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light. The MM is identified by an intermediate tone, alternate
dots 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 twodot
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 be 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 on 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 course 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 be established accurately enough on a wellexecuted
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
7-43
Figure 7-39. Localizer Course Indications. To follow indications displayed in the aircraft, start from A and proceed through E.
7-44
Figure 7-40. Illustrates a GS receiver indication and aircraft displacement. An analog system is on the left and the same indication on
the Garmin PFD on the right.
proper rate of descent, and adjust power to maintain proper
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 centerlines.
Deflection of the GS needle indicates the position of the
aircraft with respect to the glide path. When the aircraft is
above the glide path, the needle is deflected downward. When
the aircraft is below the glide path, the needle is deflected
upward. [Figure 7-40]
ILS Errors
The ILS and its components are subject to certain errors,
which are listed below. Localizer and GS signals are subject to
the same type of bounce from hard objects as space waves.
1. Reflection. Surface vehicles and even other aircraft
flying below 5,000 feet above ground level (AGL)
may disturb the signal for aircraft on the approach.
2. False courses. In addition to the desired course, GS
facilities inherently produce additional courses at
higher vertical angles. The angle of the lowest of these
false courses will occur at approximately 9°–12°. An
aircraft flying the LOC/GS course at a constant altitude
would observe gyrations of both the GS needle and GS
warning flag as the aircraft passed through the various
false courses. Getting established on one of these
false courses will result in either confusion (reversed
GS needle indications) or in the need for a very high
descent rate. However, if the approach is conducted
at the altitudes specified on the appropriate approach
chart, these false courses will not be encountered.
Marker Beacons
The very low power and directional antenna of the marker
beacon transmitter ensures that the signal will not be received
　
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