777 AIRCRAFT MAINTENANCE MANUAL
RA SYSTEM -OPERATION -2
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RA SYSTEM -RF PROCESSING
General
The RA transceiver (xcvr) transmits a frequency modulated/ continuous wave (FM/CW) signal through the transmit antenna. This signal is a CW carrier that sweeps linearly from 4235 MHz to 4365 MHz 145 times a second.
Transmit and Receive
The transmit signal reflects from the ground and the receive antenna receives the reflected signal. The receiver mixes the transmit and receive signals to make a difference frequency. The difference frequency is proportional to the signal travel time to the ground and back. The altitude processor converts the difference frequency to radio altitude. The radio altitude goes out on two ARINC 429 data buses. These buses supply radio altitude and status information to these:
*
Left AFDC
*
AIMS cabinets
*
Weather radar.
*
TCAS computer
*
GPWC.
Input Program Pins
The system select program pin input sets the system modulation rate and system identification. The modulation rate for the left RA system is 145 Hz, the center at 150 Hz, the right at 155 Hz.
The aircraft installation delay (AID) program pin is grounded to 80 feet selection. This calibrates the system so that the radio altitude is zero feet at touchdown. It compensates for the:
*
Length of the antenna cables
*
Fuselage to ground distance
*
Flare angle.
When grounded, the continuous data program pin selects non-interrupted RA data output at all times.
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777 AIRCRAFT MAINTENANCE MANUAL
RA SYSTEM -RF PROCESSING
EFFECTIVITY BEJ ALL 34-33-00
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D633W101-BEJ Jan 05/2002
RA SYSTEM -MONITOR/TEST
BITE Module
The built-in test equipment (BITE) module monitors the circuits in the RA transceiver (xcvr) for faults. The flight fault memory stores the number of faults for each flight leg. Shop personnel use the automatic test equipment (ATE) connector to read the fault memory contents.
Test
A test switch on the front of the RA xcvr starts an RA self-test. LEDs show the results of the self-test.
Maintenance personnel start a ground test from the maintenance access terminal (MAT). When the ground test starts, a test command goes from the left AIMS cabinet to the RA xcvr. The test command starts the test. When the left AIMS cabinet fails, the right AIMS cabinet supplies the test command.
The xcvr ground test and self-tests do a check of these:
*
Transceiver internal status
*
ARINC 429 output transmitters status
*
Antennas and coaxial cables.
The results of the ground test go to the left and right AIMS cabinets.
System status information goes to the AIMS cabinets continuously. This is for fault monitoring of the RA system.
The primary flight computers (PFCs) supply a test inhibit signal to the xcvr through the AIMS. This prevents a RT test when the ground speed is more than 40 knots or the airplane is in the air.
The air/ground relay supplies a discrete signal to the RA RT altitude processor and control section of the RA xcvr. This discrete prevents an RA test in flight. This discrete is also an alternate way to define flight legs for the nonvolatile fault memory in the RA xcvr.
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777 AIRCRAFT MAINTENANCE MANUAL
RA SYSTEM -MONITOR/TEST
EFFECTIVITY BEJ ALL 34-33-00
Page 29
D633W101-BEJ Jan 05/2002
RA SYSTEM -SELF TEST
Operation
Push and hold the test switch on the front of the RA xcvr to start a self test. All the LED status indicators come on red for a few seconds. After this period, the LRU STATUS LED comes on green, the XMIT ANT FAIL LED and REC ANT FAIL LED go off for a few seconds. After that, all LEDs go off for a few more seconds. Finally, the applicable LEDs come on to show a normal (green) or fault (red) condition.
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