The Instrument Landing System (ILS) is a precision approach to a particular runway. A
precision approach gives both vertical guidance (the glideslope) and horizontal guidance (the
localizer). There are four main elements to the ILS:

The Localizer - provides course guidance along the extended runway centerline.
The Glideslope - provides vertical guidance. The usual slope is 3° to the horizontal.
 Marker beacons - provide accurate range fixes along the approach path. Usually there is
an outer and a middle marker.
 Approach lights, VASI (Visual Approach Slope Indicator), and other lighting, such as
touchdown zone lighting, runway lighting, etc. The purpose of the lighting is to assist in
the transition from instrument to visual flight.

There may be other aids, such as a Non Directional Bearing (NDB) or DME (Distance
Measuring Equipment), available.
The outer marker may be replaced by a range marker, such as a compass locator or a DME
distance on some approaches. The middle marker may be replaced by a compass locator, but,
DME may not be substituted for the middle marker.
When there is an NDB co-located with an outer marker, it is indicated on the approach chart by
LOM (locator outer marker). A compass locator (otherwise known as NDB) collocated with the
middle marker is indicated by LMM.
The intersection between the localizer and the glideslope is the ideal path of approach and is
called the glidepath.
Visual information is provided since the ILS may be flown in conditions of poor visibility. The
purpose of the visual aids, such as approach lighting, touchdown zone lighting, and runway
lighting, is to assist the pilot is transitioning from instruments to visual flight.
The illustration below shows the various parts of the ILS approach.
1. Localizer plane - the localizer is normally intercepted first and the glideslope is
intercepted from beneath
2. Glideslope
3. Glidepath
4. Outer Marker - since the altitude of the glideslope where it intercepts the OM is given on
the IAP (Instrument Approach Chart), it should be used as a check to confirm altitude
readings and glideslope operation
5. Middle Marker - typically the middle marker intersects the glideslope at or near the
Decision Height (DH) and is the last point you should commence a missed approach if
not visual
THE LOCALIZER
The cockpit instrument used for flying the ILS is usually the same as your NAV 1 radio in small
aircraft and the HSI in the light and heavy jets. The needle is the same one used for VOR (VHF
omnidirectional radio) navigation. You must identify the localizer the same way you do a VOR,
by its Morse Code identifier. The ident is always a four letter code beginning with I.
The localizer transmits a highly directional beam on a frequency between 108.10 and 111.95
MHz. The specific frequency can be found on the approach plates or in the A/FD. There are 40
localizer frequencies available and all of them have an odd number as the first digit after the
decimal point. The localizer is actually two overlapping lobes of radio energy. These lobes get
wider as they get further from the localizer antenna array, which is located near the end of the
runway opposite the approach end.
The beam is adjusted so that the width of a full scale deflection of the localizer needle is 700 ft at
the approach runway threshold. The angular width of the localizer varies between 3 and 6
degrees. The typical width of the full scale deflection is 5°, or 2.5° either side of the centerline.
The localizer is accurate within the sectors indicated in the illustration from an altitude of 1000 ft
above the highest terrain along the course to 4500 ft above the elevation of the antenna. Correct
indications are guaranteed only within this service airspace. Outside this space, the correct signal
is not assured and it is possible you will not receive the coded ident.
The main function of the localizer is to provide azimuth (horizontal) guidance along the
approach. This is also known as the localizer front course. Many localizers also transmit a signal
that is usable from the opposite direction known as the back course. The back course does not
have an associated glideslope and is suppressed in some countries, such as the United Kingdom
and Australia.
The indication on the cockpit instrument is the same as that of a VOR, a right, left or centered
needle. The procedure for getting on course is also the same, fly toward the needle. Full scale
deflection of the CDI needle (Course Deviation Indicator) is approximately 2.5°. Movement of
the CDI is therefore four times more sensitive (0.5° per dot) than the with the VOR (2° per dot).
Unlike the VOR, the localizer is a single fixed course. Due to this, the OBS has no affect on the
CDI. It is a good practice to set the OBS to the inbound heading of the localizer as a reminder.
Again, there is NO heading information given, only position information.
If the localizer fails, the whole ILS approach is unauthorized and is not permitted. If only the
glideslope fails, the approach can still be made as a localizer approach, with higher minimums
required.
