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instrument is connected to both the
static and dynamic pressure systems.
It's similar to the altimeter inside,
except that the capsule is fed directly
with dynamic pressure, and its size
varies in direct proportion to it. The
needle, connected to the capsule, will
read airspeed directly. Some aircraft,
such as the Bell 407 helicopter, have
a dampened needle, which will
indicate the speed you have been, and
not the speed you are at.
The instrument may be calibrated in
knots or mph, that is, a rate of change
of distance per unit of time. There
are several variations, however:
· Indicated airspeed (IAS) is read
directly, without corrections.
· Calibrated airspeed (CAS) is the
IAS corrected for instrument and
position errors, which are highest
at low speeds (IAS and CAS will
be about the same at speeds
above cruise). It's known by
older pilots as the Rectified
Airspeed (RAS).
· True Air Speed (TAS) is the CAS
corrected for altitude and
temperature (remember its
original calibration is based on
70 Canadian Private Pilot Studies
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.
Various markings are quite 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 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. A white arc, on
an aeroplane, is for flap operation.
If the pitot tube becomes blocked,
you will see a gradual increase in
speed with height (it's mostly a
barometer, and therefore an
altimeter inside). If the static is
blocked, it will under-read in a climb
and over-read in a descent (the best
way to remember this is that it will
always indicate more slowly than if it
were working properly).
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" equals 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'.
Vertical Speed Indicator
There is a capsule inside this, too,
but it is connected only to the static
system. However, there is a restrictor,
or calibrated leak between the inside
and outside of the capsule that
makes the pressure outside lag
behind that on the inside.
The capsule is compressed or
expanded one way or another and
the "suitable linkage" transfers the
movements to the dial to show
climb or descent:
It is both a trend and a rate
instrument, showing a direction of
movement (up or down), and how
fast you're doing it, in hundreds of
feet per minute.
It suffers from lag error, which may
last up to 6-8 seconds before the air
inside and outside the capsule
stabilizes, and reversal error, which
occurs when abrupt changes cause
Instruments 71
movement briefly in the opposite
direction.
Compass
The Earth has its own magnetic
field, which resembles a doughnut,
in that the lines of force are more or
less parallel with the curvature of the
Earth but increase their angle
towards the Poles until they move
vertically downwards in a circle
surrounding the true pole:
As the compass needle tries to
follow the lines of force, you will
find it trying to dip near the Poles, to
a point where it is almost vertical
and actually unreadable. This is why
true tracks and headings are flown in
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