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ensure that the needle points vaguely
where you expect it to. The TEST
button spins the needle 90° away
from its tuned position, and return,
to indicate a good signal.
You can improve reception by offtuning
slightly, which reduces station
interference, or using a higher
frequency to reduce static.
BFO means Beat Frequency Oscillator.
The BFO switch also uses the sense
aerial by itself to detect the
modulated Morse identifier. Hearing
this by itself helps you tell if there is
any fading (night effect) or noise
(thunderstorms, interference). The
tone you hear when this switch is
activated is actually put there by the
ADF receiver, since a carrier wave
by itself cannot be heard.
Aside from continuously listening to
the ID, the only way of knowing
about problems is seeing the needle
rotate to the right if the signal is not
received – there are no Off flags.
The fixed card display (goniometer)
consists of a compass rose with 0°
representing the nose of the aircraft
at the top of the instrument, and a
needle that points to where the
signal is coming from (including
thunderstorms if they are stronger
than what you are tuned into). Thus,
if a station is ahead, the needle will
point to 0°, or 180° if it is behind.
However, if you made no allowance
for wind, and just pointed the nose
of the aircraft at the station, you
would actually follow a curved path
towards it. Allowing for drift lets you
keep a straight track, needed to keep
on the airway (see Tracking, below).
Unfortunately, working with fixed
cards involves a little maths. First of
all, though, here are a few
definitions:
·  Magnetic Heading – the angle
between the longitudinal axis
and magnetic North.
·  Relative Bearing - the angle
between the aircraft’s
longitudinal axis and the NDB,
288 Canadian Professional Pilot Studies
which is what you read directly
from a fixed card ADF.
·  Magnetic Track or Bearing – the
angle between aircraft position
and the NDB, either to or from.
Take note of this formula (you will
need it in the exam):
MH + RB = BTS
In other words, the magnetic
heading plus the relative bearing
gives you the bearing to the station.
Taking the example below, the
formula would read:
324 + 46 = 010
You can 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
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
Electricity & Radio 289
will already know the bearing to
station (BTS), because it will be on
　
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