pages: 1: ILS Basic Components | 2:
More ILS Components
The ILS approach is the most common precision approach. The technical
aspects of the ILS are well covered in the AIM and you are encouraged
to spend the time necessary with this section to thoroughly understand
the operation, specifications, and limitations of the ILS. You will
probably fly more ILS approaches than all the rest of the approach
You will be tested numerous times on the workings of the ILS during
your flying career. The tests are not graded on a curve. They are
strictly pass/fail. If you pass, you land safely. Fail, you bend
an airplane or worse.
From AIM 1-1-10. INSTRUMENT LANDING SYSTEM (ILS)
- The ILS is designed to provide an approach path for
exact alignment and descent of an aircraft on final approach
to a runway.
- The ground equipment consists of two highly directional transmitting
systems and, along the approach, three (or fewer) marker beacons.
The directional transmitters are known as the localizer and
- The system may be divided functionally into three parts:
- Guidance information - localizer, glideslope
- Range information - marker beacon, DME
- Visual information - approach lights, touchdown and centerline
lights, runway lights
- Compass locators located at the Outer Marker (OM) or
Middle Marker (MM) may be substituted for marker beacons.
DME, when specified in the procedure, may be substituted for
- Where a complete ILS system is installed on each end of a
runway; (that is, the approach end of Runway 4 and the approach
end of Runway 22) the ILS systems are not in service simultaneously.
- The localizer transmitter operates on one of 40 ILS channels
within the frequency range of 108.10 to 111.95 MHz. Signals
provide the pilot with course guidance to the runway centerline
- The approach course of the localizer is called the front
course and is used with other functional parts, for example,
glideslope, marker beacons, etc. The localizer signal is transmitted
at the far end of the runway. It is adjusted for a course width
of (full-scale fly left to full-scale fly right) or 700 feet
at the runway threshold
- The course line along the extended centerline of the runway,
in the opposite direction to the front course is called the
|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 out bound
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.
- Identification is in International Morse Code and
consists of a three letter identifier preceded by the letter
I (••) transmitted on the localizer frequency.
- 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 operational service
- To 10 degrees either side of the course along a radius of
18 NM from the antenna, and:
- From 10 to 35 degrees either side of the course along a
radius of 10 NM
- Unreliable signals may be received outside these areas.
- LOCALIZER TYPE DIRECTIONAL AID
- The Localizer type Directional Aid (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
degrees. The LDA is not aligned with the runway. Straight-in
minimums may be published where alignment does not exceed 30
degrees between the course and runway. Circling minimums only
are published where this alignment exceeds 30 degrees.
- The UHF glideslope 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 "glidepath" means that portion of the glideslope
that intersects the localizer.
|False glideslope signals may exist
in the area of the localizer back course approach which
can cause the glideslope flag alarm to disappear and present
unreliable glideslope information. Disregard all glideslope
signal indications when making a localizer back course approach
unless a glideslope is specified on the approach and landing
- The glideslope transmitter is located between 750 feet and
1,250 feet from the approach end of the runway (down the runway)
and offset 250 to 650 feet from the runway centerline. It transmits
a glidepath beam 1.4 degrees wide. The signal provides descent
information for navigation down to the lowest authorized decision
height (DH) specified in the approved ILS approach procedure.
The glidepath may not be suitable for navigation below the lowest
authorized DH and any reference to glidepath indications below
the height must be supplemented by visual reference to the runway
environment. Glidepaths with no published DH are usable to runway
- The glidepath projection angle is normally adjusted to 3 degrees
above horizontal so that it intersects the MM at about 200 feet
and the OM at about 1,400 feet above the runway elevation. The
glideslope is normally usable to the distance of 10 NM. However,
at some locations, the glideslope has been certified for an
extended service volume which exceeds 10 NM.
- Pilots must be alert when approaching the glidepath interception.
False courses and reverse sensing will occur at angles considerably
greater than the published path.
- Make every effort to remain on the indicated glidepath.
|Avoid flying below the glidepath
to assure obstacle/terrain clearance is maintained.
- The published glideslope threshold crossing height
(TCH) DOES NOT represent the height of the actual glidepath
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 glideslope antenna
should be, if that aircraft remains on a trajectory formed by
the four mile to middle marker glidepath segment.
- Pilots must be aware of the vertical height between the aircraft's
glideslope 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
feet, depending on the type of aircraft.
- DISTANCE MEASURING EQUIPMENT (DME)
- 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) final approach fix (FAF)
- 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 segments
- As a FAF for BC approaches
- As a substitute for the OM
page 2: More ILS Components