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Taking the above example, the
formula would read:
324 + 46 = 010
Get the relative bearing like this:
BTS – MH = RB
If you split the display into two
halves, based on a line between 0°
and 180°, and call the right half plus,
and the left minus, you can use the
needle’s position in either to find the
130 JAR Private Pilot Studies
track to a station. For example, if the
needle is in the right half (the plus
segment), add the heading to the
relative bearing to get the track. If it
is in the left, take it away (work the
needle back from zero in this case).
Whilst turning right, the aircraft
heading will increase while the relative
bearing decreases, and vice versa. As
long as you remain on the same
bearing, the amount of heading
change will always equal the change
of ADF indication.
RMI
The Radio Magnetic Indicator is a
combination of ADF indicator and
slaved compass that replaces the
fixed card with one that moves, so
the top of the instrument represents
the aircraft heading and the needle
points to the actual bearing (or
reciprocal, if you look at the other
end), which saves you doing the
calculations above in your head. In
other words, it always displays the
present heading and bearing, and
does some of the work you would
have to do with a fixed display.
There will also be a repeater needle
from the VORs giving you the same
information relative to the stations
they are tuned to.
In the above example, the heading is
324°, and the Bearing To Station (BTS)
is 010°.
As a point of interest, the VOR
needle on an RMI will always read
correctly if any deviation occurs –
the heading and ADF reading will be
in error by the amount of deviation.
Position Fix
For a fixed card ADF, find the
relative bearing to each station and
add them to your heading to get the
tracks to the stations. Then find the
reciprocals and plot them outwards.
Along an airway, to find where you
are in relation to an intersection, you
will already know the bearing to
station (BTS), because it will be on
the map. Using the formula:
RB = BTS – MH
you can find what the needle will
indicate when you get there.
Time to Station
As with the VOR, note the seconds
taken to go through a number of
degrees on the relative bearing, and
divide the time just noted by the
number of degrees gone through to
get the time in minutes.
Then use groundspeed (or TAS in
emergency) to find your distance.
Tracking
To find an intercept heading, just
add or subtract the intercept angle to
the track you wish to establish, as
with an airway. It’s common to use
90° inbound and 45° outbound, but
use whatever ATC and
circumstances (or exam questions)
dictate (30° is nice). Note the track,
and add or subtract your heading, as
Electricity & Radio 131
appropriate, to get the expected
relative bearing when on track,
which you will be when the needle
of a fixed card points to it. With an
RMI, just watch the needle.
When drifting, the needle will always
point to the side of the aircraft the
wind is coming from, so corrections
inbound should always be made that
way, ensuring that the needle actually
goes to the other side of the
longitudinal axis once a corrected
heading is established. For example,
if you want to track 090°, and the
wind is coming from the right, to be
on track you want to end up in a
situation where the heading is an
equal amount of degrees the other
side of the lubber line as the needle
is, such as a heading of 110° (plus 20
of the lubber line), looking for a
340° relative bearing (minus 20 of
the lubber line).
Or, to use another example, for a
track of 090°, your heading might be
070° while the ADF needle points to
110° (heading minus 20°, looking for
plus 20° from the needle). If you are
going the same way as your track,
the needle will tell you which way to
go. If it is on the left, your track will
be on the left, and vice versa. Just turn
whichever way until the needle reads
the desired intercept on the opposite
side. A good ploy is to allow the drift
to happen until you get a positive
reading, say 10° port, double it the
other way (20° starboard), and when
you are back on track, reduce by half
(i.e. 10° in this case) to hold it.
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