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distance away. However, it is not
transmitted at the same strength all
the way round – the amplitude is
also varied to produce a polar
diagram called a limacon, which is
similar in shape to the cardioid used
by the ADF (below), but without an
absolute null point. It rotates 30
124 Canadian Private Pilot Studies
times per second. The phase of this
signal in the aircraft depends on the
bearing from the station (which is
probably why it's called a variphase
signal). Both signals are in phase at
0°, or North, 90° out when East, and
so on. For each degree moved, the
signal changes phase for the same
amount, in both frequency and
amplitude, which is how your
direction is determined. Because the
signal is frequency and amplitude
modulated, it is classed as an A9W
signal. Just to complicate matters,
Doppler VOR has its modulations
the other way round.
Overhead the beacon, you will be in
a cone of confusion that exists with any
antenna – this is an area where no
signal is received, so the TO/FROM
flags disappear and the alarm flag
comes up (in the case of the VOR,
the cone is 100° across). During this
station passage, you should ignore the
signal. There will also be ambiguities
abeam the beacon - at a point 90°
either side of the selected radial there
is a zone of ambiguity where the flag
will not show at all, and the
indications should therefore not be
relied upon.
Inside your aircraft will be a large
black box somewhere in the back,
with a remote indicator in the
cockpit:
Once you select a radial by turning
the Omni Bearing Selector (the small
knob just under the dial), the Course
Deviation Indicator needle will be in
the centre, or either side of the
centreline, up to 10° away from the
radial, so each dot left or right
represents 2°, if there are 4 dots on
your display (2 ½° if there are 3).
When in the middle, you will be on
the radial, which traditionally is from
the station when on the same side,
shown by To/From Flag, which, on
later instruments, will be a small
white triangle pointing in the
relevant direction. If the indicator
shows To, you are on the reciprocal, or
on the other side (in the above
example, the radial is N, or 360°,
because the To flag is showing).
This is a common trap in exam
questions – if you are tracking
inbound on a radial, remember to set
the reciprocal at the top of the display,
since radials go from the station.
All you have to do then is watch the
needle – if you are going away from
a station on a radial, and the needle
is pointing left, then you fly left until
it centres:
Electricity & Radio 125
If you are going to the station, then
you fly right. The thing to remember
is that the needle always points to where
the radial is, which is nothing to do with
the heading of the aircraft (remember
this for exams). All you do is follow
the needle (when coming down the
ILS, you follow the cross formed by
the localiser and glideslope needles).
In short, the radial is where the
needle is, and you do not necessarily
turn that way to get to it -
sometimes, having the needle on the
left means turn right! Only if your
heading is the same direction as the
OBS will it be on the correct side.
For any radial, there are boundaries
formed by the CDI and the
To/From indicator, forming
quadrants around the station (that is,
there are four distinct areas). You
will be in one of them.
Common routes between stations
may use a shared radial which may
be identified with a Victor number if
they are also used as an airway, such
as V-24.
You can therefore take the
indications from two VOR stations,
draw the lines of position (i.e. bearings)
from the compass roses and the
intersection point is your position:
Remember to ensure that the CDI is
centred in both cases and the FROM
flags are showing.
When tracking along an air route,
tune and identify the station you are
going from, track the selected radial
until near the mid-point, then tune
and identify the next station. The
To/From flag should change over.
If you have to use another VOR to
provide a fix as a reporting point
along the air route, select the
required radial, and when the needle
is centred you are over the fix:
126 Canadian Private Pilot Studies
If you have to check whether you
　
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