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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.