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casing turns around the gyro, which
has a compass card mounted on it. It
is also air driven.
Because of wander (real or apparent
- see above), unless you have a slaved
compass (meaning automatic), you
should align this instrument with the
magnetic compass every 15 minutes
or so, remembering, of course, to do
it in level, unaccelerated flight. You
may get erroneous readings if the
aircraft adopts unusual attitudes.
Turn Coordinator
This is actually a combination of two
instruments, one power driven, and
the other not. A small aircraft tilts to
indicate whether you are banking, so
it is a useful backup to the artificial
horizon, especially since the gyro is
electrically operated and not affected
if the suction system fails (although
it gives you a rudimentary indication
of bank, turns without the other
instruments are done with timing).
The instrument is sensitive to yaw
and roll, because the gyro's axis is
tilted upwards by about 30-35°.
When the wings in the little aircraft
hit one of the lower marks you are in
a Rate 1 turn, which takes two
minutes to go through 360°, making
3° per second (you can also add 7 kts
to 10% of your airspeed to get a
rough guide to the bank angle).
Underneath is a ball in a clear tube
containing fluid, for damping
purposes, called an inclinometer:
76 JAR Private Pilot Studies
It is subject to gravity and centrifugal
force, and will be thrown one way or
another if the aircraft is not in a
coordinated turn. In a slip, the rate of
turn is too slow for the bank, so the
centrifugal force will be less, and the
ball will not be thrown out so much.
It will therefore be on the inside of
the turn (decrease the angle or
increase the rate to correct, or both):
In a skid, the turn is too fast, so
more centrifugal force causes the
ball to be displaced more, to the
outside of the turn:
Correction is the opposite of the
slip, above.
The turn indicator's gyro is laterally
mounted, so it can tilt about the
longitudinal axis. A linkage joins it to
the pointer, and there is a restraining
spring between it and the instrument
case. There are mechanical stops to
keep it from going more than 45°
either side of the centre. As it is
electrically driven, it is not affected
by suction failure. Although it gives
you a rudimentary indication of
bank, turns without the other
instruments are done with timing.
Rate of turn indications are only
accurate at the speed for which the
instrument has been calibrated,
though these are not serious (around
5%). The angle of bank to obtain a
given rate of turn increases with
TAS, but you shouldn't need to
make any calculations – the
instrument reads correctly
automatically. If the gyro rotates too
slowly, the device will have less
inertia and be less rigid, so it will tilt
less and indicate a slower rate of turn
than you are actually doing.
A Question
1. You've just lost all your electrics,
and you are left with a map and the
E2B compass. Can they be relied on
to get you home?
An Answer
1. You can rely on the map, but the
compass will have been swung with
the electrics on, so all the local
magnetic fields will have changed
with them off. You can therefore
expect large deviation errors.
Weather
Around the Earth is a collection of
gases, called the atmosphere. 21% of it,
luckily for us, is oxygen, but 78% is
nitrogen, with 1% of odds and ends,
like argon, that need not concern us
here, plus bits of dust and the odd
pollutant. What is important,
however, is varying amounts of
water vapour which will produce
clouds. Because it weighs fiveeighths
of an equivalent amount of
dry air, it will also reduce your
engine's punch, but that’s the subject
of another chapter. The nitrogen, as
an inert gas, is there to keep the
amount of oxygen down, since it is
actually quite corrosive.
The atmosphere is split into four
concentric gaseous areas. Starting
from the bottom, these are the
troposphere, stratosphere, mesosphere and
thermosphere, although the last two are
not important right now. The first
two are, however, and the boundary
between them is called the tropopause,
a freezing layer of dry air. Its height
over the Equator is around 60,000
feet, more than it is at the Poles
(35,000), because the air is warmer
there and has expanded, taking the
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