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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
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
Instruments 71
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
those areas.
As the magnetic pole and lines of
force do not coincide with either the
true poles or lines of longitude, there
is a system of accounting for magnetic
variation, discussed in Navigation.
A direct reading compass has a
pivoted magnet that is free to align
itself with the horizontal component
of the Earth's magnetic field. It must
have certain properties to be able to
do this, namely:
· Horizontality. The needle must
dip as little as possible. This is
done by making its centre of
gravity lie below the pivot point,
with pendulous magnets, which
opposes the vertical component
of the Earth's magnetic force
(Z). Although there is still a
residual dip, if it is less than 3°
at mid-latitudes, it is OK. There
is also a collar and sleeve
assembly that stops it falling
apart when inverted.
· Sensitivity. This can be improved
by increasing the length and/or
the pole strength of the magnet.
However, two short magnets
will do just as well, and they can
also be employed as the weights
under the pivot point
mentioned above. Pole strength
can be increased by using
special alloys. In addition, you
could use a jewelled pivot to
reduce friction, a suspension
fluid which both lubricates it
and reduces the effective weight
of the whole assembly.
· Aperiodicity. The ability to settle
quickly after a disturbance,
which is helped by the
suspension liquid. The two
magnets employed above are
also useful here, as they keep
the mass of the assembly near
the pivot, reducing inertia. Light
alloys reduce inertia even more.
Being magnetic, the compass will be
affected by all the fields generated by
the aircraft itself, causing a
phenomenon called Deviation, which
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