INSTRUMEN LANDING SYSTEM
Provided by: Ali-Dalirian
 The instrument landing system ( ILS ) has been in existence for
over 40 years, but, it is today still the most accurate approach
and landing aid that is used regularly by the airliners.
 The system provides pilot with an accurate means of carrying
out an instrument approach to a
runway, giving precision
guidance both in the horizontal and the vertical planes enabling
the aircraft to carry out automatic landing.
 ILS provides pilot with visual instructions in the cockpit to fly the
aircraft down on a predetermined glide path and extended
runway center-line course to his decision altitude/ height.
 At DA/H, the pilot decides to land, if he has the required visual
references and sufficient room to maneuver the aircraft for a
safe touchdown, or follows the published missed approach
procedure .
 Therefore, the ILS is designated to provide an approach path for
exact alignment and descent on final approach to a runway. It
provides highly accurate course and glideslope to a given
runway.
1
ILS GROUND EQUIEMENT AND VISUAL AIDS CONCIST OF:
a. Two transmitting systems (localizer for course guidance and
glideslope for vertical guidance).
b. Range information consist of marker beacons (outer, middle and
inner), DME.
c. Visual information consist of approach lights, touchdown lights,
runway centerline lights and runway lights.
LOCALIZER Is the primary piece of ILS equipment located at the far end of
the runway approximately 1000 feet ( 300 meters ) , opposite the
approach end, providing course ( horizontal ) guidance.
 The localizer transmitter operates on one of 40 ILS channels
within the VHF frequency of 108.00 to 111.95 MHZ, using the odd
tenth frequencies, for example, 108.1 – 108.15 – 108.3 – 108.35 –
108.5 – 108.55 - ……… .
 The identification consist Of three-letter
morse code preceded by the letter ‟ I ”,
transmitted on localizer frequency.
 The localizer sends out two patterns modulated at 150 HERTS to
the right of an airplane inbound, and 90 HERTS to the left of the
approaching airplane, covering both left and right side of the
inbound traffic.
 These two signal patterns, overlap along the line formed by an
extension of the centerline of the runway, which is called course.
This course, is identified by the airplane receiving equipment as
the area of equal signal strength between the two modulated
patterns.
 The difference in depth between the 90 HZ and 150 HZ
modulation is measured in order to establish the aircraft position
in relation to the localizer beam on an ILS.
2
NOTE 1: These two overlapping lobes transmit on the same VHF
frequency with different modulations and describes the ILS localizer
radiation pattern.
NOTE 2: The error of an ILS localizer beam are due to the ground
reflections.
(The localizer sends out two patterns modulated at 150 HERTS to the
right and 90 HERTS to the left of the approaching airplane)
 Course information is accurate to at least 18 N.M from the
antenna. At 18 N.M, the minimum reception altitude is 1000 feet
above the highest terrain and the maximum reception altitude is
4500 feet above the elevation of the antenna.
 The operation service or off-course indications at 10 N.M is 35°on
either side of the course. From 10 to 18 N.M, this angle reduces
to 10°either side.
3
 The angular width of the localizer is between three and six
degrees, as necessary to provide a linear width of approximately
700 feet at the runway approach threshold.
(The operation service or off-course indications of localizer)
localizer presentation –
 The ILS localizer information can be displayed on a course
deviation indicator, on the horizontal situation indicator ( HSI ) .
4
 Using the vertical needle of HSI (CDI), for localizer signals, the
total span of the CDI is 5°or 2.5° offset for full scale deflection
on each side of the course.
 With a five dot display on either side of the CDI on the ILS
localizer display, each dot represents approximately 0.5°.
 If an aircraft is flying downwind outside the coverage of the ILS,
the CDI indications will be unreliable in azimuth and elevation.
 If the heading of the HSI is frozen, this does not influence the
indication of the ILS localizer needle, and, it still shows
whether the aircraft is located to the left, to the right or on the
localizer center line, therefore,
 If the aircraft is lined up on the localizer, the localizer needle will
be centered.
 NOTE 1 : When an off flag appears in front of the CDI, it
indicates that the signal strength is not sufficient and the needle
indications are unreliable.
 Momentary off flag indications, or, CDI needle deflections, or
both may occur when obstructions or other aircraft pass
between the transmitting antenna and the receiving aircraft and
may be disregarded.

NOTE 2 : CDI movement is more rapid during localizer tracking,
so, the pilot must make smaller corrections than with VOR.
These corrections also must be made more quickly.
GLIDE SLOPE
 The glide slope transmitter which provides vertical guidance
(descent information) for the aircraft approaching, is located on
one side of the runway between 750 feet to 1,250 feet (400
meters ) upwind from the approach end of the runway (down the
runway) and offset 150 meters from the runway centerline.
5
 It operates on one of 40 ILS channels within the frequency range
of 329.15 MHZ, to 335.00 MHZ UHF band which is paired to the
associated VHF localizer. So, when the pilot tunes the VHF
localizer
frequency,
he/she
automatically
channels
the
appropriate UHF glide slope frequency.
 The glide path beam is 1.4 degree wide vertically (0.7°above and
0.7°
below).