FLYING THE LOCALIZER
The localizer is a performance instrument and should be included in your scan when flying an
ILS approach. Any required corrections should be made with small, coordinated turns. You
should concentrate on the HI and AI for attitude flying with an occasional glance at the CDI to
check tracking. Do not chase the CDI.
Since the localizer beam is narrow, an intercept should be made at 30° or less. Otherwise, you
have a very good chance of flying through the localizer. If an LOM or other aid is available, it
should be monitored during the intercept to help you anticipate the turn onto the localizer.
Once the CDI starts moving, indicating you are approaching the centerline, you should
immediately turn to the localizer course. If there is a crosswind, a wind correction angle should
be estimated. Your goal is to fly a course that will keep you centered on the localizer.
The localizer acts as a command instrument while on the approach, so fly toward the needle.
Since the course narrows as you approach the runway, the corrections necessary will become
smaller and smaller. Winds normally will change as you get nearer the ground, therefore a
number of small corrections is normal when flying the ILS. A good rule of thumb is to restrict
heading corrections to 5° at the beginning of the approach and only 2° near the end. If you are
flying an aircraft with a heading bug, the bug is about 12° wide (plus or minus 6°), therefore
most course corrections should be made within the width of the bug. The following illustration
shows left of course, on course, and right of course indications respectively.
You should initially steer a heading that stops the needle from moving. Then wait a few seconds
to see if the CDI moves. If it does, make a small correction to center the CDI. You should be on
a steady course before reaching the outer marker. Any changes required after passing the outer
marker should be made with changes of 2° or less.
When continuing past the localizer and tracking outbound, as in a missed approach, the CDI
continues to be a command instrument, unless you reverse course. If you reverse course, the CDI
is a non-command instrument and you would fly away from the needle to regain the course.
This same situation is encountered when tracking outbound on the localizer front course, which
is sometimes required in an approach. The indicator will still indicate the angular distance off
course, but you would fly away from the CDI in order to center the needle. When flying with
reverse sensing, think of the required action as "pulling" the needle back on course. If there is a
LOM (locator outer marker) available, use it when flying outbound to help orient yourself and
stay on track.
Flying the localizer with the HSI (horizontal situation indicator) is very similar to using the
regular indicator. There are however several advantages to using the HSI. First, the HSI reduces
the scan required, since it combines heading and localizer information in one instrument.
Another advantage is the fact that the HSI uses a slaved compass card (it turns when the aircraft
turns). Therefore, the HSI always functions as a command instrument, even when tracking
outbound, provided you set the localizer heading in the HSI.
Interception of the localizer is also simplified since you are given a clear visual of where you are
in relation to the localizer and the intercept angle is presented on the HSI. The following
illustration shows left of course, on course, and right of course indications on the HSI.
The Glideslope
The glideslope typically gives a 3° approach to the runway and intersects the runway
approximately 1000 ft from the threshold. The 3° slope means a descent of about 300 ft per
nautical mile is required to stay on the glideslope. This gives a reasonable rate of descent for
most aircraft, e.g. 600 fpm at 120 kts and 450 fpm at 90 kts groundspeed. The approximate
height above the ground for the glideslope at any given point can be approximated by
multiplying the distance from the runway in nautical miles by 300. i.e. at 3 nm, the glideslope
should be 900 ft.
Another handy calculation to be familiar with is how to figure a descent rate that will keep you at
the correct rate for any given groundspeed. The rule of thumb here is 5 times your groundspeed.
The glideslope antenna is usually situated 750-1250 ft down the runway from the threshold and
offset by about 300 ft to the side of the runway. The distance from the threshold is to assure
adequate wheel clearance over the threshold. The aiming point when flying the ILS is not the
"numbers" but the designated touchdown zone, where the glideslope intersects the runway.
Like the localizer, the glideslope signal is composed of two overlapping lobes. The area of
overlap is about 1.4° or plus or minus 0.7° from the centerline. The glideslope is calibrated to 10
nm, but, you may receive it further out. Descent should not be started until the localizer is
intercepted.
The cockpit indicator for the glideslope is the horizontal needle on the VOR indicator and
usually some type of vertical ribbon gauge on an HSI. Like the VOR, the red "Off" flag should
not be visible when a usable signal is received. The glideslope frequency is paired with the
localizer frequency and is automatically set when you tune the localizer.
