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AIM 

4/20/23

 

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Navigation Aids

4.

Pilots are cautioned that even when the critical areas are considered to be protected, unless the official

weather observation including controller observations indicates a ceiling less than 200 feet or RVR less than 2000
feet, ATC may still authorize a preceding arriving, departing, or missed approach aircraft to pass through or over
the localizer critical area and that this may cause signal disturbances that could result in an undesired aircraft state
during the final stages of the approach, landing, and rollout.

5.

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 and be prepared to intervene as necessary. (See FIG 1

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8.)

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.

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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 identifier transmitted in Morse Code on the SDF frequency. The

appropriate instrument approach chart will indicate the identifier used at a particular airport.

3/15/07

7110.65R CHG 2

AIM

10/5/23