Aviation localizer7/6/2023 The most critical period of an instrument approach, particularly during low ceiling/visibility conditions, is the point at which the pilot must decide whether to land or execute a missed approach. At 10 NM from the point of touchdown, this represents a vertical distance of approximately 1,500 feet, narrowing to a few feet at touchdown.Īpproach Lighting Systems (ALS)Normal approach and letdown on the ILS is divided into two distinct stages: the instrument approach stage using only radio guidance, and the visual stage, when visual contact with the ground runway environment is necessary for accuracy and safety. The system provides no vertical guidance for approaches on the back course. Unlike the localizer, the GS transmitter radiates signals only in the direction of the final approach on the front course. At locations where standard minimum obstruction clearance cannot be obtained with the normal maximum GS angle, the GS equipment is displaced farther from the approach end of the runway if the length of the runway permits or the GS angle may be increased up to 4°. The GS projection angle is normally adjusted to 2.5°to 3.5° above horizontal, so it intersects the MM at about 200 feet and the OM at about 1,400 feet above the runway elevation. The course projected by the GS equipment is essentially the same as would be generated by a localizer operating on its side. The GS equipment is housed in a building approximately 750 to 1,250 feet down the runway from the approach end of the runway and between 400 and 600 feet to one side of the centerline. The glidepath is the straight, sloped line the aircraft should fly in its descent from where the GS intersects the altitude used for approaching the FAF to the runway touchdown zone. Glideslope (GS)GS describes the systems that generate, receive, and indicate the ground facility radiation pattern. With no more than one quarter scale deflection maintained, the aircraft will be aligned with the runway. This sensitivity permits accurate orientation to the landing runway. With this course width, a full-scale deflection shows when the aircraft is 2.5° to either side of the centerline. The localizer course is very narrow, normally 5°. The localizer includes a voice feature on its frequency for use by the associated ATC facility in issuing approach and landing instructions. For example, the ILS localizer at Springfield, Missouri, transmits the identifier ISGF. The ILS identification is preceded by the letter “I” (two dots). Each localizer facility is audibly identified by a three-letter designator transmitted at frequent regular intervals. The localizer course width is defined as the angular displacement at any point along the course between a full “fly-left” (CDI needle fully deflected to the left) and a full “fly-right” indication (CDI needle fully deflected to the right). The localizer provides course guidance, transmitted at 108.1 to 111.95 MHz (odd tenths only), throughout the descent path to the runway threshold from a distance of 18 NM from the antenna to an altitude of 4,500 feet above the elevation of the antenna site. These are called the front and back courses, respectively. This unit radiates a field pattern, which develops a course down the centerline of the runway toward the middle markers (MMs) and outer markers (OMs) and a similar course along the runway centerline in the opposite direction. LocalizerThe localizer (LOC) ground antenna array is located on the extended centerline of the instrument runway of an airport, located at the departure end of the runway to prevent it from being a collision hazard. For example, the compass locator may be used with NDB approaches. These facilities may be used as a part of the ILS system, as well as part of another approach. ILS Components Ground ComponentsThe ILS uses a number of different ground facilities.
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