2. System Description
__________________
 A. Keyboard (Ref. Fig. 001) The MCDU is linked to both FMGCs, CFDS, ATSU, AIDS and optional or spare systems (e.g. SATCOM). When the MCDU MENU multi-purpose key is pushed, the menu page is displayed. The operator pushes the line select key to select a system : communication is broken with the active system and established with the associated one.
 B. General Display Rules
 (1) Non-valid and missing data For all display fields only valid data will be displayed. If the data are not valid for display or not available for display, the display field will contain dashes. When a new page is called up or a data is entered, it may require that the computer performs calculations before all the data to be displayed are available. In these cases, the screen is firstly blanked and then the portion of the display that is available and can be displayed is presented to the pilot immediately. However, the data fields that are not available would be left as dashes and filled in as the data become available.

 MCDU Keyboard
 Figure 001

 (2)
 Small and large fonts On the MCDU there are two sizes of fonts. Generally, all data on the data line (line adjacent to line select key) are displayed in large font and data on the label line are displayed in small font. However, the following are exceptions :

 -Data with default values or computer calculated values which could be changed by the pilot are displayed in small font when the computer supplied data are being used and in large font when the data are pilot entered.
 -If two data entries are dependent upon each other, the independent data (those defaulted or latest input by the pilot) are displayed in large font and the dependent data are in small font.

 (3)
 Special symbols There are special symbols on MCDU displays that indicate certain functions to the pilot. Certain symbols may give a clue to the pilot as to what is available or if there is more information than is already displayed. The following symbols are used in the displays:

 < or > If this symbol appears next to a line select key (LSK) it indicates that pushing this line select key will only cause the MCDU to display a different page (it is the call-up of a page), the name of which is displayed besides the symbol.
 *  This symbol appears next to a line select key it indicates that pushing this LSK will change the parameters which affect the active situation of the aircraft, or causes the activation of a function (it is the call-up of a function). The page displayed may also change as a result.
 <-- --> One of these symbols appearing beside a line select key indicates that the pilot can push the key to select a data, without activating a function.
 .- -.
 '--' This indicates that the user may enter a data here (can be optional). Occasionally this data may be computed and entered by the computer as a result of another pilot action.
 .--.
 '--'  This indicates that the data are necessary for the user to be input in order for the computer to perform the specific function.
 ------ This indicates that the data will be displayed by the computer when it has enough information to do so. The user does not normally enter this information. However, under certain circumstances he may be able to alter the data.
 ARROWS
 UP and
 DOWN  These arrows (up or down) indicate that vertical slewing is available. They will appear in columns 23 and 24 in the scratchpad.
 ---->  This arrow, in column 23 of the title line indicates that
 horizontal slewing is available from this page. Pushing the
 NEXT PAGE key will horizontally slew the page.

 TRIANGULAR
 SYMBOL  This symbol beside a fixed waypoint on the flight plan page, indicates that the A/C must overfly the fixed waypoint. This symbol is mostly used by the FMGC.

 (4) Color code

 -------------------------------------------------------------------------------
| DATA | MCDU COLOR |
 |----------------------------------------------------|------------------------|
 | Titles, comments, symbols of slewing, dashes, minor| White |
 | messages, call-up of page, scratchpad data entry, | | 
R  | F-PLNs legs, secondary waypoints, active waypoints,| | 
R 
R  | to waypoints, destination waypoints, active HDG SEL| | 
R  | legs, active TRK SEL legs, NAV armed legs, | | 
R  | missed approach legs, alternate legs, offset legs | |
 |----------------------------------------------------|------------------------|
　
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