*
Passenger address/cabin interphone (PA/CI) controller allows menu control to communicate to the cabin

*
Airplane maintenance menu access on the multi-functional displays (MFD)

*
Text entry for the flight deck communication function (FDCF) formats.

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FMCS -CDU FUNCTIONAL DESCRIPTION

Alternate Control Panel Functions

The left and right CDUs give an alternate control panel function for the onside electronic flight instrument system (EFIS) control panel. The center CDU acts as a left or right CDU when either CDU fails. The CDUs have controls for the display select panel (DSP). The EFIS control panel and the DSP lose their functionality when a CDU acts as a backup for that position.
Alternate LNAV Function
Each CDU can be an alternate navigation source. The active FMCF supplies map and flight plan data to the CDUs. The left and right CDUs can display the active FMCF map and flight plan data in two ways.
The left and right CDUs can retransmit the active map data and send it to the onside navigation display (ND) through the PDF when the NAV source switch is in the CDU position. The CDUs also show FMCF pages.
The CDUs can reformat and transmit the map and flight plan data to the onside ND via the PDF when the NAV source switch is in the normal position. This occurs if the FMCFs fail. If this happens, no FMCF pages are available.
EFFECTIVITY BEJ ALL
All CDUs can show the ALTN LEGS, ALTN NAV PROGRESS, and ALTN NAV RADIO pages for display when the active FMCF or both FMCFs fail. This lets the crew review and change the lateral flight plan through CDU line select keys and keyboard entries. The center CDU can replace the left or right CDU for map display on the ND when the left or right CDU fails.
The CDUs supply great circle distance calculations and other data the same way as the FMCF. The CDUs use this data from the air data inertial reference unit (ADIRU) to supply navigation outputs:
*
Altitude

*
Ground speed

*
Latitude and longitude

*
Mag heading

*
Roll angle

*
True heading

*
North and east velocities.

The CDUs do not calculate radio position or receive global positioning system (GPS) inputs to help in navigation calculations. The CDUs do not calculate performance data.
Alternate LNAV Guidance
The CDU receives flight plan data from the active FMCF partition and uses actual airplane position from the ADIRU to calculate lateral steering commands. The CDUs send lateral steering commands to the AFDS when:
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FMCS -CDU FUNCTIONAL DESCRIPTION

*
The alternate navigation mode is active

*
A flight plan exists

*
LNAV request from the MCP.

The AFDS selects the left, center, or right CDU (priority with all CDUs valid) lateral guidance command for roll control. The CDU that sends commands to the AFDS also sends map data to the captain ND.
The CDUs do not give guidance for holding patterns and lateral path offsets. They also do not give runways or airports for display.
Alternate LNAV Engage and Disengage Logic
The CDU LNAV engage logic starts with these conditions:
*
LNAV mode request from the AFDS

*
The current FMCF flight plan or a manual flight plan entry

*
The airplane position satisfies the LNAV capture criteria (capture criteria is a crosstrack distance of 2.5 nautical miles or less, or a crosstrack distance that the CDU LNAV function can initiate a path capture when on an intercept heading)

*
Active FMCF not valid

*
ADIRU roll data is valid

*
ADIRU position data is valid.

The LNAV disengage logic is set when the AFDS inputs are invalid or the AFDS disengages.
Alternate (Back-up) Navigation Radio Tuning
The left and right CDUs can do alternate navigation radio tuning. The center CDU is not a backup for the left or right CDU for alternate navigation radio tuning.
The CDUs (left and right) send a tuning source discrete and frequency to the onside radios when the active FMCF fails. The left CDU sends a tuning source discrete and frequency to the left and center navigation radios. The right CDU sends a tuning source discrete to the right radios.
The navigation radios transfer tuning control to the CDUs when they receive the tuning source discrete. The CDUs transmit the last tuning data to their onside navigation radios from the active FMCF.
The ALTN NAV RADIO page permits manual entries to tune the onside navigation radios. The frequencies go to the navigation radio on an ARINC 429 bus.
　
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