The glideslope is a command instrument, i.e. fly toward the needle. The following illustration
shows the too high, on glideslope, and too low on both the Cessna glideslope indicator and the
HSI from the Lear 45.
The only time you would see reverse sensing on the glideslope is during inverted flight -- not a
recommended procedure and leads to very short rollouts on landing.
A full scale fly-up indication means that the aircraft is 0.7° or more below the glideslope. Like
the VOR, the glideslope deflection is shown by dots. The recommended procedure is to not go
below a half scale deflection in order to assure ground clearance. A full scale deflection of the
glideslope indicator is not acceptable on an ILS approach.
The full scale deflection of the glideslope is 1.4°; the localizer is 5°; and the VOR is 20°. This
means that the glideslope is 3 times more sensitive than the localizer and 12 times more sensitive
than the VOR.
The glideslope is only approved for navigational use down to the DH (Decision Height) for any
particular ILS. Any reference to the glideslope after that point must be supplemented by visual
reference. Category 1 ILS is approved for use down to a DH of 200 ft HAT (Height Above
Touchdown). Cat 2 ILS has a DH of 100 ft and Cat 3 has a 0 DH.
If the glideslope fails, the approach can usually still be flown as a localizer only approach, but,
the minimums will be higher. The allowable descent profile will be indicated on the approach
chart and will use such things as the markers or DME (Distance Measuring Equipment) to
indicate the stepdowns in altitude. If only the full ILS is allowed, the chart will be marked "LOC
ONLY N/A".
FLYING THE GLIDESLOPE
The key to flying the glideslope is a constant rate of descent. Applying our rule of thumb from
earlier is one way to decide on the proper rate of descent for a given groundspeed. Be aware that
your groundspeed may change during the approach due to changes in the wind direction or
velocity.
The glideslope should be held with small pitch changes. The process is similar to bracketing a
track, but in the vertical plane rather than the horizontal. For example, if you are a little low,
raise the pitch slightly until you are back on the glideslope and then reduce the pitch slightly.
Remember that you are only dealing with 1.4° full deflection, so not much change is necessary.
Airspeed is maintained with power. The changes in pitch may require slight changes in airspeed,
which should remain constant. Variations of ±5 kts are acceptable. A typical power adjustment is
made in increments of 100 rpm or 1 inch of manifold pressure for a piston engine. Strong or
gusty winds may require greater changes.
Energy management is a key to flying a good ILS approach. If you are a little low and fast,
increasing pitch will slow you down as well as increasing your altitude. Conversely, if you are a
little high and a little slow, decreasing pitch and flying the aircraft down will decrease altitude
and increase speed. Remember, hold the glideslope with pitch and hold airspeed with power.
Like the localizer, the glideslope indicator will become more sensitive as you near the runway
and your corrections should become smaller and smaller.
MARKER BEACONS
The marker beacons, also called fan markers because of the signal shape, are highly focused
vertical signals which can only be received when directly overhead. It is not possible to "track" a
marker beacon like you can other navaids.
The typical ILS has two marker beacons positioned along the localizer. The outer marker (OM)
is positioned between 4 and 7 nm from the runway threshold and the middle marker (MM) is at
3500 ft from the threshold.
The airborne equipment is the marker beacon receiver which has both aural and visual indicators.
A series of low frequency (400 Hz) dashes, transmitted at 2 per second, and a flashing blue light
indicate passage of the OM. The MM is indicated by a series of medium pitched (1300 Hz)
alternating dots and dashes and a flashing amber light. There is sometimes also an inner marker
(IM) between the MM and the threshold with has high pitched (3000 Hz) dots at 6 per second
and a flashing white light.
Marker beacons (fan markers) are also sometimes used as enroute fixes along airways and have a
dot dash dot pattern at 3000 Hz. The same white light that marks the IM is used for the fan
markers. Some localizer back courses also use the white light and a pattern of dot dot, dot dot,
dot dot at 3000 Hz
The marker beacons are very narrow and hence are only heard for a few seconds as the aircraft
passes overhead.
OTHER GLIDESLOPE CHECKS
Not all ILS approaches have an OM or MM. In this case, you can check your altitude at a known
point, such as the FAF (Final Approach Fix) or with DME checkpoints. DME stepdowns are
often provided through the use of an required altitude at a specific DME fix.
