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  Tuesday 16 October 2018 21:52 GMT  

Aviation Theory

Instrument Landing System (ILS)

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pages: 1: ILS Basic Components | 2: More ILS Components

The ILS approach is the most common precision approach. The technical aspects of the ILS are well covered in the AIM and you are encouraged to spend the time necessary with this section to thoroughly understand the operation, specifications, and limitations of the ILS. You will probably fly more ILS approaches than all the rest of the approach types combined.

You will be tested numerous times on the workings of the ILS during your flying career. The tests are not graded on a curve. They are strictly pass/fail. If you pass, you land safely. Fail, you bend an airplane or worse.


    1. The ILS is designed to provide an approach path for exact alignment and descent of an aircraft on final approach to a runway.
    2. The ground equipment consists of two highly directional transmitting systems and, along the approach, three (or fewer) marker beacons. The directional transmitters are known as the localizer and glideslope transmitters.
    3. The system may be divided functionally into three parts:
      1. Guidance information - localizer, glideslope
      2. Range information - marker beacon, DME
      3. Visual information - approach lights, touchdown and centerline lights, runway lights
    4. Compass locators located at the Outer Marker (OM) or Middle Marker (MM) may be substituted for marker beacons. DME, when specified in the procedure, may be substituted for the OM.
    5. Where a complete ILS system is installed on each end of a runway; (that is, the approach end of Runway 4 and the approach end of Runway 22) the ILS systems are not in service simultaneously.
    1. The localizer transmitter operates on one of 40 ILS channels within the frequency range of 108.10 to 111.95 MHz. Signals provide the pilot with course guidance to the runway centerline
    2. The approach course of the localizer is called the front course and is used with other functional parts, for example, glideslope, marker beacons, etc. The localizer signal is transmitted at the far end of the runway. It is adjusted for a course width of (full-scale fly left to full-scale fly right) or 700 feet at the runway threshold
    3. The course line along the extended centerline of the runway, in the opposite direction to the front course is called the back course.
    4. CAUTION
      Unless the aircraft's ILS equipment includes reverse sensing capability, when flying inbound on the back course it is necessary to steer the aircraft in the direction opposite the needle deflection when making corrections from off-course to on-course. This "flying away from the needle" is also required when flying out bound on the front course of the localizer. DO NOT USE BACK COURSE SIGNALS for approach unless a BACK COURSE APPROACH PROCEDURE is published for that particular runway and the approach is authorized by ATC.
    5. Identification is in International Morse Code and consists of a three letter identifier preceded by the letter I () transmitted on the localizer frequency.
    6. The localizer provides course guidance throughout the descent path to the runway threshold from a distance of 18 NM from the antenna between an altitude of 1,000 feet above the highest terrain along the course line and 4,500 feet above the elevation of the antenna site. Proper off-course indications are provided throughout the following angular areas of operational service volume:
      1. To 10 degrees either side of the course along a radius of 18 NM from the antenna, and:
      2. From 10 to 35 degrees either side of the course along a radius of 10 NM
      Illustration showing localizer coverage angles
      Figure 1-1-6
    7. Unreliable signals may be received outside these areas.
    8. top

    1. The Localizer type Directional Aid (LDA) is of comparable use and accuracy to a localizer but is not part of a complete ILS. The LDA course usually provides a more precise approach course than the similar Simplified Directional Facility (SDF) installation, which may have a course width of 6 or 12 degrees. The LDA is not aligned with the runway. Straight-in minimums may be published where alignment does not exceed 30 degrees between the course and runway. Circling minimums only are published where this alignment exceeds 30 degrees.
    1. The UHF glideslope transmitter, operating on one of the 40 ILS channels within the frequency range 329.15 MHz, to 335.00 MHz radiates its signals in the direction of the localizer front course. The term "glidepath" means that portion of the glideslope that intersects the localizer.
    2. CAUTION
      False glideslope signals may exist in the area of the localizer back course approach which can cause the glideslope flag alarm to disappear and present unreliable glideslope information. Disregard all glideslope signal indications when making a localizer back course approach unless a glideslope is specified on the approach and landing chart.
    3. The glideslope transmitter is located between 750 feet and 1,250 feet from the approach end of the runway (down the runway) and offset 250 to 650 feet from the runway centerline. It transmits a glidepath beam 1.4 degrees wide. The signal provides descent information for navigation down to the lowest authorized decision height (DH) specified in the approved ILS approach procedure. The glidepath may not be suitable for navigation below the lowest authorized DH and any reference to glidepath indications below the height must be supplemented by visual reference to the runway environment. Glidepaths with no published DH are usable to runway threshold.
    4. The glidepath projection angle is normally adjusted to 3 degrees above horizontal so that it intersects the MM at about 200 feet and the OM at about 1,400 feet above the runway elevation. The glideslope is normally usable to the distance of 10 NM. However, at some locations, the glideslope has been certified for an extended service volume which exceeds 10 NM.
    5. Pilots must be alert when approaching the glidepath interception. False courses and reverse sensing will occur at angles considerably greater than the published path.
    6. Make every effort to remain on the indicated glidepath.
    7. CAUTION
      Avoid flying below the glidepath to assure obstacle/terrain clearance is maintained.
    8. The published glideslope threshold crossing height (TCH) DOES NOT represent the height of the actual glidepath on-course indication above the runway threshold. It is used as a reference for planning purposes which represents the height above the runway threshold that an aircraft's glideslope antenna should be, if that aircraft remains on a trajectory formed by the four mile to middle marker glidepath segment.
    9. Pilots must be aware of the vertical height between the aircraft's glideslope antenna and the main gear in the landing configuration and, at the DH, plan to adjust the descent angle accordingly if the published TCH indicates the wheel crossing height over the runway threshold may not be satisfactory. Tests indicate a comfortable wheel crossing height is approximately 20 to 30 feet, depending on the type of aircraft.
    1. When installed with the ILS and specified in the approach procedure, DME may be used:
      1. In lieu of the OM
      2. As a back course (BC) final approach fix (FAF)
      3. To establish other fixes on the localizer course
    2. In some cases, DME from a separate facility may be used within Terminal Instrument Procedures (TERPS) limitations:
      1. To provide ARC initial approach segments
      2. As a FAF for BC approaches
      3. As a substitute for the OM


... page 2: More ILS Components

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