The Instrument Landing System (ILS) is a precision approach
to a particular runway. A precision approach gives both vertical
guidance (the glideslope) and horizontal guidance (the localizer).
The section on the ILS from the AIM was included in an earlier chapter
for a more thorough discussion of the technical aspects of the ILS.
There are four main elements to the ILS:
- the localizer - provides course guidance along the extended
- the glideslope - provides vertical guidance. The usual
slope is 3° to the horizontal.
- marker beacons - provide accurate range fixes along the
approach path. Usually there is an outer and a middle marker.
- approach lights, VASI (Visual Approach Slope Indicator),
and other lighting, such as touchdown zone lighting, runway lighting,
etc. The purpose of the lighting is to assist in the transition
from instrument to visual flight.
There may be other aids, such as a compass locator (NDB)
or DME, available.
The outer marker may be replaced by a range marker, such as a compass
locator or a DME distance on some approaches. The middle marker
may be replaced by a compass locator, but, DME may not be substituted
for the middle marker.
When there is a compass locator collocated with an outer marker,
it is indicated on the approach chart by LOM. A compass locator
collocated with the middle marker is indicated by LMM.
The intersection between the localizer and the glideslope is the
ideal path of approach and is called the glidepath.
Visual information is provided since the ILS may be flown in conditions
of poor visibility. The purpose of the visual aids, such as approach
lighting, touchdown zone lighting, and runway lighting, is to assist
the pilot is transitioning from instruments to visual flight.
The illustration below shows the various parts of the ILS approach.
- Localizer plane - the localizer is normally intercepted first
and the glideslope is intercepted from beneath
- Outer Marker - since the altitude of the glideslope where it
intercepts the OM is given on the IAP, it should be used as a
check to confirm altitude readings and glideslope operation
- Middle Marker - typically the middle marker intersects the glideslope
at or near the Decision Height and is the last point you should
commence a missed approach if not visual
The cockpit instrument used for flying the ILS is usually the same
as your NAV 1 radio in small aircraft and the HSI in the light and
heavy jets. The needle is the same one used for VOR navigation.
You must identify the localizer the same way you do a VOR, by its
Morse Code identifier. The ident is always a four letter
code beginning with I.
The localizer transmits a highly directional beam on a frequency
between 108.10 and 111.95 MHz. The specific frequency can be found
on the approach plates or in the A/FD. There are 40 localizer frequencies
available and all of them have an odd number as the first digit
after the decimal point. The localizer is actually two overlapping
lobes of radio energy. These lobes get wider as they get further
from the localizer antenna array, which is located near the end
of the runway opposite the approach end.
The beam is adjusted so that the width of a full scale deflection
of the localizer needle is 700 ft at the approach runway threshold.
The angular width of the localizer varies between 3 and 6 degrees.
The typical width of the full scale deflection is 5°, or 2.5° either
side of the centerline.
The localizer is accurate within the sectors indicated in the illustration
from an altitude of 1000 ft above the highest terrain along the
course to 4500 ft above the elevation of the antenna. Correct indications
are guaranteed only within this service airspace. Outside this space,
the correct signal is not assured and it is possible you will not
receive the coded ident.
The main function of the localizer is to provide azimuth
(horizontal) guidance along the approach. This is also known
as the localizer front course. Many localizers also transmit a signal
that is usable from the opposite direction known as the back course.
The back course does not have an associated glideslope and is suppressed
in some countries, such as the United Kingdom and Australia.
The indication on the cockpit instrument is the same as that of
a VOR, a right, left or centered needle. The procedure for
getting on course is also the same, fly toward the needle. Full
scale deflection of the CDI needle is approximately 2.5°.
Movement of the CDI is therefore four times more sensitive (0.5°
per dot) than the with the VOR (2° per dot).
Unlike the VOR, the localizer is a single fixed course. Due to
this, the OBS has no affect on the CDI. It is a good practice to
set the OBS to the inbound heading of the localizer as a reminder.
Again, there is NO heading information given, only position information.
If the localizer fails, the whole ILS approach is unauthorized
and is not permitted. If only the glideslope fails, the approach
can still be made as a localizer approach, with higher minimums
FLYING THE LOCALIZER
The localizer is a performance instrument and should be included in
your scan when flying an ILS approach. Any required corrections should
be made with small, coordinated turns. You should concentrate on the
HI and AI for attitude flying with an occasional glance at the CDI
to check tracking. Do not chase the CDI.
Since the localizer beam is narrow, an intercept should be made
at 30° or less. Otherwise, you have a very good chance of flying
through the localizer. If an LOM or other aid is available, it should
be monitored during the intercept to help you anticipate the turn
onto the localizer.
Once the CDI starts moving, indicating you are approaching the
centerline, you should immediately turn to the localizer course.
If there is a crosswind, a wind correction angle should be estimated.
Your goal is to fly a course that will keep you centered on the
The localizer acts as a command instrument while on the approach,
so fly toward the needle. Since the course narrows as you approach
the runway, the corrections necessary will become smaller and smaller.
Winds normally will change as you get nearer the ground, therefore
a number of small corrections is normal when flying the ILS. A good
rule of thumb is to restrict heading corrections to 5° at the beginning
of the approach and only 2° near the end. If you are flying an aircraft
with a heading bug, the bug is about 12° wide (plus or minus 6°),
therefore most course corrections should be made within the width
of the bug. The following illustration shows left of course, on
course, and right of course indications respectively.
You should initially steer a heading that stops the needle from
moving. Then wait a few seconds to see if the CDI moves. If it does,
make a small correction to center the CDI. You should be on a steady
course before reaching the outer marker. Any changes required after
passing the outer marker should be made with changes of 2° or less.
When continuing past the localizer and tracking outbound, as in
a missed approach, the CDI continues to be a command instrument,
unless you reverse course. If you reverse course, the CDI is a non-command
instrument and you would fly away from the needle to regain the
This same situation is encountered when tracking outbound on the
localizer front course, which is sometimes required in an approach.
The indicator will still indicate the angular distance off course,
but you would fly away from the CDI in order to center the needle.
When flying with reverse sensing, think of the required action as
"pulling" the needle back on course. If there is a LOM available,
use it when flying outbound to help orient yourself and stay on
Flying the localizer with the HSI is very similar to using
the regular indicator. There are however several advantages to using
the HSI. First, the HSI reduces the scan required, since it combines
heading and localizer information in one instrument. Another advantage
is the fact that the HSI uses a slaved compass card (it turns when
the aircraft turns). Therefore, the HSI always functions as a command
instrument, even when tracking outbound, provided you set the localizer
heading in the HSI.
Interception of the localizer is also simplified since you are
given a clear visual of where you are in relation to the localizer
and the intercept angle is presented on the HSI. The following illustration
shows left of course, on course, and right of course indications
on the HSI.
This concludes Part 1 of ILS Navigation.
on in part 2: The Glideslope