Instrument Landing System

Guidance Information:

Range Information

Localizer Type Directional Aid (LDA):

• The 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°
• 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
  • Any signals received outside of this range shall be considered unusable for navigation
• The antenna may be offset from the runway centerline and 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

ILS Minimums:

• 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 signal interferences may occur when surface vehicles, aircraft, or both are operated near the localizer or glide slope antennas
• At controlled airports ATC issues control instructions to avoid interfering operations within ILS critical areas during the hours the Airport Traffic Control Tower (ATCT)

• ILS Critical Areas:

  • Ground control will advise you when to hold short of critical areas
  • Issued when ceilings are less than 800-2
• When official weather observation is a ceiling of less than 800 feet and/or visibility 2 miles:

  • 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:

    • Do not authorize 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 side-stepping to land on another runway
• When official weather observation is at or above ceiling 800 feet and/or visibility 2 miles:
  • No critical area protective action is provided under these conditions
  • A flight crew desiring autoland or coupled approaches must advise ATC under these conditions, to ensure that the ILS critical areas are protected when the aircraft is inside the ILS MM
    • Pilot: "Denver Tower, United 1153, Request [Autoland/Coupled] Approach [Runway]"
    • ATC: "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
• NOTE: 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

Instrument Landing System Procedure:

• Request the ILS approach from ATC
• Follow the assigned clearance or radar vectors, as assigned by ATC
• Complete any appropriate checklists, ensuring the aircraft is in the landing configuration prior to glide slope interception
• Upon intercepting the glide slope, reduce power, and adjust pitch to maintain glide slope
  • The glideslope angle can be found on the instrument approach chart however, it is generally around 3 degrees
• Make small adjustments as required
  • Corrections will become more sensitive as you get closer to the airport so it is imperative to stabilize your approach as early as possible
  • To recapture the glideslope, corrections should be within a degree or two of pitch, or a couple hundred feet per minute on the rate of descent
  • To recapture the localizer, stay inside of the heading bug, if available, or 5 degrees left or right of center if none is
• Upon reaching decision height:
  • Continue the approach if the runway environment is in sight, or;
  • Execute the published missed approach procedure if ATC does not direct a different procedure

Radar Vectors:

• 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

Conclusion:

• 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

References:

• Aeronautical Information Manual (1-1-9) Instrument Landing System (ILS)
• Aeronautical Information Manual (1-1-10) Simplified Directional Facility (SDF)
• Aeronautical Information Manual (5-4-5) Instrument Approach Procedure Charts
• Aeronautical Information Manual (5-4-13) ILS Approaches to Parallel Runways
• Aeronautical Information Manual (5-4-14) Parallel ILS/MLS Approaches (Dependent)
• Aeronautical Information Manual (5-4-15) Simultaneous Parallel ILS/MLS Approaches (independent)
• CFI Notebook.net - Airport Markings and Signs
• CFI Notebook.net - Airport Traffic Control Tower (ATCT)
• Instrument Flying Handbook (9-35) Instrument Landing System (ILS)