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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°:
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
within ±1° - the system should shut
down automatically if it gets outside
that. Phase comparison error should not
be more than ±3°, and station errors
should be within ±1°. The nominal
accuracy is ±5°.
Problems
Although the VOR is less subject to
static and interference than an NDB,
and it is much more accurate, the
transmissions depend on line of
Electricity & Radio 127
sight, and there is a zone of
ambiguity at 90° to a radial,
mentioned above. In addition,
certain propeller or rotor RPM
settings can cause fluctuations up to
±6° (change the RPM slightly before
saying the instrument is U/S).
Time to Station
You often need to know the time it
will take to get to a station, which is
simply found by turning 90° from
the inbound radial and noting the
seconds taken to go through a number
of them. To get the time in minutes,
divide the time just noted by the
number of radials (degrees) gone
through. All you need do then is use
the groundspeed (or TAS in
emergency) to find your distance.
For time to station, the formula is:
Time (mins) = Mins x 60
Degrees
On the whizzwheel, set the minutes
on the outer scale, and the degrees
on the inner one. Read the answer
on the outer scale opposite the 60
arrow.
For the distance, try:
Distance = Mins x GS
Degrees
ADF/NDB
An Automatic Direction Finder (ADF),
also known as a radio compass, is a
device in an aircraft that picks up
signals broadcast on the Medium
wave band by Non Directional Beacons
(NDBs), so called because they
radiate in all directions.
Transmissions are not dependent on
line of sight, so the system is good
for long distance travel, although it
does have a few problems,
mentioned below. It is possible to
get 1,000 nm range over sea and 300
nm over land if the power is high
enough, but since better systems
have come along, NDBs are now
used as enroute navaids on airways,
homing beacons for instrument
approaches and markers for the
Instrument Landing System (ILS), with a
typical range of about 35 nm.
NDBs are checked for accuracy to
within ± 5° for approach and 10°
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