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(oC), scale.
3.20.1. Although aircraft thermometers are usually calibrated in oC, it is sometimes necessary to
interconvert Fahrenheit and centigrade temperatures. The following formulas may be used:
oF = (1.8 X oC) + 32o
oC = (oF - 32o) / 1.8
3.20.2. Temperature error is the total effect of scale error and heat of compression error. Scale error is
simply an erroneous reading of the pointer under standard conditions. It is difficult for a crewmember to
evaluate this error without sensitive testing equipment. With this in mind, the reading of the indicator is
considered correct and is called indicated air temperature (IAT).
3.20.3. The second error, heat of compression error, causes the instrument to read too warm. Heating
occurs at high speeds from friction and the compression of air on the forward edge of the temperature
probe. Thus, the IAT is always corrected by a minus correction factor to produce true air temperature
(TAT). Heat of compression increases with TAS. The TAT can be obtained from the aircraft flight
manual.
Section 3G— Airspeed
3.21. Basics. Airspeed is the speed of the aircraft in relation to the airmass surrounding that aircraft.
3.22. Pitot-Static System. Accurate airspeed measurement is obtained by means of a pitot-static system.
The system consists of: (1) a tube mounted parallel to the longitudinal axis of the aircraft in an area that
is free of turbulent air generated by the aircraft, and (2) a static source that provides still, or undisturbed,
air pressure.
3.22.1. Ram and static pressures may be taken from a single pitot-static tube or from completely separate
sources. A pitot-static tube usually has a baffle plate, as shown in Figure 3.21, to reduce turbulence and
to prevent rain, ice, and dirt from entering the tube. There may be one or more drain holes in the bottom
of the tube to dispose of condensed moisture. A built-in electrical heating element, controlled by a
switch inside the aircraft, prevents the formation of ice in the tube.
106 AFPAM11-216 1 MARCH 2001
3.22.2. Reasonable care should be taken with the pitot-static system to ensure continuous, reliable
service. The drain holes should be checked periodically to ensure they are not clogged. At the
completion of each flight, a cover is placed over the intake end of the tube to prevent foreign objects and
moisture from collecting in the tube.
Figure 3.21. Operating Principle of the Airspeed Indicator.
3.23. Principles of Operation of Airspeed Indicators. The heart of the airspeed indicator is a
diaphragm which is sensitive to pressure changes. Figure 3.21 shows it located inside the indicator case
and connected to the ram air source in the pitot tube. The indicator case is sealed airtight and connected
to the static pressure source. The differential pressure created by the relative effects of the impact and
static pressures on the diaphragm causes it to expand or contract. As the speed of the aircraft increases,
the impact pressure increases, causing the diaphragm to expand. Through mechanical linkage, the
expansion is displayed as an increase in airspeed. This principle is used in the IAS meter, the TAS meter,
and the Machmeter.
3.24. Airspeed Definitions. There are many reasons for the difference between IAS and TAS. Some of
the reasons are the error in the mechanical makeup of the instrument, the error caused by incorrect
installation, and the fact that density and pressure of the atmosphere vary from standard conditions.
3.24.1. Indicated Airspeed (IAS). IAS is the uncorrected reading taken from the face of the indicator. It
is the airspeed that the instrument shows on the dial.
3.24.2. Basic Airspeed (BAS). BAS is the IAS corrected for instrument error. Each airspeed indicator
has its own characteristics which cause it to differ from any other airspeed indicator. These differences
may be caused by slightly different hairspring tensions, flexibility of the diaphragm, accuracy of the
scale markings, or even the effect of temperature on the different metals in the indicator mechanism. The
effect of temperature introduces an instrument error due to the variance in the coefficient of expansion of
AFPAM11-216 1 MARCH 2001 107
the different metals comprising the working mechanisms. This error can be removed by the installation
of a bimetallic compensator within the mechanical linkage. This bimetallic compensator is installed and
properly set at the factory, thereby eliminating the temperature error within the instrument. The accuracy
of the airspeed indicator is also affected by the length and curvature of the pressure line from the pitot
tube. These installation errors must be corrected mathematically. Installation, scale, and instrument
errors are all combined under one title called instrument error. Instrument error is factory-determined to
　
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