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The turn indicator's spin axis is
laterally mounted (across the
aircraft), so it can tilt about the
Instruments 251
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.
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.
Turn And Bank Indicator
This instrument (see pictures above)
also has an inclinometer underneath,
but it has a vertical needle instead of
a horizontal small aircraft. As such it
will only give you the rate of turn,
since it is only sensitive to yaw.
It is also electrically driven, with the
axis across the aircraft (so it spins up
and away from you), each end of the
spindle held in place with a spring.
Thus, it can only move in two axes,
none of them vertical. The spin rate
is 10,000 RPM.
As the springs stretch to cope with
gyro movement around the
longitudinal axis, they apply a force
that produces a secondary precession
equal to the rate of turn. In other
words, a turn makes the gyro move,
to create a primary precession that in
turn stretches a spring that creates
another one in the same direction as
the original force.
All errors cause under-reading,
except when the rate of turn is less
than 1, when rotor speed is too fast
than normal, and springs are slack.
The ball is sensitive to gravity and
centrifugal force.
Flight Director
This is a combination of the artificial
horizon, localiser & glideslope (see
Radios), radio altimeter for DH and
warning flags for instrument failure,
so not only does it tell you which
way up you are, it also tells you
where you are going, during an
approach, at least. The idea is to
keep as much information as
possible in a small area to reduce the
scan rate. It doesn’t tell the aircraft
what to do – rather, command bars
show you what control inputs are
required. All you need to do is align
the small aircraft in the middle with
them as they move.
More sophisticated flight directors
that take their input from multiple
computers are called Flight
Management Systems.
Horizontal Situation Indicator
The HSI is also a combination
instrument that displays heading and
navigation information. In other
words, it is a gyro compass with the
VOR and needle included. There
may also be a DME readout. It
allows you a vertical perspective of
your position in relation to navaids:
252 Canadian Professional Pilot Studies
EFIS
The Electronic Flight Instrument System
replaces the traditional ones with
CRT displays, hence the occasional
reference to Glass Cockpit Displays:
These have no moving parts, and
can be switched to show different
instruments, or duplicate
information, which is helpful if one
fails. In emergency, you can isolate
some instruments for closer scrutiny.
Instrument Flying Procedures
Although accurate flying is
important, good IFR work is based
on correct procedures, and a
methodical routine, which allow
anticipation. Given two marginal
candidates flying a test within the
accepted limits, the one with better
procedural tactics and a systematic
way of working will be in a better
position for a pass, if the examiner is
in a good mood.
The instrument experience you get on your
basic licence course does not qualify you for
proper instrument flying! You must learn
to overcome many of your body's
limitations to do it properly.
ATC always change things to help
traffic flow, and they often tell you
to just revert to the previous
clearance when the panic's over, so
get into the habit of writing
everything down, particularly radio
frequencies, as you may need to
return to the previous one if you
can't make contact on the new one
(also, change to the new on one the
other box, if you have one). Write
down the levels you are cleared to. If
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