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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:
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
Electricity & Radio 125
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.
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:
If you have to check whether you
have passed it or not, in this case,
the needle will be to the left of the
station is on the left, and vice versa if
you are not there yet, assuming your
heading is the same as the OBS.
Otherwise, the needle will point the
opposite way if you have already
gone past (oops!).
To intercept an inbound radial, tune
and identify the VOR station, select
the reciprocal of the desired radial,
by turning the OBS until you get a
TO reading. Fly to whichever side
the needle is displaced, turning the
shortest way to a heading 90° away
from it, until the needle starts to
move, at which point reduce the
intercept angle to 45°:
126 JAR Private Pilot Studies
As the needle centres, reduce the
intercept angle again and maintain
the track with suitable adjustments
for drift.
Do the same outbound, except look
for a FROM reading.
A good rule (inbound and
outbound) is to subtract the
intercept angle if the needle goes
left, and add if it goes right to find
the heading to steer. For example,
280°-90°=190°.
To bracket for drift, turn onto a zero
wind heading and see what the drift
actually is. Get back on track, make a
large correction the opposite way
and see what happens. Get back on
track and half the original correction.
Keep doing this until the correct
heading is found.
Testing
Some airfields have low power test
equipment (2 watts) transmitting on
114.8 (usually), identified with the
ATIS, so have a pen ready to save
you writing it down again later (the
ID may just be a series of dots). The
system is intended for ground use –
although it can be used when
airborne, there will be certified
airborne check points, but you could
always get to a position on a known
radial and check the readings. As you
move the OBS, you can expect the
usual indications relating to the
bearing selected (which is why two
transmitters are used, to save you
moving the aircraft to the radials).
With the needle centred, the
instrument should read 000° FROM
or 180° TO at any point within the
airport, with an accuracy of ± 4° (±
6° when airborne).
In fact, transmitter error (or FM/AM
synchronisation, at least) should be
　
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