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Instrument Flying Handbook
Navigation Systems
Instrument Approach Systems

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Instrument Flying


Table of Contents

Chapter 1. Human Factors
Chapter 2. Aerodynamic Factors
Chapter 3. Flight Instruments
Chapter 4. Section I
Airplane Attitude Instrument
Using Analog Instrumentation
Chapter 4. Section II
Airplane Attitude Instrument
Using an Electronic Flight

Chapter 5. Section I
Airplane Basic
Flight Maneuvers
Using Analog Instrumentation
Chapter 5. Section II
Airplane Basic
Flight Maneuvers
Using an Electronic Flight

Chapter 6. Helicopter
Attitude Instrument Flying

Chapter 7. Navigation Systems
Chapter 8. The National
Airspace System

Chapter 9. The Air Traffic
Control System

Chapter 10. IFR Flight
Chapter 11. Emergency

Standard two-bar VASI.
Figure 7-38. Standard two-bar VASI.

ILS Airborne Components
Airborne equipment for the ILS system includes receivers
for the localizer, GS, marker beacons, ADF, DME, and the
respective indicator instruments.

The typical VOR receiver is also a localizer receiver with
common tuning and indicating equipment. Some receivers
have separate function selector switches, but most switch
between VOR and LOC automatically by sensing if odd
tenths between 108 and 111.95 MHz have been selected.
Otherwise, tuning of VOR and localizer frequencies is
accomplished with the same knobs and switches, and the CDI
indicates "on course" as it does on a VOR radial.

Though some GS receivers are tuned separately, in a typical
installation the GS is tuned automatically to the proper
frequency when the localizer is tuned. Each of the 40 localizer
channels in the 108.10 to 111.95 MHz band is paired with a
corresponding GS frequency.

When the localizer indicator also includes a GS needle, the
instrument is often called a cross-pointer indicator. The
crossed horizontal (GS) and vertical (localizer) needles are
free to move through standard five-dot deflections to indicate
position on the localizer course and glide path.

When the aircraft is on the glide path, the needle is horizontal,
overlying the reference dots. Since the glide path is mach
narrower than the localizer course (approximately 1 .4° from
full up to full down deflection), the needle is very sensitive
to displacement of the aircraft from on-path alignment. With
the proper rate of descent established upon GS interception,
very small corrections keep the aircraft aligned.

The localizer and OS warning flags disappear from view on
the indicator when sufficient voltage is received, to actuate the
needles. The flags show when an unstable signal or receiver
malfunction occurs.

The OM is identified by a low-pitched tone, continuous dashes
at the rate of two per second, and a purple/blue marker beacon
light. The MM is identified by an intermediate tone, alternate
doth and dashes at the rate of 95 dot/dash combinations per
minute, and an amber marker beacon light. The IM, where
installed, is identified by a high-pitched tone, continuous dots
at the rate of six per second, and a white marker beacon light.
The back-course marker (BCM), where installed, is identified
by a high-pitched tone with two dots at a rate of 72 to 75 two-
dot combinations per minute, and a white marker beacon light.
Marker beacon receiver sensitivity is selectable as high or low
on many units. The low-sensitivity position gives the sharpest
indication of position and should he used during an approach.
The high-sensitivity position provides an earlier warning that
the aircraft is approaching the marker beacon site.

ILS Function
The localizer needle indicates, by deflection, whether the
aircraft is right or left of the localizer centerline, regardless of
the position or heading of the aircraft. Rotating the OBS has
no effect no the operation of the localizer needle, although
it is useful to rotate the OBS to put the LOC inbound course
under the course index. When inbound on the front course, or
outbound on the back course, the course indication remains
directional. (See Figure 7-39, aircraft C, D, and E.)

Unless the aircraft has reverse sensing capability and it is in
use, when flying inbound on the back coarse or outbound
on the front course, heading corrections to on-course are
made opposite the needle deflection. This is commonly
described as "flying away from the needle." (See Figure 7-39,
aircraft A and B.) Back course signals should not be used
for an approach unless a back course approach procedure
is published for that. particular runway and the approach is
authorized by ATC.

Once you have reached the localizer centerline, maintain
the inbound heading until the CDI moves off center. Drift
corrections should be small and reduced proportionately as
the course narrows. By the time you reach the OM, your drift
correction should he established accurately enough on a well-
executed approach to permit completion of the approach,
with heading corrections no greater then 2°.

The heaviest demand on pilot technique occurs during
descent from the OM to the MM, when you maintain
the localizer course, adjust pitch attitude to maintain the
proper rate of descent, and adjust power to maintain propel
airspeed. Simultaneously, the altimeter must be checked
and preparation made for visual transition to land or for a
missed approach. You can appreciate the need for accurate
instrument interpretation and aircraft control within the ILS
as a whole, when you notice the relationship between CDI
and glide path needle indications, and aircraft displacement
from the localizer and glide path center lines.