Instrument Landing System


Guidance Information:


  • The localizer (LOC) provides lateral course guidance during an approach to landing
  • Instrumentation operates between 108 - 111.95 MHz, on one of 40 channels which transmits from a ground node
    • The approach course of the localizer is called the front course and is used with other functional parts, e.g., glide slope, marker beacons, etc.
      • Some localizers may transmit the course line along the extended centerline of a runway, in the opposite direction to the front course which is called the back course
      • Opposite ILS' are not in service simultaneously (i.e., the approach end of Runway 4 and the approach end of Runway 22)
      • 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 outbound 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
  • Adjusted for a course width of (full scale fly-left to a full scale fly-right) of 700' at the runway threshold
    • Vertical guidance is provided up to 18 NM from the antenna between 1,000' above the highest terrain along the course line and 4,500' above antenna elevation
    • Lateral guidance [Figure 1]:
      • To 10° either side of the course along a radius of 18 NM from the antenna; and
      • From 10 to 35° either side of the course along a radius of 10 NM

Signals may be received outside of the advertised vertical and lateral limits, but are considered unreliable

  • Identification is in International Morse Code and consists of a three-letter identifier preceded by the letter I (- -) transmitted on the localizer frequency
    • 3 letter id preceded by I (I-DAB)
    • The Morse code pattern is identified on the approach plate

Instrument Flying Handbook, Localizer Course
Figure 1: Instrument Flying Handbook, Localizer Course

Glide Slope/Glide Path:

  • The UHF glide slope 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 glide path means that portion of the glide-slope that intersects the localizer
    • CAUTION: False glide slope signals may exist in the area of the localizer back course approach which can cause the glide slope flag alarm to disappear and present unreliable glide slope information. Disregard all glide slope signal indications when making a localizer back course approach unless a glide slope is specified on the approach and landing chart
  • Transmitter is located 750 - 1,250' from the approach end offset 250 - 650' from centerline
  • It transmits a glide path beam 1.4° wide (vertically)
  • The signal is considered accurate down to the lowest decision height (DH) published on an ILS approach procedure
    • Any reference to glidepath indications below that height must be supplemented by visual reference to the runway environment
    • Glide-paths with no published DH are usable to runway threshold
  • Normally adjusted to 3° above horizontal intersecting the Middle Marker (MM) at 200' and the Outer Marker (OM) at 1400' above the runway elevation
  • Usable 10 NM but may be extended, however, at some locations, the glide slope has been certified for an extended service volume which exceeds 10 NM
  • Be alert for false glide slopes and reverse sensing when interceptiong the glideslope
    • False courses and reverse sensing can occur when intercepting the ILS at angles considerably greater than the published path
    • It is therefore extremely important to maintain glide-slope to assure obstacle/terrain clearance is maintained
  • The published glide slope threshold crossing height (TCH) DOES NOT represent the height of the actual glide path 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 glide slope antenna should be, if that aircraft remains on a trajectory formed by the four-mile-to-middle marker glidepath segment
    • TCH does not coincide with GS altitude over threshold but rather with the antennas should be (think bigger aircraft)
  • Pilots must be aware of the vertical height between the aircraft's glide slope 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', depending on the type of aircraft

The TCH for a runway is established based on several factors including the largest aircraft category that normally uses the runway, how airport layout effects the glide slope antenna placement, and terrain. A higher than optimum TCH, with the same glide path angle, may cause the aircraft to touch down further from the threshold if the trajectory of the approach is maintained until the flare. Pilots should consider the effect of a high TCH on the runway available for stopping the aircraft

Range Information

Distance Measuring Equipment:

  • When installed with the ILS and specified in the approach procedure, DME may be used:
    • In lieu of the OM;
    • As a back course (BC) FAF; and
    • To establish other fixes on the localizer course
  • In some cases, DME from a separate facility may be used within Terminal Instrument Procedures (TERPS) limitations:
    • To provide ARC initial approach segment;
    • FAF for BC approaches; and
    • Substitute for OM

Aeronautical Information Manual, Marker Passage Indications
Figure 2: Marker Passage Indications

Marker Beacons:

