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upward or downward trend, thereby helping maintain a
constant altitude.
Some of the more complex VSIs, called instantaneous vertical
speed indicators (IVSI), have two accelerometer-actuated air
pumps that sense an upward or downward pitch of the aircraft
and instantaneously create a pressure differential. By the time
the pressure caused by the pitch acceleration dissipates, the
altitude pressure change is effective.
Dynamic Pressure Type Instruments
Airspeed Indicator (ASI)
An ASI is a differential pressure gauge that measures the
dynamic pressure of the air through which the aircraft is
flying. Dynamic pressure is the difference in the ambient
static air pressure and the total, or ram, pressure caused by
the motion of the aircraft through the air. These two pressures
are taken from the pitot-static system.
3-9
Figure 3-11. Mechanism of an Airspeed Indicator.
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, TAS
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 case. 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
the instrument pointer to indicate the TAS on the subdial.
[Figure 3-12]
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 ASIs are calibrated in knots, or
nautical miles per hour; some 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 POH/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.
3-10
Figure 3-12. A true airspeed indicator allows the pilot to correct
IAS for nonstandard temperature and pressure.
Figure 3-13. A Machmeter shows the ratio of the speed of sound to
the TAS the aircraft is flying.
Figure 3-14. A maximum allowable airspeed indicator has a movable
pointer that indicates the never-exceed speed, which changes with
altitude to avoid the onset of transonic shock waves.
Most high-speed aircraft are limited to a maximum Mach
number at which they can fly. This is shown on a Machmeter
as a decimal fraction. [Figure 3-13] For example, if the
Machmeter indicates .83 and the aircraft is flying at 30,000
feet where the speed of sound under standard conditions is
589.5 knots, the airspeed is 489.3 knots. The speed of sound
varies with the air temperature. If the aircraft were flying at
Mach .83 at 10,000 feet where the air is much warmer, its
airspeed would be 530 knots.
Maximum Allowable Airspeed
Some aircraft that fly at high subsonic speeds are equipped
with maximum allowable ASIs like the one in Figure 3-14.
This instrument looks much like a standard air-speed indicator,
calibrated in knots, but has an additional pointer colored red,
　
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