Surface vehicles and even other aircraft flying below 5,000 feet above ground level (AGL) may disturb the signal for aircraft on the approach. Localizer and glide-slope signals are subject to the same type of bounce from hard objects as space waves. The ILS and its components are subject to certain errors, which are listed below. It also tempts one to "dive and drive." You can, and should, fly a Continuous Descent Final Approach for most Localizer Back Course approaches. The Localizer Back Course provides several opportunities to make mistakes setting the back course instead of the front course or forgetting to press the flight director's BC button, for example. The Localizer Back Course provides an accurate centerline. Pilots must be instrument rated and the aircraft be equipped with the appropriate airborne equipment to execute back course localizer approaches. The ALS provides lights that will penetrate the atmosphere far enough from touchdown to give directional, distance, and glide path information for safe visual transition. At this point, the approach should be continued by reference to the runway touchdown zone markers. As the runway threshold is approached, the visual glide path will separate into individual lights. 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. 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. For example, the outer locator at Dallas/Love Field (DAL) is identified as “DA.” The middle locator at DAL is identified by the last two letters “AL.” The coding identification of the outer locator consists of the first two letters of the three-letter identifier of the associated LOC. When used in conjunction with an ILS front course, the compass locator facilities are collocated with the outer and/or MM facilities. The back-course marker, where installed, indicates the back-course FAF.Ĭompass locators are low-powered NDBs and are received and indicated by the ADF receiver. It indicates the point at which an aircraft is at the decision height on the glide path during a Category II ILS approach. The inner marker (IM), where installed, is located on the front course between the MM and the landing threshold. The MM is located approximately 3,500 feet from the landing threshold on the centerline of the localizer front course at a position where the glide-slope centerline is about 200 feet above the touchdown zone elevation. The OM is located on the localizer front course 4 to 7 miles from the airport to indicate a position at which an aircraft, at the appropriate altitude on the localizer course, will intercept the glide path.A marker beacon may also be installed to indicate the FAF on the ILS back course. A third beacon, the inner, is used where Category II operations are certified. Two VHF marker beacons, outer and middle, are normally used in the ILS system.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 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. The localizer (LOC) ground antenna array is located on the extended centerline of the instrument runway of an airport, remote enough from the opposite (approach) end of the runway to prevent it from being a collision hazard.
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