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system.
• Erratic instrument readouts.
• Weak transmissions and poor radio reception.
• St. Elmo’s Fire.
Traditional Navigation Systems
Nondirectional Radio Beacon (NDB)
The nondirectional radio beacon (NDB) is a ground-based
radio transmitter that transmits radio energy in all directions.
The ADF, when used with an NDB, determines the bearing
from the aircraft to the transmitting station. The indicator
may be mounted in a separate instrument in the aircraft
panel. [Figure 7-2] The ADF needle points to the NDB
ground station to determine the relative bearing (RB) to the
transmitting station. It is the number of degrees measured
clockwise between the aircraft’s heading and the direction
from which the bearing is taken. The aircraft’s magnetic
heading (MH) is the direction the aircraft is pointed with
respect to magnetic north. The magnetic bearing (MB) is the
direction to or from a radio transmitting station measured
relative to magnetic north.
NDB Components
The ground equipment, the NDB, transmits in the frequency
range of 190 to 535 kHz. Most ADFs will also tune the AM
broadcast band frequencies above the NDB band (550 to
1650 kHz). However, these frequencies are not approved
for navigation because stations do not continuously identify
themselves, and they are much more susceptible to sky
wave propagation especially from dusk to dawn. NDB
stations are capable of voice transmission and are often used
for transmitting the automated weather observing system
(AWOS). The aircraft must be in operational range of the
NDB. Coverage depends on the strength of the transmitting
station. Before relying on ADF indications, identify the
station by listening to the Morse code identifier. NDB stations
are usually two letters or an alpha-numeric combination.
ADF Components
The airborne equipment includes two antennas, a receiver,
and the indicator instrument. The “sense” antenna (nondirectional)
receives signals with nearly equal efficiency
from all directions. The “loop” antenna receives signals
better from two directions (bidirectional). When the loop
and sense antenna inputs are processed together in the ADF
radio, the result is the ability to receive a radio signal well in
all directions but one, thus resolving all directional ambiguity.
The indicator instrument can be one of four kinds: fixedcard
ADF, rotatable compass-card ADF, or radio magnetic
7-4
Figure 7-3. Relative bearing (RB) on a fixed-card indicator. Note
that the card always indicates 360°, or north. In this case, the
relative bearing to the station is 135° to the right. If the aircraft
were on a magnetic heading of 360°, then the magnetic bearing
(MB) would also be 135°.
Figure 7-4. Relative bearing (RB) on a movable-card indicator. By
placing the aircraft’s magnetic heading (MH) of 045° under the
top index, the relative bearing (RB) of 135° to the right will also
be the magnetic bearing (no wind conditions) which will take you
to the transmitting station.
indicator (RMI) with either one needle or dual needle. Fixedcard
ADF (also known as the relative bearing indicator (RBI))
always indicates zero at the top of the instrument, with the
needle indicating the RB to the station. Figure 7-3 indicates
an RB of 135°; if the MH is 045°, the MB to the station is
180°. (MH + RB = MB to the station.)
The movable-card ADF allows the pilot to rotate the
aircraft’s present heading to the top of the instrument so
that the head of the needle indicates MB to the station and
the tail indicates MB from the station. Figure 7-4 indicates
a heading of 045°, MB to the station of 180°, and MB from
the station of 360°.
The RMI differs from the movable-card ADF in that it
automatically rotates the azimuth card (remotely controlled
by a gyrocompass) to represent aircraft heading. The RMI
has two needles, which can be used to indicate navigation
information from either the ADF or the VOR receiver. When
a needle is being driven by the ADF, the head of the needle
indicates the MB TO the station tuned on the ADF receiver.
The tail of the needle is the bearing FROM the station. When
a needle of the RMI is driven by a VOR receiver, the needle
indicates where the aircraft is radially with respect to the
VOR station. The needle points to the bearing TO the station,
as read on the azimuth card. The tail of the needle points to
the radial of the VOR the aircraft is currently on or crossing.
Figure 7-5 indicates a heading of 005°, the MB to the station
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