VISUAL COMPONENTS
Various visual components are a part of the ILS approach and are used to help in the transition
from instrument to visual flight. Approach Light Systems (ALS) extend from the runway
threshold out from the approach end of the runway. ALS act as a lead in to the runway and are a
standardized series of red and white lights, consisting of extended centerline lighting and
crossbars at specific intervals. They extend 2400-3400 ft for precision instrument approach
runways and 1400-1500 ft for non precision.
Another form of approach lighting is the Sequenced Flashing Lights (SFL), or Runway
Alignment Indicator Lights (RAIL). This appears as a ball of white lights traveling toward the
runway along the extended centerline at a rate of two flashes per second. The SFL and RAIL are
sometimes referred to as the "rabbit".
The threshold is marked with a row of green lights. Some runways may have flashing strobes on
either side of the threshold. These are called Runway End Identifier Lights (REIL).
The Visual Approach Slope Indicator (VASI) may be seen in several forms. There are 2 bar, 3
bar, PAPI (Precision Approach Path Indicator), PVASI (Pulsating Visual Approach Slope
Indicator), Tri-color VASI and T-VASI. The purpose of all of these is to provide visual
glideslope information during the last phase of the ILS approach. The 2 bar VASI is seen in
FS98 and consists of two rows of lights, usually at 500 and 1000 ft from the threshold. When you
are low, both bars are red. When on glideslope, they are red over white. When high, they are
both white.
Flying the ILS
The first thing to do before starting an ILS approach is to familiarize yourself with the relevant
approach chart. Even though the chart is available during the approach, you should be familiar
with the basic picture of the approach. This will improve your situational awareness and make
things a lot easier during the approach.
The minimums and the DH should be checked. The Missed Approach Procedure should be
reviewed. You should also check your fuel quantity to determine whether or not you have
sufficient fuel to make one or more attempts before heading for your alternate. Make sure to
allow for reserves.
These steps would be completed well before reaching the airport. This will allow you to
concentrate on flying the approach and not having to worry about other details during that time.
For your trip, you will follow the normal procedures for enroute navigation. All clearances
should be written down and read back to ATC as you proceed. All navaids should be identified
before being used. If a holding pattern might be required, plan the correct entry based on your
heading when you reach the holding point.
The closer you get to the airport, the heavier the workload. Besides flying the approach, you still
have to attend to the normal procedures for landing, including the descent checklist, the pre
landing checklist, configuration and airspeed changes, communications and navigation.
Once you have completed the enroute part of your flight, you will transition to the approach by
using some combination of STARs, radar vectors, or other navigation techniques. Once you start
the ILS approach, you will continue down the localizer and glideslope until reaching the DH. At
the DH, if you are visual, you will complete the flight with a landing. If you do not break out of
the clouds at or above the DH, or you do not have the required visibility, you will execute the
missed approach procedure and then either make another attempt at landing or divert to an
alternate.
It should be remembered that the missed approach is not an emergency procedure, but, a normal
part of the approach which provides a safe path away from the airport if conditions are below
minimums or you decide to execute the missed approach for other reasons. The missed approach
should be commenced without delay and efficiently once it becomes necessary. If you decide to
"go missed" before reaching the DH, you should continue on the localizer course until reaching
the point where the missed approach procedure begins. Then follow the missed approach
procedure or ATC instructions.
In finishing up the lesson, let's consider the various indications you may see as you fly the ILS
approach. There are only nine basic indications during the approach. One is the correct glidepath.
The other eight require a correction in either altitude, course, or both. All the combinations are
shown in the following illustrations and include indications for the Cessna (top) and the Lear 45
HSI (bottom). The numbers in the illustration correspond to the numbered cells in the table
below.
1 On GS, On Localizer
2 On GS, Left
3 On GS, Right
4 High, Left
5 High, On Localizer
6 High, Right
7 Low, Left
8 Low, On Localizer
9 Low, Right
References
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ICAO Annex 10 Volume 1, Radio Navigation Aids, Fifth Edition — July 1996
Aeronautical Information Manual, FAA – February 11, 2010
Digital Terminal Procedures, FAA – May 2010
http://www.navfltsm.addr.com/index.htm
http://en.wikipedia.org/wiki/Instrument_landing_system