Aeronautical Information Manual (AIM), page 61

Index   60 -- Page 61 -- 62

FIG 1-1-5
Service Volume Lower Edge
Standard High and Low



0 5 10 15

1-1-9. Instrument Landing System (ILS)

a. General

1. The ILS is designed to provide an approach
path for exact alignment and descent of an aircraft on
final approach to a runway.

2. The ground equipment consists of two highly
directional transmitting systems and, along the
approach, three (or fewer) marker beacons. The
directional transmitters are known as the localizer
and glide slope transmitters.

3. The system may be divided functionally into
three parts:
(a) Guidance information: localizer, glide
(b) Range information: marker beacon,
DME; and
(c) Visual information: approach lights,
touchdown and centerline lights, runway lights.

4. Precision radar, or compass locators located
at the Outer Marker (OM) or Middle Marker (MM),
may be substituted for marker beacons. DME, when
specified in the procedure, may be substituted for the

5. Where a complete ILS system is installed on
each end of a runway; (i.e., the approach end of

20 25 30 35 40

Runway 4 and the approach end of Runway 22) the
ILS systems are not in service simultaneously.

b. Localizer

1. The localizer transmitter operates on one of
40 ILS channels within the frequency range of
108.10 to 111.95 MHz. Signals provide the pilot with
course guidance to the runway centerline.

2. The approach course of the localizer is called
the front course and is used with other functional
parts, e.g., glide slope, marker beacons, etc. The
localizer signal is transmitted at the far end of the
runway. It is adjusted for a course width of (full scale
fly-left to a full scale fly-right) of 700 feet at the
runway threshold.
3. The course line along the extended centerline
of a runway, in the opposite direction to the front
course is called the back course.
Unless the aircraft's ILS equipment includes reverse
sensing capability, when flying inbound on the back
course it is necessary to steer the aircraft in the direction
opposite the needle deflection when making corrections
from off-course to on-course. This "flying away from the
needle" is also required when flying outbound on the
front course of the localizer. Do not use back course
signals for approach unless a back course approach
procedure is published for that particular runway and the
approach is authorized by ATC.

Navigation Aids 1-1-7

Page 61 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   60 -- Page 61 -- 62