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Measurement of Aircraft Speed and Altitude

Accurate measurements of speed and altitude are essential to the safe and
efficient operation of aircraft. Accurate speed measurements, for example, are
needed to avoid loss of control at low speeds (stall condition) and to prevent
exceedance of the aerodynamic and structural limitations of the aircraft at
high speeds, whereas accurate altitude measurements are needed to insure clear-
ance of terrain obstacles and to maintain prescribed vertical separation minima
along the airways.

The instruments that are used to measure speed and altitude include the
altimeter, the airspeed indicator, the true-airspeed indicator, the Machmeter.
and the rate-of—climb (or vertical-speed) indicator. All these instruments are
actuated by pressures, while one, the true-airspeed indicator, is actuated by
air temperature as well.

Two basic pressures, static pressure and total pressure, are used to actu—
ate the instruments. The static pressure is the atmospheric preSSure at the
flight level of the aircraft, while the total pressure is the sum of the static
pressure and the impact pressure, which is the pressure developed by the forward
speed of the aircraft.

The static pressure is used to actuate both the altimeter and the rate-cf-
climb indicator. Although this pressure varies from day to day, the decrease in
static pressure with height is generally cont;nuous at any one time and place.
Accordingly, a pressure-height relation based on average atmospheric condi ions
has been adopted as a standard (see “standard atmosphere" in chapter [11).
Measurements of static pressure are then used to provide indications of height
in terms of pressure altitude (chapter XII) and indications of vertical speed
in terms of rate of change in the pressure altitude.

For the three forward-speed indicators, impact pressure is derived as
differential pressure from measurements of total pressure and static pressu
in accordance with equation (1.1). The airspeed indicator 1: actuated solely
to impact pressure and is calibrated to indicate true airspeed at sea-level
density in the standard atmosphere: at altitude, however, the indicated airs;eed
is lower than the true airspeed (chapter III). The true-airspeed indicator. on
the other hand. combines the measurement of impact pressure with measurements of
static pressure and temperature to ind'cate true airspeed independent of il:i-
tude.