The mechanism of the ASI in Figure 3-11 consists of a thin,
corrugated phosphor bronze aneroid, or diaphragm, that
receives its pressure from the pitot tube. The instrument
case is sealed and connected to the static ports. As the
pitot pressure increases or the static pressure decreases, the
diaphragm expands. This dimensional change is measured by
a rocking shaft and a set of gears that drives a pointer across
the instrument dial. Most AS1s are calibrated in knots, or
nautical miles per hour; sonic instruments show statute miles
per hour, and some instruments show both.
Types of Airspeed
Just as there are several types of altitude, there are multiple
types of airspeed: Indicated Airspeed (IAS), Calibrated
Airspeed (CAS), Equivalent Airspeed (EAS), and True
Airspeed (TAS).
indicated Airspeed (IAS)
IAS is shown on the dial of the instrument, uncorrected for
instrument or system errors.
Calibrated Airspeed (CAS)
CAS is the speed at which the aircraft is moving through
the air, which is found by correcting IAS for instrument
and position errors. The IOH/AFM has a chart or graph to
correct IAS for these errors and provide the correct CAS for
the various flap and landing gear configurations. |
Equivalent Airspeed (EAS)
EAS is CAS corrected for compression of the air inside the
pitot tube. EAS is the same as CAS in standard atmosphere
at sea level. As the airspeed and pressure altitude increase,
the CAS becomes higher than it should be, and a correction
for compression must be subtracted from the CAS.
True Airspeed (TAS)
TAS is CAS corrected for nonstandard pressure and
temperature. TAS and CAS are the same in standard
atmosphere at sea level. Under nonstandard conditions, IAS
is found by applying a correction for pressure altitude and
temperature to the CAS.
Some aircraft are equipped with true ASIs that have a
temperature-compensated aneroid bellows inside the
instrument ease. This bellows modifies the movement of
the rocking shaft inside the instrument case so the pointer
shows the actual TAS.
The TAS indicator provides both true and IAS. These
instruments have the conventional airspeed mechanism,
with an added subdial visible through cutouts in the regular
dial. A knob on the instrument allows the pilot to rotate the
subdial and align an indication of the outside air temperature
with the pressure altitude being flown. This alignment causes
die instrument pointer to indicate the TAS on the subdial.
[Figure 3-12]
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