  • 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
  • Ordinarily, there are two marker beacons associated with an ILS, the OM and MM
  • Locations with a Category II ILS also have an Inner Marker (IM)
  • When an aircraft passes over a marker, the pilot will receive the indications [Figure 2]
    • The OM normally indicates a position at which an aircraft at the appropriate altitude on the localizer course will intercept the ILS glide path
    • The MM indicates a position approximately 3,500 feet from the landing threshold. This is also the position where an aircraft on the glide path will be at an altitude of approximately 200 feet above the elevation of the touchdown zone
    • The IM will indicate a point at which an aircraft is at a designated decision height (DH) on the glide path between the MM and landing threshold
  • A back course marker normally indicates the ILS back course final approach fix where approach descent is commenced
  • Precision radar or compass locators (OM, MM) may be substituted for marker beacons
  • DME, when specified in the procedure, may be substituted for the OM

Compass Locators:

  • Similar to an NDB, a compass locator is a low to medium powered beacon which is the precursor to modern day marker beacons
  • Compass locators provide the same basic information to the pilot as a marker beacon
  • 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
  • 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
  • Provide transition from en-route to approach phase
  • 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 and usually associated with TWEB
  • OM: First 2 letters of loc id group
  • MM: Last 2 letters of loc id group

Instrument Flying Handbook, Localizer Coverage Limits
Figure 1: Instrument Flying Handbook, Localizer Coverage Limits

Localizer Type Directional Aid (LDA):

  • While not part of a complete ILS, the LDA is similar to a localizer, as the name suggests
  • 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°
  • LDA is not aligned with the runway
  • If less than 30° off: straight in minimums published
  • If more than 30° off: circle to land minimums published
  • Some approaches have glide slopes referred to Approaches with Vertical Guidance (APVs)
    • These are annotated in the plan view of the instrument approach chart with a note, "LDA/Glide-slope"
    • These procedures fall under a newly defined category of approaches called Approach with Vertical Guidance (APV)
    • LDA minima for with and without glide-slope is provided and annotated on the minima lines of the approach chart as S-LDA/GS and S-LDA
    • Because the final approach course is not aligned with the runway centerline, additional maneuvering will be required compared to an ILS approach
    • Note that approaches with vertical guidance are just that, guidance
      • These approaches are non-precision approaches, not precision

Simplified Directional Facility:

  • The SDF provides a final approach course similar to that of the ILS localizer but no glide slope information
  • Operates on the same localizer frequency (108.10 to 111.95)
  • Techniques and procedures used in an SDF instrument approach are essentially the same as those in a localizer except the final approach course may not be aligned with the runway and the course may be wider, resulting in less precision
    • Note that as the approach course originates at the antenna site, an approach which is continued beyond the runway threshold will lead the aircraft to the SDF offset position rather than along the runway centerline
  • Usable to 35° either side of course line, same as a localizer
  • The antenna may be offset from the runway centerline so the final approach course and the runway bearing should be referenced on the IAP chart
  • Normally not more than 3° however if the approach is continued beyond the runway threshold you will be in a bad position to land
  • The signal is fixed at either 6 or 12° as necessary to provide maximum fly-ability and optimum course quality
  • Identification is provided through a 3-letter Morse code which is indicated on the instrument approach chart for a particular airport


  • The lowest authorized ILS minimums, with all required ground and airborne systems components operative, are:
  • CAT I:
    • DH: 200'
    • RVR: 2,400' (1,800 w/ TDZ and centerline lighting) or (with Autopilot or FD or HUD, RVR 1,800')
  • Special Authorization Category I:
    • DH: 150'
    • RVR: 1,400'
    • HUD to DH
  • CAT II:
    • DH: 100'
    • RVR: 1,200' with auto-land or HUD to touchdown and noted on authorization, RVR 1,000'
  • Special Authorization Category II with Reduced Lighting:
    • DH: 100
    • RVR: 1,200 with auto land or HUD to touchdown and noted on authorization
      • Touchdown zone, centerline lighting, and ALSF-2 are not required);
  • CAT IIIa:
    • No DH or DH below 100'
    • RVR not less than 700'
  • CAT IIIb:
    • No DH or DH below 50'
    • RVR Less than 700' but not less than 150'
  • CAT IIIc:
    • No DH
    • No RVR minimum
  • Note that special authorization and equipment required for Categories II and III

Inoperative ILS Components:

  • Inoperative localizer: ILS not authorized
  • Inoperative glide-slope: ILS reverts to a non-precision, localizer approach, if you have a failure of the GS after the FAF inform the controller you are switching to a localizer approach while climbing or descending to the MDA (no longer a DH)
  • 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

