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D. Functional Description - G/S Antenna Switching (Fig. 3)
(1)
The left G/S antenna coaxial relay receives 28v dc from the ILS ANT SW L circuit breaker and is controlled by a discrete from the proximity switch electronic unit (PSEU). The PSEU applies a ground to the relay to select the left G/S capture antenna when the nose landing gear is up. The relay is deenergized to select the left G/S track antenna when the nose landing gear is down and locked. The selected antenna provides rf signals to the left ILS receiver.
(2)
The left G/S antenna coaxial relay also provides a discrete to the left FCC and to all three EIU's for determining which antenna is the source of G/S rf signals. When the relay is deenergized, the discrete is a ground.
(3)
The operation of the right G/S antenna coaxial relay is the same as the left, receiving 28v dc from the ILS ANT SW R circuit breaker, using the PSEU for control and sending output to the right ILS receiver, to the right FCC, and to all three EIU's.
(4)
The operation of the center G/S antenna coaxial relay is similar to the left, receiving 28v dc from the ILS ANT SW C circuit breaker, and sending output to the center ILS receiver, to the center FCC, and to all three EIU's. However, the discrete that controls the switching is provided by the modular avionic and warning electronic assembly (MAWEA). This discrete is a ground if the nose landing gear is not down and locked.
E. Functional Description - Receiver Tuning (Fig. 4)
(1)
The ILS receivers can be tuned automatically or manually. The ILS receivers are normally tuned automatically by the FMC if a flight plan has been previously entered (Ref 34-61-00). The following paragraphs describe the manual tuning procedure.
(2)
The CDU is used to input tuning data. The tuning procedure depends on whether or not the FMC's are operational.
(3)
If at least one FMC is operational, all three receivers can be tuned at once using the left or right CDU as follows:
(a)
Press the NAV RAD key on the CDU to show the NAV RADIO page.
(b)
Enter the desired frequency into the scratchpad area below the PRESELECT dashed line, using the alphanumeric keypad.
(c)
Press the line select key 4L next to the ILS frequency display. The new frequency value is moved from the scratchpad area of the CDU screen to the current ILS frequency line. The FMC accepts this frequency and tunes all three receivers.
(d)
If the line select key 6L next to the PRESELECT dashed line is pressed, the frequency is loaded into the PRESELECT area for later use.
(4)
If neither FMC is operational, only one receiver can be tuned at a time using its onside CDU as follows:
(a)
Press the NAV RAD key on the left CDU to show the ALTN NAV RADIO page.
(b)
Enter the desired frequency into the scratchpad area below the PRESELECT dashed line, using the alphanumeric keypad.
EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú
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MAINTENANCE MANUAL
(c)
Press the line select key 4L next to the ILS frequency display. The new frequency value is moved from the scratchpad area of the CDU screen to the current ILS frequency line. The selected frequency input is sent directly to the left receiver.
(d)
The right and center receivers are tuned in a similar manner using the right and center CDU's, respectively.
F. Functional Description - Self-Test Displays (Fig. 5)
(1)
During a system self-test the receiver outputs a sequence of four data modes for display on the flight instruments:
(a)
Invalid data (failure warning) for approximately 3 seconds
(b)
No computed data (NCD) for approximately 2 seconds
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