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formula, which should be
remembered for the exams:
NM = 1.23 X ÅHeight AGL
It’s simple Pythagoras, since it
involves right-angled triangles.
Technically, the Height AGL above is
the height of both antennae (i.e.
transmitter and receiver), in feet, but
this seems to be ignored for exams.
In real life, the results from the
formula above will vary if the
transmitter is weak, there is
something in the way, or the receiver
is not working properly.
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
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 (well, you
will in the exam, anyway), 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
Electricity & Radio 285
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 minutes on
the outer scale, and 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.
An L NDB is under 50 watts in
power, an M is between 50-2,000
and an H is over 2,000 watts. Power
affects NDB range (in nm) by a
factor of 5 times the square root of
the power in watts:
Range (nm) = 5 x ÅPower(W)
The medium (and long) frequencies
used are between 190-1750 KHz,
but that part of the radio spectrum
also includes commercial radio
stations. Their use in IFR work is
not allowed because of the problems
of identification, and there are no
guarantees of consistency of service,
but they are useful to listen to on
long journeys (and yes, the needle
still points to the station).
NDBs are flight checked for
accuracy to within ± 5° for approach
and 10° enroute (exam question). If
there is no ID, but the system
otherwise appears to behave
normally, the NDB is undergoing
calibration or maintenance (another
　
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