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The localiser transmits two
overlapping lobes of radio energy on
VHF, the one on the left during
approach being A2 modulated at 90
Hz, and the other at 150 Hz. If the
depth of modulation of both lobes
appears to be the same, the receiver
assumes you are on track. The
greater the difference in modulation
depths, the more the indicator
needle (below) is displaced. The
glidepath transmissions are done in a
similar way, but on UHF, with the
frequencies paired with those used
for the localiser (the upper lobe is
the 90 Hz one). Unfortunately,
ground reflections from the lower
lobe produces side-lobes which may
give false glidepath indications.
Luckily, these should be above the
real glidepath, but you should still be
aware of them. Watch for
inordinately high rates of descent,
and check altitudes against distances.
Glidepath calculations in exams are
simply variations on the 1 in 60 rule.
The localiser is checked for accuracy
within 10° either side of course up to
18 nm, and 35° within 10 nm. It is 4
times more sensitive than the VOR,
so full deflection is 2 ½ °, as opposed
to 10° ( ½ ° per dot), with a width of
700 feet at the threshold. If the
localiser is more than 30° away from
the centreline of the runway, it is
known as an offset localiser.
The signals received in the cockpit
are translated onto an instrument
like this:
The vertical needle shows whether
you are left or right and the
horizontal one tells you whether you
are high or low. In the above
example, you are high on the
glideslope and left of the localiser, so
you "chase the cross" to get back on,
down and to the right.
Here is a picture of it combined with
other stuff in a Horizontal Situation
Indicator (HSI):
The two black bars on either side (in
the centre, above) are the glideslope
indicators, and the rest of the
instrument behaves like an RMI – in
the example above, the aircraft
Electricity & Radio 297
heading is 175°, and so the localiser
is on the left. The runway heading is
200°, so the allowance for drift is
25° port, on the glideslope.
Note that the OBS serves no
function when an instrument that
displays OR readings is also used for
the ILS.
The frequency range for the localiser
lies between 108-112 MHz, on odd
decimals, within which there will be
40 channels. The glideslope has the
same number, between 329-335
MHz, with the frequencies paired.
Glidepath means any part of the
glideslope intersecting the localiser.
While the localiser is transmitted
from the far end of the runway, the
glideslope is generated between 750-
1250 feet from the threshold. Its
signals are only valid down to the
lowest authorised Decision Height.
The Threshold Crossing Height (TCH) is
the height that the glideslope
antenna should be to ensure that the
wheels don't hit the ground if they
hang too far below the cockpit.
When tuning in, the Morse Code
ident will usually start with an I.
However, in mountainous areas, a
track guidance localiser helps arriving
aircraft, which uses X, meaning that
the glidepath is unuseable (there
probably won't be one there at all).
Three beacons (at 75 MHz) are used
on the way in so you can tell how far
you've got to go, the Outer Marker at
about 7 miles, which often coincides
with an NDB, and the Middle Marker
at about ½ nm (where you should
be at 200 feet), plus an inner marker
just before the threshold, although
these are not present in Canada. In
the cockpit, the outer marker will
produce a flashing blue light within a
couple of degrees of the overhead
(it's actually geared for a number of
seconds at a particular speed). The
middle marker will be amber and the
inner one white, if used. They all
beep as well, using different tones
(400, 1800 and 3000 MHz if you
really want to know). You may, of
course, be lucky and have a DME to
help with distance information.
298 Canadian Professional Pilot Studies
You need to know the effects of
sudden changes in wind velocity
when coming down a 3° glidepath at
constant speed and power. A strong
headwind causes a decrease in
groundspeed and rate of descent,
and a tailwind does the opposite.
Where one runway has an ILS at
each end, only one will be used at
any time. If you have to use one at
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