ILS Outputs
The multi-mode receivers send the deviation on two ARINC 429 outputs. Both bus outputs go to an output monitor that compares the output data to the transmit data to ensure they are the same. The buses then go to the:
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AIMS cabinets
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Onside autopilot flight director computer (AFDC)
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Ground proximity warning computer (GPWC) (from the left MMR only).
The audio or Morse code station identifier goes to the audio output circuits then to the audio management unit (AMU). When the Morse code station identifier is present, the detector sends it to the processor to transmit it on the data buses.
When the navigation displays (NDs) receive the digital Morse code station identifier, it replaces the ILS frequency with the station identifier.
ILS Discrete Inputs
The onside AFDC sends a tune inhibit discrete to inhibit tuning of the receivers when G/S and LOC are active with the autopilot engaged. The AFDCs also supply an ILS tune inhibit when these conditions are true:
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A flight director is on
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G/S and LOC are active
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The airplane is below 500 feet of radio altitude.
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ILS -FUNCTIONAL DESCRIPTION
The source destination identifier (SDI) program pins identify the receivers as being in the left, right, or center position.
The air/ground relay sends an air or ground discrete to the receiver to inhibit tests in the air and to supply flight leg data as a backup to the AIMS cabinet flight legs data.
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ILS -LOCALIZER ANTENNA SWITCHING
Operation
The localizer antenna switch change the inputs between the VOR antenna and the localizer antennas in the nose radome.
The antenna switches get 28v dc from their respective circuit breakers. The autopilot flight director computers (AFDCs) send a localizer antenna request discrete to control the position of the antenna switches.
The localizer antenna switches send an antenna acknowledge discrete to the AFDCs to give the commanded switch position.
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ILS -GLIDESLOPE ANTENNA SWITCHING
General
The glideslope (G/S) antenna switches change the receiver glideslope antenna inputs between the G/S antenna in the nose radome and the G/S track antenna on the aft nose gear doors.
The antenna switches get 28v dc from their respective circuit breakers. The autopilot flight director computers (AFDCs) supply a glideslope antenna request discrete to control the position of the antenna switches.
The G/S antenna switches send an antenna acknowledge discrete to the AFDCs to give the commanded switch position.
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ILS -TUNING
General
There are three ways to tune the multi-mode receivers (MMRs):
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Automatically by the AIMS cabinets
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Manually by the left, right, or center CDU
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By the CDUs as an alternate tuning source.
AIMS Auto Tune
The flight management computing function (FMCF) of the right AIMS cabinet automatically tunes the receivers when the ILS station is part of the active flight plan. The left AIMS FMCF tunes the receivers through the right AIMS cabinet, if the right FMCF fails.
Manual Tuning
You use the left or right CDUs to manually tune the receivers. When you enter an ILS frequency or identifier into the CDU, the CDU frequency becomes the active frequency. It overrides the AIMS FMCF auto tune input. The CDU delete function deletes the CDU frequency selection and the AIMS FMCF auto tune becomes active. The center CDU tunes the receivers if the left or right CDUs fail.
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