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Aviation Theory

Instrument Landing System (ILS) [page 2]

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

    1. ILS marker beacons have a rated power output of 3 watts or less and an antenna array designed to produce an elliptical pattern with dimensions, at 1,000 feet above the antenna, of approximately 2,400 feet in width and 4,200 feet in length. Airborne marker beacon receivers with a selective sensitivity feature should always be operated in the "low" sensitivity position for proper reception of ILS marker beacons.
    2. Ordinarily, there are two marker beacons associated with an ILS, the OM and MM. Locations with a Category II and III ILS also have an inner marker (IM). When an aircraft passes over a marker, the pilot will receive the following indications:
      Table 1-1-3
    3.      ----------------------
            MARKER  CODE   LIGHT
             OM     ---    BLUE
             MM     .-.-   AMBER
             IM     ....   WHITE
             BC     .. ..  WHITE
      1. The OM normally indicates a position at which an aircraft at the appropriate altitude on the localizer course will intercept the ILS glidepath.
      2. The MM indicates a position approximately 3,500 feet from the landing threshold. This is also the position where an aircraft on the glidepath will be at an altitude of approximately 200 feet above the elevation of the touchdown zone.
      3. The inner marker (IM) will indicate a point at which an aircraft is at a designated decision height (DH) on the glidepath between the MM and landing threshold.
    4. A back course marker normally indicates the ILS back course final approach fix where approach descent is commenced.
    1. Compass locator transmitters are often situated at the MM and OM sites. The transmitters have a power of less than 25 watts, a range of at least 15 miles and operate between 190 and 535 kHz. At some locations, higher powered radio beacons, up to 400 watts, are used as OM compass locators. These generally carry Transcribed Weather Broadcast (TWEB) information.
    2. Compass locators transmit two letter identification groups. The outer locator transmits the first two letters of the localizer identification group, and the middle locator transmits the last two letters of the localizer identification group.

  4. Table 1-1-4 - Frequency Pairs Allocated for ILS
    Localizer   Glideslope
    108.10      334.70
    108.15      334.55
    108.30      334.10
    108.35      333.95
    108.50      329.90
    108.55      329.75
    108.70      330.50
    108.75      330.35
    108.90      329.30
    108.95      329.15
    109.10      331.40
    109.15      331.25
    109.30      332.00
    109.35      331.85
    Localizer   Glideslope
    109.50      332.60
    109.55      332.45
    109.70      333.20
    109.75      333.05
    109.90      333.80
    109.95      333.65
    110.10      334.40
    110.15      334.25
    110.30      335.00
    110.35      334.85
    110.50      329.60
    110.55      329.45
    110.70      330.20
    110.75      330.05
    Localizer   Glideslope
    110.90      330.80
    110.95      330.65
    111.10      331.70
    111.15      331.55
    111.30      332.30
    111.35      332.15
    111.50      332.90
    111.55      332.75
    111.70      333.50
    111.75      333.35
    111.90      331.10
    111.95      330.95
    1. The lowest authorized ILS minimums, with all required ground and airborne systems components operative, are:
      1. Category I - Decision Height (DH) 200 feet and Runway Visual Range (RVR) 2,400 feet (with touchdown zone and centerline lighting, RVR 1,800 feet).
      2. Category II - DH 100 feet and RVR 1,200 feet.
        {New-96-6 Revised June 20, 1996}
      3. Category IIIa - No DH or DH below 100 feet and RVR not less than 700 feet.
        {New-96-6 Added June 20, 1996}
      4. Category IIIb - No DH or DH below 50 feet and RVR less than 700 feet but not less than 150 feet.
      5. Category IIIc - No DH and no RVR limitation.
        NOTE: Special authorization and equipment required for Category II and III
      6. top

