because the system is disabled by the weight-
RECEIVER-TRANSMITTER. This system
uses the RT-1042/APN-194(V) as its receiver-
transmitter (fig. 2-10, view B). It is an airtight unit
that contains all the electronic components for the
generation, detection, and time difference
computations of the radar pulses.
There are two altitude modes of operation-one
for low level and one for high level. The low-level
mode is for altitudes less than 1,000 feet. In this
mode, the RT transmits a very narrow, low-powered
pulse to get maximum range resolution. The
high-level mode is for altitudes above 1,000 feet. In
this mode, the output pulse is a wider, high-powered
pulse, which ensures sufficient ground return energy
There are two ranging modes of operationone
for search and one for tracking. In the search mode,
the system successively examines increments of
range with each cycle of operation until the complete
altitude range is searched for ground return. When
the range is found, the system switches to the track
mode. In the track mode, the system locks onto and
tracks the leading edges of the ground return pulses.
It then sends continuous altitude information to the
In most aircraft with this system, there is an
Interference Blanker, MX-9132A/APN-194(V),
located next to the RT (fig. 2-10, view B). This
blanker attenuates any RF from direct antenna
leakage and provides isolation of the receiver from
the transmit antenna.
LOW ALTITUDE AUDIBLE ALARM. Some
of the aircraft with the APN-194(V) system have the
BZ-157A low altitude audible alarm installed (fig.
2-10, view C). In the EA-6A aircraft, this box is on
the ICS relay box behind the EWOs seat. This alarm
will apply a tone to the intercommunication system
when the aircraft falls below the altitude that the limit
bug is set. The tone is a 2-second tone alternating
between 700 and 1700 Hz at 2-Hz intervals. This
alarm will also sound when there is an unreliable
condition in the system. This tone alternates between
700 and 1700 Hz at 8-Hz intervals. The alarm will
also send a signal to the indicator to cause the OFF
flag to appear.
The unreliable condition warning
signal takes precedence over the low altitude warning
The BZ-157A has three switches on it. The first
one is the volume control, which controls the volume
of the alarm. The other two are press-to-test switches.
Depressing the LOW ALT switch will cause the alarm
to be heard. Depressing the UNREL switch will
cause the alarm to be heard and the OFF flag to
LOW ALTITUDE WARNING LIGHT. The
low altitude warning light is mounted on the pilots
instrument panel. This light will illuminate whenever
the aircraft falls below the altitude that the limit bug is
RADAR ALTIMETER WARNING SET
The radar altimeter warning set (RAWS) works in
conjunction with the radar altimeter systems that do
not have the BZ-157A alarm unit. The P3-C aircraft
uses the AN/APQ-107 system. This system provides
the pilot and copilot with warning signals whenever
any of the following conditions exists:
. Aircraft flies below preselected altitudes.
. Input power to radar altimeter fails.
. RAWS warning circuit indicates unreliability.
When the aircraft descends to 380 (±20) feet
(high-altitude index), the radar altimeter signal to
RAWS is interrupted. This causes the AUTOPILOT/
RADAR ALTM warning lights to flash and a 1-kHz
interrupted tone to be heard over the ICS. Both
signals occur at a rate of two pulses-per-second for a
3-second duration. This also happens as the aircraft
descends through 170 feet (low-altitude index). The
warnings will continue as long as the aircraft remains
below the low index. When the nosegear is down or
the flaps are in the approach, takeoff, or landing
position, the warning signals are disabled.
If the radar altimeter receiver signal is too weak to
provide reliable altitude information or the altimeter
malfunctions, the RAWS will give both warnings.
This occurs unless the signals are disabled by the
nosegear or flap position.
There are two RAWS press-to-test switches
located in the aircraft. One on the RAWS itself, and
one on the forward load center. If you depress the
switch on the RAWS, it will actuate both signals,
unless the nosewheel is down and locked-then only
the aural warning occurs. Depressing the switch on
the forward load center results in both the visual and
the aural warnings.