft use a barometric altimeter to determine height. Some aircraft use additional
types of altimeters, including encoding and radar altimeters that are specialized equipment
used for mission requirements. The barometric altimeter is an aneroid barometer which
converts pressure differences to a direct readout in feet.
Altimeter readings must include a reference in order to be useful. Altimeter readings for a
barometric altimeter use the current barometric pressure at Mean Sea Level (MSL) as the
reference. Prior to an aircraft's departure, the airfield tower controller tells the pilot the local
altimeter setting, which is the barometric pressure at Mean Sea Level for the airfield.
Airfields are normally higher than Mean Sea Level; and when the pilot sets the local
altimeter setting in the Kollsman window of the aircraft's altimeter, the altimeter will indicate
the airfield's elevation above Mean Sea Level. For example, if the aircraft is in Denver,
Colorado, the altimeter will indicate approximately 5,600 feet while the aircraft is still on the
ground since the elevation at Denver is 5,600 feet MSL. The altitude shown on the
altimeter is called Indicated Altitude.
Altimeters are subject to errors caused by installation, mechanical misalignment,
positioning of the pressure-sensing ports on the aircraft, and age/wear. These errors are
grouped into one category called Instrument Error. Instrument error is determined by
noting the difference between known airfield elevation and Indicated Altitude (on altimeter)
prior to takeoff when the current airfield altimeter setting is SET. For example, an aircraft
altimeter showing an Indicated Altitude of 80 feet at NAS Pensacola, where the airfield
elevation is 30 feet MSL, would have an instrument error of +50 feet. You cannot correct
for instrument error; and for this reason, if the total altimeter error is in excess of 75 feet,