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original calibration is based on
the standard atmosphere). The
slide rule part of the flight
computer is used to calculate
these, discussed below. On
average, the TAS increases by
2% over the IAS for every 1,000
feet. Refer to the Performance
chapter for a discussion on the
effects of air density on TAS.
·  Equivalent Airspeed (EAS) is CAS
compensated for compressibility,
or factors coming from high
speeds above around 300 kts.
Various markings on the ASI are
useful if you don't have the flight
manual to hand. The green arc covers
the range of speeds for normal
operations. The yellow arc is the
Instruments 241
caution range (that is, not to be used
for long periods of time), and the red
line is the speed not to be exceeded,
VNE (on a turbine, the equivalent is a
red and white barber pole affair that
moves with temperature and other
things affecting the speed of sound).
A white arc, on an aeroplane, is the
range for flap operation.
If the pitot tube gets blocked, you
will see a gradual increase in speed
with height (it's an altimeter inside).
If the static is blocked, it will underread
in a climb and over-read in a
descent (that is, it will indicate more
slowly than if it were working).
To find TAS, start with the CAS and
Pressure Altitude. You will also need
the temperature which, in an exam,
may involve a conversion from
Fahrenheit to Centigrade, and from
miles per hour to knots. For
example, given an altimeter setting
of 30.40", an indicated altitude of
3450', an OAT of 41°F and an IAS
of 138 mph, find the TAS in knots.
For the moment we will take CAS as
118 kts, having converted 138 mph
to 120 kts and looked it up on an
imaginary graph. If there isn't a
graph, the question will contain the
information required. 41°F also
converts to 5°C. The PA is found in
the usual way, remembering that 1"
is equal to 1,000'. The difference
between 29.92" and 30.40" is .48, or
480 feet, which gives 2970' when
subtracted from 3450' (29.92 is the
"higher" figure in terms of distance
above ground).
The TAS is 122 kts, and the Density
Altitude (out of interest) at 2500'.
Machmeter
This is an instrument that displays
aircraft speed in relation to the local
speed of sound. It will only be found
in high speed aircraft, because they
will suffer from the effects of flying
in that range (see High Speed Flight).
The Mach number is TAS compared
to the local speed of sound, and is
found with this formula:
Mach # = TAS
S
where S is the speed of sound which,
luckily, varies only with temperature,
and can be found with this formula:
S = 38.94 X Å°Kelvin
The result is in knots, and to get
Kelvin, add 273 to the Centigrade.
In other words, the speed of sound
is proportional to the square root of
the absolute temperature, so height
doesn't really have much to do with
it. For example, at MSL in ISA, it
will be 661.32 kts. It is the same at
FL100 if the OAT is 15°C.
242 Canadian Professional Pilot Studies
On the whizzwheel (computer), if
you set the Mach Index opposite the
temperature, you can read the speed
of sound directly against the inner
scale 1.0.
In the above picture, the Index is in
a window at the bottom. TAS 280
corresponds to a Mach No of .424.
You can go the other way and find
the TAS from the OAT and Mach
number. The formula is simply:
TAS = S X Mach No
You need the temperature to find S
first (see above). On the computer,
line up the Mach Index with the
OAT in the TAS window and look
for the TAS on the outer scale
opposite the Mach No.
At high speeds, compressibility
becomes important, meaning that
the temperature will become
artificially increased at speeds above
about 300 kts, because air that
cannot get out of the way fast
enough is compressed against the
undisturbed air around the airframe
(this is already accounted for in the
CR flight computer, so don't add any
figures again from charts or tables).
The dynamic pressure is measured
by an airspeed capsule inside the
machmeter, while the static pressure
is measured by an aneroid capsule.
They are at right angles to each other
and a complex linkage detects their
movement ratios. In other words, an
ASI and an altimeter (in the same
casing) feed their movements to a
　
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