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Instrument Approach Systems – ILS Function

in Navigation Systems

The localizer needle indicates, by deflection, whether the aircraft is right or left of the localizer centerline, regardless of the position or heading of the aircraft. Rotating the OBS has no effect on the operation of the localizer needle, although it is useful to rotate the OBS to put the LOC inbound course under the course index. When inbound on the front course, or outbound on the back course, the course indication remains directional. (See Figure 7-39, aircraft C, D, and E.)


Figure 7-39. Localizer Course Indications. To follow indications displayed in the aircraft, start from A and proceed through E.

Figure 7-39. Localizer Course Indications. To follow indications displayed in the aircraft, start from A and proceed through E. [click image to enlarge]

Unless the aircraft has reverse sensing capability and it is in use, when flying inbound on the back course or outbound on the front course, heading corrections to on-course are made opposite the needle deflection. This is commonly described as “flying away from the needle.” (See Figure 7-39, aircraft A and B.) Back course signals should not be used for an approach unless a back course approach procedure is published for that particular runway and the approach is authorized by ATC.

Once you have reached the localizer centerline, maintain the inbound heading until the CDI moves off center. Drift corrections should be small and reduced proportionately as the course narrows. By the time you reach the OM, your drift correction should be established accurately enough on a well-executed approach to permit completion of the approach, with heading corrections no greater then 2°.

The heaviest demand on pilot technique occurs during descent from the OM to the MM, when you maintain the localizer course, adjust pitch attitude to maintain the

proper rate of descent, and adjust power to maintain proper airspeed. Simultaneously, the altimeter must be checked and preparation made for visual transition to land or for a missed approach. You can appreciate the need for accurate instrument interpretation and aircraft control within the ILS as a whole, when you notice the relationship between CDI and glide path needle indications, and aircraft displacement from the localizer and glide path centerlines.

Deflection of the GS needle indicates the position of the aircraft with respect to the glide path. When the aircraft is above the glide path, the needle is deflected downward. When the aircraft is below the glide path, the needle is deflected upward. [Figure 7-40]

Figure 7-40. Illustrates a GS receiver indication and aircraft displacement. An analog system is on the left and the same indication on the Garmin PFD on the right.

Figure 7-40. Illustrates a GS receiver indication and aircraft displacement. An analog system is on the left and the same indication on the Garmin PFD on the right. [click image to enlarge]

 

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