because the system is disabled by the weight- on-wheels switch. 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 for tracking. There are two ranging modes of operation—one 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 indicator. 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 EWO’s 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 signal. 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 appear. LOW ALTITUDE WARNING LIGHT.— The low altitude warning light is mounted on the pilot’s instrument panel. This light will illuminate whenever the aircraft falls below the altitude that the limit bug is set. 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. 2-11