The Instrument Landing System (ILS) provides an approach path for exact alignment and descent of an aircraft on final approach to a runway
The basic components of an ILS are the localizer, glide slope, and Outer Marker (OM) and, when installed for use with Category II or Category III instrument approach procedures, an Inner Marker (IM)
The directional transmitters are known as the localizer, which provides lateral guidance and a glide slope which provides vertical guidance
The system is not fool proof however, and so pilots must be aware of the factors which can cause ILS Distortion to ensure you are receiving reliable signals
Guidance Information:
Localizer:
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
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
The localizer signal is transmitted at the far end of the runway
Adjusted for a course width of (full scale fly-left to a full scale fly-right) of 700' at the runway threshold
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 the operational service volume: [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
The areas described in and depicted in [Figure 2] represent a Standard Service Volume (SSV) localizer. All charted procedures with localizer coverage beyond the 18 NM SSV have been through the approval process for Expanded Service Volume (ESV), and have been validated by flight inspection
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
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
The glide slope is normally usable to the distance of 10 NM
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
Note that 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
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
ILS systems may have an associated OM. An MM is no longer required. Locations with a Category II ILS also have an Inner Marker (IM)
Due to advances in both ground navigation equipment and airborne avionics, as well as the numerous means that may be used as a substitute for a marker beacon, the current requirements for the use of marker beacons are:
An OM or suitable substitute identifies the Final Approach Fix (FAF) for nonprecision approach (NPA) operations (for example, localizer only); and
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. An MM is no longer operationally required. There are some MMs still in use, but there are no MMs being installed at new ILS sites by the FAA; and
An IM, where installed, indicates the point at which an aircraft is at decision height on the glide path during a Category II ILS approach. An IM is only required for CAT II operations that do not have a published radio altitude (RA) minimum
A back course marker normally indicates the ILS back course final approach fix where approach descent is commenced
The following means may be used to substitute for the OM:
Compass locator; or
Precision Approach Radar (PAR); or
Airport Surveillance Radar (ASR); or
Distance Measuring Equipment (DME), Very High Frequency Omni-directional Range (VOR), or Nondirectional beacon fixes authorized in the Standard Instrument Approach Procedure; or
A suitable RNAV system with Global Positioning System (GPS), capable of fix identification on a Standard Instrument Approach Procedure
Marker Passage Indications
Marker Passage Indications
Marker Beacons:
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
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"
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
Issued when official weather observation is a ceiling of less than 800 feet and/or visibility 2 miles:
Requires the pilot to keep the entire aircraft clear of the associated marking, and remain on the safe side of the ILS Critical Area
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"
Safety works best when we help each other, so if the weather is at or below 800' or 2 miles and ATC hasn't issued you an ILS hold, it doesn't hurt to hold short and ask anyway
Remember, ILS Critical Areas are only mandatory when ATC issues a hold. If the field is uncontrolled, there is no requirement to hold short of the ILS Critical Area
That said, be mindful of the weather and inbound aircraft, and consider holding short if conditions warrant
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 that 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
This single ILS frequency is found in the top left of the approach plate
A list of which frequencies are paired together can be found in the Aeronautical Information Manual under paragraph 1-1-9, Instrument Landing System
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 (FAF), 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
At the outer Marker (OM) or Locator Outer Marker (LOM):
Check the altimeter crossing the OM/LOM
Perform the 5Ts:
Time: Note the time
Turn: to track the inbound course
Twist: Verify that the OBS is set to inbound course:
Throttle: Maintain approach speed
Talk: Make required reports
At 1000' above DA(H), call out "1000 above DA(H)"
At 500' above DA(H), call out "500 above DA(H)," and complete a GUMP check
At 100' above DA(H), call out "100 above DA(H)"
Upon reaching DA(H):
Continue the approach if the runway environment is in sight, or;
Descend no lower than 100 above touchdown zone elevation when referencing the approach light system without seeing the red terminating bars or red side-row bars
Call out "missed approach," and execute the published missed approach procedure if ATC does not direct a different procedure
With the runway in sight and in a position from which a descent to a landing on the indented runway can be made at a normal rate of descent using normal maneuvers
Callout, "Runway in Sight, Landing"
Maintain the localizer and glideslope during the visual descent to a point over the runway where the glideslope must be abandoned to accomplish a normal landing
At or below VFE, set the flaps as appropriate for landing
Instrument Flying Handbook, Instrument Landing System
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
