(5) Slewing functions (Ref. Fig. 002)
 (a) Horizontal slewing Certain functions need multiple pages for the display. These pages are arranged in a "book" form (page A, B, C...). The A page is displayed upon pushing the mode key. The other pages (B, C, etc.) are called up by using the horizontal slewing function (NEXT PAGE function key) When page B is displayed there is no special key to display page A (a prompt PREV PAGE > can be used). The displays that use horizontal slewing are closed loop displays. This means that when the last page is displayed and the NEXT PAGE function key is pushed, the display changes to the A page. When horizontal slewing is available, an arrow will be displayed in column 23 of the title line.

 Horizontal and Vertical Slewing
 Figure 002

 When the book contains more than 2 pages instead of the NEXT PAGE function key, it is possible to use the line select key adjacent to NEXT PAGE > or PREV PAGE > indications to select the other pages of the book.
 (b)
 Vertical slewing Certain pages in the computer contain more data than will fit on a single page (a page longer than the six pairs of lines available on the display). In this case data are arranged on a "drum". In these instances vertical slewing can be used to scroll the display up or down. Each time a vertical slew key is pushed, this scrolls the MCDU display by one line pair, or by one page. A line pair is a line adjacent to a line select key and its label line. Holding the vertical slew key pushed scrolls the MCDU display at a rate of 2 line pairs per second. The arrows will be displayed in column 23 and/or 24 of the scratchpad when vertical slewing is available. Vertical and horizontal slewings may be available together for certain functions. In this case, if a display has been slewed up or down, then the horizontal slewing is performed for the two pages of the "book" and calling next page of the book will enable access to the next page at the same slew position as previous page.

 (c)
 Value adjustment Another use for vertical slew keys is incrementing data by a set amount in a particular data field. When this is available, arrows will be displayed adjacent to the data label that would be affected if the vertical slew keys were pushed. When a vertical slew key is held pushed, the slewing will take place at a rate of 2 increments or decrements per second.

 (6) General display of a page
 -Only the selectable functions or page call-ups are displayed. This is to avoid cluttering of the screen with not usable prompts.
 -On a page, some prompts may be displayed following a logic dependent on external parameters (flight phase, ground/airborne, ...). In such cases, the architecture of the page is not modified by the deletion or addition of a prompt (prompts present at the same place) e.g. lateral revision pages at origin and destination airports. (Ref. Fig. 003)
 -Prompts that are used on several pages have to be displayed at the same place on the different pages.

 Example of Display
 Figure 003

 ex : . < RETURN in 6L

R  . PRINT * in 5R . INSERT * in 6R.
 (7) Acknowledgement of user action Each time the MCDU user pushes a line select key or a mode key, he must receive an acknowledgement of his action. The screen has to be blanked as soon as the subsystem has received this action, before it displays a new page or a new message. The new page also displays an acknowledgement of the pilot action (insertion of a value in a line, message, etc.). Examples :
 -
The user wants to enter a value into a field, he may have the following indications: OUT OF RANGE: if the value is not within the range as expected by the system. FORMAT ERROR: if the value has not the correct format.

 -
The NOT ALLOWED indications may be displayed if an incorrect line

 select key is pushed. In case of the call-up of a function, the acknowledgement is made by deletion of the star "*" next to the function name when the call-up is taken into account by the system. If the system is not available a message is displayed (PRINTER NOT READY).
 C. Page Hierarchy (Ref. Fig. 004, 005, 006, 007, 008, 009) Each FM function of the FMGC may be called by pushing a mode key. A page is then displayed where the function may be performed. From this main page, it is possible to call other pages where the sub-functions may be performed. These sub-pages are either displayed by pushing the applicable line select key or automatically following a data entry. Sometimes the only way to exit a page is to push another mode key. On page hierarchy figure, N/P means NEXT PAGE and arrows mean that page slewing is possible
　
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