AIM 

10/5/23 

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

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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

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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

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letter identifier transmitted in Morse Code on the SDF frequency. The 

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

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18 

Navigation Aids