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AIM

10/12/17

1−1−13

Navigation Aids

rized when weather or visibility conditions are less

than ceiling 800 feet and/or visibility 2 miles.

4. Pilots are cautioned that vehicular traffic not

subject to ATC may cause momentary deviation to

ILS course or glide slope signals. Also, critical areas

are not protected at uncontrolled airports or at airports

with an operating control tower when weather or

visibility conditions are above those requiring

protective measures. Aircraft conducting coupled or

autoland operations should be especially alert in

monitoring automatic flight control systems. 

(See FIG 1−1−7.)

NOTE−

Unless otherwise coordinated through Flight Standards,

ILS signals to Category I runways are not flight inspected

below the point that is 100 feet less than the decision

altitude (DA). Guidance signal anomalies may be

encountered below this altitude.

1−1−10. Simplified Directional Facility

(SDF)

a. The SDF provides a final approach course

similar to that of the ILS localizer. It does not provide

glide slope information. A clear understanding of the

ILS localizer and the additional factors listed below

completely describe the operational characteristics

and use of the SDF.

b. The SDF transmits signals within the range of

108.10 to 111.95 MHz.

c. The approach techniques and procedures used

in an SDF instrument approach are essentially the

same as those employed in executing a standard

localizer approach except the SDF course may not be

aligned with the runway and the course may be wider,

resulting in less precision.

d. Usable off−course indications are limited to

35 degrees either side of the course centerline.

Instrument indications received beyond 35 degrees

should be disregarded.

e. The SDF antenna may be offset from the runway

centerline. Because of this, the angle of convergence

between the final approach course and the runway

bearing should be determined by reference to the

instrument approach procedure chart. This angle is

generally not more than 3 degrees. However, it should

be noted that inasmuch as the approach course

originates at the antenna site, an approach which is

continued beyond the runway threshold will lead the

aircraft to the SDF offset position rather than along

the runway centerline.

f. The SDF signal is fixed at either 6 degrees or

12 degrees as necessary to provide maximum

flyability and optimum course quality.

g. Identification consists of a three−letter identifi-

er transmitted in Morse Code on the SDF frequency.

The appropriate instrument approach chart will

indicate the identifier used at a particular airport.