ILS Course Distortion:

  • All pilots should be aware that disturbances to ILS localizer and glide slope courses may occur when surface vehicles or aircraft are operated near the localizer or glide slope antennas
  • Most ILS installations are subject to signal interference by either surface vehicles, aircraft, or both
  • ATC issues control instructions to avoid interfering operations within ILS critical areas at controlled airports during the hours the Airport Traffic Control Tower (ATCT)
  • Ceiling less than 800' and/or visibility 2 miles:
    • ILS Critical Areas:
      • Ground control will advise you when to hold short of critical areas
      • Issued when ceilings are less than 800-2
      • 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 if the weather is better than 800-2
    • Localizer Critical Area:
      • Except for aircraft that land, exit a runway, depart, or execute a missed approach, vehicles and aircraft are not authorized in or over the critical area when an arriving aircraft is inside the outer marker (OM) or the fix used in lieu of the OM
      • Additionally, when conditions are less than reported ceiling 200 feet or RVR less than 2,000 feet, do not authorize vehicles or aircraft operations in or over the area when an arriving aircraft is inside the MM, or in the absence of a MM, 1/2 mile final
    • Glide slope Critical Area:
      • Except for aircraft on approach, vehicles or aircraft operations in or over the area when an arriving aircraft is inside the ILS outer marker (OM), or the fix used in lieu of the OM, unless the arriving aircraft has reported the runway in sight and is circling or sideāˆ’stepping to land on another runway
  • Weather Conditions at or above ceiling 800' and/or visibility 2 miles
    • No critical area protective action is provided under these conditions
    • 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
      • Example: Denver Tower, United 1153, Request Autoland/Coupled Approach (runway)
      • ATC replies with: United 1153, Denver Tower, Roger, Critical Areas not protected
  • Aircraft holding below 5,000' 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' and/or visibility 2 miles
  • Pilots are cautioned that vehicular traffic not subject to ATC may cause momentary deviation to ILS course or glide slope 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

Unless otherwise coordinated through Flight Standards, ILS signals to Category I runways are not flight inspected below the point that is 100 feet less than the decision altitude (DA). Guidance signal anomalies may be encountered below this altitude

Frequency Pairs Allocated for ILS:

  • Frequencies are paired to give localizer and glide-slope information on a single frequency
  • As far as you, the pilot are concerned, you only need to input the VHF frequency, and the UHF will be tied to that

Localizer MHz Glide Slope
108.10 334.70
108.15 334.55
108.3 334.10
108.35 333.95
108.5 329.90
108.55 329.75
108.7 330.50
108.75 330.35
108.9 329.30
108.95 329.15
Localizer MHz Glide Slope
109.1 331.40
109.15 331.25
109.3 332.00
109.35 331.85
109.50 332.60
109.55 332.45
109.70 333.20
109.75 333.05
109.90 333.80
109.95 333.65
Localizer MHz Glide Slope
110.1 334.40
110.15 334.25
110.3 335.00
110.35 334.85
110.5 329.60
110.55 329.45
110.70 330.20
110.75 330.05
110.90 330.80
110.95 330.65
Localizer (MHz) Glide Slope
111.10 331.70
111.15 331.55
111.30 332.30
111.35 332.15
111.50 332.9
111.55 332.75
111.70 333.5
111.75 333.35
111.90 331.1
111.95 330.95

Instrument Flying Handbook, Instrument Landing System
Figure 3: Instrument Flying Handbook, Instrument Landing System


  • On dogleg to final when within 5-7 NM of the FAF execute the above procedures accomplish the same procedures as above starting with step 6
  • If above the localizer minimums and you lose glide-slope then you may request the localizer if you lost a localizer approach, however if you lose glide-slope below localizer minimums go mist and if you lose the localizer in any situation, go missed


  • Some final approach fixes may be designated with cross radials using VORs however de-selecting the ILS is NOT an acceptable method of identifying the FAF
  • Where a complete ILS system is installed on each end of a runway; (i.e., the approach end of Runway 4 and the approach end of Runway 22) the ILS systems are not in service simultaneously
  • Pilots should be aware of the possibility of momentary erroneous indications on cockpit displays when the primary signal generator for a ground-based navigational transmitter is inoperative
    • Pilots should disregard any navigation indication, regardless of its apparent validity, if the particular transmitter was identified by NOTAM or otherwise as unusable or inoperative