    1. Inoperative localizer: When the localizer fails, an ILS approach is not authorized.
    2. Inoperative glideslope: When the glideslope fails, the ILS reverts to a nonprecision localizer approach.
      REFERENCE: See the Inoperative Component Table in the U.S. Government Terminal Procedures Publication (TPP), for adjustments to minimums due to inoperative airborne or ground system equipment.
    1. All pilots should be aware that disturbances to ILS localizer and glideslope courses may occur when surface vehicles or aircraft are operated near the localizer or glideslope antennas. Most ILS installations are subject to signal interference by either surface vehicles, aircraft or both. ILS CRITICAL AREAS are established near each localizer and glideslope antenna.
    2. ATC issues control instructions to avoid interfering operations within ILS critical areas at controlled airports during the hours the Airport Traffic Control Tower (ATCT) is in operations as follows:
      1. Weather Conditions - Less than ceiling 800 feet and/or visibility 2 miles.
        1. LOCALIZER CRITICAL AREA - Except for aircraft that land, exit a runway, depart or miss approach, vehicles and aircraft are not authorized in or over the critical area when an arriving aircraft is between the ILS final approach fix and the airport. Additionally, when the ceiling is less than 200 feet and/or the visibility is RVR 2,000 or less, vehicle and aircraft operations in or over the area are not authorized when an arriving aircraft is inside the ILS MM.
        2. GLIDE SLOPE CRITICAL AREA - Vehicles and aircraft are not authorized in the area when an arriving aircraft is between the ILS final approach fix and the airport unless the aircraft has reported the airport in sight and is circling or sidestepping to land on a runway other than the ILS runway.
      2. Weather Conditions - At or above ceiling 800 feet and/or visibility 2 miles.
        1. No critical area protective action is provided under these conditions.
        2. A flight crew, under these conditions, should advise the tower that it will conduct an AUTOLAND or COUPLED approach to ensure that the ILS critical areas are protected when the aircraft is inside the ILS MM.
    3. Aircraft holding below 5000 feet between the outer marker and the airport may cause localizer signal variations for aircraft conducting the ILS Approach. Accordingly, such holding is not authorized when weather or visibility conditions are less than ceiling 800 feet and/or visibility 2 miles.
    4. Pilots are cautioned that vehicular traffic not subject to ATC may cause momentary deviation to ILS course or glideslope 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. (See Figure 1-1-7)


Illustration showing schematic overview of a standard FAA ILS approach system (click for larger image)
Figure 1-1-7

NOTE - Unless otherwise coordinated through flight standards, ILS signals to Category I runways are not flight inspected below 100 feet AGL. Guidance signal anomalies may be encountered below this altitude.

Figure 1-1-7 text
ILS [FAA Instrument Landing System] Standard Characteristics and Terminology
I LS approach charts should be consulted to obtain variations of individual systems.

VHF Localizer - Provides Horizontal Guidance
108.10 to 111.95 MHz. Radiates about 100 watts. Horizontal polarization. Modulation frequencies 90 and 150 Hz. Modulation depth on-course 20% for each frequency. Code identification (1020 Hz, 5%) and voice communication (modulated 50%) provided on same channel. 1000 feet typical [distance from departure end of runway]. Localizer transmitter building is offset 250 feet minimum from center of antenna array and within 90 degrees plus or minus 30 degrees from approach end. Antenna is on centerline and normally is under 50:1 clearance plane.

UHF Glideslope Transmitter - Provides Vertical Guidance
329.3 to 335.0 MHz. Radiates about 5 watts. Horizontal polarization, modulation on path 40% for 90 Hz and 150 Hz. The standard glideslope angle is 3.0 degrees. It may be higher depending on local terrain.

Middle Marker - Indicates approximate Decision Height Point
Modulation 1300 Hz, 95%. Keying: 95 alternate dot and dash combinations per minute. Amber light.
Located 3000 to 6000 feet from threshold

Outer Marker - Provides Final Approach Fix for Nonprecision Approach
Modulation 400 Hz, 95%. Keying: Two dashes per second. Blue light.
Located 4 to 7 miles from end of runway, where glideslope intersects the procedure turn (minimum holding) altitude, plus or minus 50 feet vertically.
Sited to provide 55 feet (plus/minus 5 feet) runway threshold crossing height,
[Displaced laterally] 250 to 600 feet from centerline of runway.

All marker transmitters approximately 2 watts of 75 MHz modulated about 95%.

Course width varies between 3 to 6 degrees, tailored to provide 700 feet at threshold (full scale limits).

NOTE: Compass locators, rated at 25 watts output 190 to 535 kHz, are installed at many outer and some middle markers. A 400 Hz or a 1020 Hz tone, modulating the carrier about 95%, is keyed with the first two letters of the ILS identification on the outer locator and the last two letters on the middle locator. At some locators, simultaneous voice transmissions from the control tower are provided, with appropriate reduction in identification percentage.


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