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AIM

10/12/17

1−1−9

Navigation Aids

specified in the procedure, may be substituted for the

OM.

5. Where a complete ILS system is installed on

each end of a runway; (i.e., the approach end of

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.

CAUTION−

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.

4. Identification is in International Morse Code

and consists of a three−letter identifier preceded by

the letter I (

 ) transmitted on the localizer

frequency.

EXAMPLE−

I−DIA

5. The localizer provides course guidance

throughout the descent path to the runway threshold

from a distance of 18 NM from the antenna between

an altitude of 1,000 feet above the highest terrain

along the course line and 4,500 feet above the

elevation of the antenna site. Proper off−course

indications are provided throughout the following

angular areas of the operational service volume:

(a) To 10 degrees either side of the course

along a radius of 18 NM from the antenna; and

(b) From 10 to 35 degrees either side of the

course along a radius of 10 NM.  (See FIG 1−1−6.)

FIG 1−1−6

Limits of Localizer Coverage

RUNWAY

RUNWAY

LOCALIZER

ANTENNA

LOCALIZER

ANTENNA

10 NM10

 N

M

18 NM18

 N

M

NORMAL LIMITS OF LOCALIZER

COVERAGE:  THE SAME AREA

APPLIES TO A BACK COURSE

WHEN PROVIDED.

NORMAL LIMITS OF LOCALIZER

COVERAGE:  THE SAME AREA

APPLIES TO A BACK COURSE

WHEN PROVIDED.

10°

10°

35°

35°

6. Unreliable signals may be received outside

these areas.

c. Localizer Type Directional Aid (LDA)

1. The LDA is of comparable use and accuracy

to a localizer but is not part of a complete ILS. The

LDA course usually provides a more precise

approach course than the similar Simplified

Directional Facility (SDF) installation, which may

have a course width of 6 or 12 degrees.

2. The LDA is not aligned with the runway.

Straight−in minimums may be published where

alignment does not exceed 30 degrees between the

course and runway. Circling minimums only are

published where this alignment exceeds 30 degrees.

3. A very limited number of LDA approaches

also incorporate a glideslope. These are annotated in

the plan view of the instrument approach chart with

a note, “LDA/Glideslope.” These procedures fall

under a newly defined category of approaches called

Approach with Vertical Guidance (APV) described in

paragraph 5−4−5, Instrument Approach Procedure

Charts, subparagraph a7(b), Approach with Vertical

Guidance (APV). LDA minima for with and without

glideslope is provided and annotated on the minima

lines of the approach chart as S−LDA/GS and

S−LDA. Because the final approach course is not

aligned with the runway centerline, additional

maneuvering will be required compared to an ILS

approach.