Like
localizer,
the
signals
consists
of
two
overlapping beams modulated at 90 hertz( above) and 150 hertz
(below).
(Like localizer, the glide slope signals consists of two overlapping
beams modulated at 90 hertz above and 150 hertz below).
 The centerline of the beam is normally adjusted to three degrees
above the horizon that intersects the MM at about 200 feet and
the OM at about 1,400 feet above the runway elevation.
6
NOTE : The glide slope is normally usable to the distance of 10 NM,
and the pilot should disregard the glide slope indications until he/she
is approaching the prescribed intercept altitude.
false glide path –
 it only will be found when intercepting the glideslope from above
( not a very good practice ), because there is a danger of
capturing a false glide path due to the multiple-lobe nature of the
glide path signals producing false glide path at angle above the
nominal glide path . therefore;
 Normal flying the ILS, is to establish on the localizer
and then, intercept the glideslope from below .
CAUTION : Avoid flying below the glide path to assure obstacle
and terrain clearance is maintained.
Glide path presentation –
 The glide path information can also be displayed on a horizontal
situation indicator ( HSI ).
(The glide slope signals activates the glide slope needle located in the
HSI with a separate off flag which appears when the signals received
is not adequate for reliable use.)
7
 Full scale deflection on a glide slope indicator, indicates that the
aircraft is 0.7° above or below the glide path .
 To calculate the correct height on the ILS approach at a
specified distance from the runway threshold, the formula is :
Height = glideslope in°× 6080 × distance in N.M ÷ 60
 To calculate the rate of descent (R.O.D ) required to maintain a
glide slope at a specified ground speed :
R.O.D = glideslope in° × ground speed × 100 ÷ 60
ILS MARKER BEACONS ILS marker beacons transmit at 75 MHZ in the VHF band provides
the pilot with distance information to the runway by radiating an
elliptical beam upward from the ground providing a pattern of
approximately 2400 feet in width and 4200 feet in length, at 1000
feet above the antenna.
 Ordinarily, there are two marker beacons associated with an ILS,
the outer marker (OM), located four to seven miles from end of
the runway, and, middle marker (MM) located at 3500 feet from
the landing threshold.
 The MM is placed in such a way that it is passed by the aircraft
following the ILS at a height approximately 200 FT .
 The inner marker located between middle marker and the landing
threshold, operates in association with ILS CAT II.
 The ILS markers are identified in the aeroplane by color light and
audio signals.
NOTE : The ILS marker beacons do not interfere with each other
because they transmit in narrow vertical beams .
8
COMPASS LOCATOR Many ILS systems use an NDB as a compass locator, which
normally is collocated with the outer marker. The combined
facility is called a compass locator at the outer marker (LOM).
 Compass locators usually have a power output of less than 25
watts, resulting in a reception of 15 miles, in the frequency of
190 to 535 KHZ.
 If a compass locator is installed at the middle marker, it is called
a LMM.
IDENTIFICATION OF COMPASS LOCATORS The compass locators transmit a two-letter morse code identifier
taken from the localizer identifier group. (LOM the first two, and
LMM the second two letters).
ILS WITH DMEOn many ILS approaches, a DME transmitter
paired with an ILS
which is frequency
is placed at or near the localizer or glide slope
transmitter, providing direct runway distance information. This DME
equipment may be used as, in lieu of OM, and establish other fixes on
the localizer course.
NOTE- To provide ARC for initial approach segment, DME from a
separate facility within the instrument approach procedure may be
used.
ILS VISUAL AIDS Approach light systems are visual aids which normally
are associated with the ILS. Other visual aids are:
 High intensity runway lights (HIRL), touchdown zone
lights, centerline lights and markings, and RVR.
9
ILS back course approach  ILS back course approaches are allowed in some countries
which enables the aircraft to make a non-precision approach on
the back beam of the localizer transmitter.
 When using the ILS back - beam approach procedure, the pilot
must set the course arrow to the localizer front-beam course
because the HSI removes the reversed sensing and the localizer
needle indications are normal.
NOTE : Back-beam approach may only be used when specified
approved procedure exists for that specific approach and a published
procedure at hand.
ILS categories :
CAT I – a DH not lower than 200ft ( 60m ); and a visibility not less
than 2400ft ( 800m ) or RVR not less than 1800ft ( 550m ) .
CAT II – a DH lower than 200ft ( 60m ) but not lower than 100ft (30m);
and a RVR not less than 300m .
CAT IIIA – a DH lower than 100ft ( 30m ), or no DH; and a RVR not
less than 200m .
CAT IIIB – a DH lower than 50ft ( 15m ), or no DH; and a RVR less
than 200m but not less than 75m .
CAT IIIC – no DH and no RVR limitation .
 The acceptance of category II or III operations will depend on
the following criteria :
A) The aircraft has suitable flight characteristics;
B) The aircraft will be operated by a qualified crew;
C) The aerodrome is suitably equipped and maintained, and;
10
D) The required safety level can be maintained .
 When CAT II / III operations are in progress, all taxiing aircraft
must remain clear of the ILS sensitive area and hold at the CAT II
and III holding points that are further away from the runway in
use than the regular CAT I holding point .
( THE END )
11