FMS CONTROL-DISPLAY UNIT:

The CDU provides a means for the crew to communicate with the FMC.  It contains pushbutton key controllers and a display screen.  The keys are of two types: - 

1. Alphanumeric keys, which can be used to enter departure and destination points and also Waypoint if not already stored on tape; they will also be used if the flight plan needs to be changed during the flight. 

2. Dedicated keys, which are used for specific functions usually connected with display.  For example, by using the appropriate key the pilot can call up flight plan, Waypoint data, flight progress, present position, etc. 

When, for example, a departure point is entered using alphanumeric keys, the information is often held in a temporary register and displayed to the pilot; this is known as a scratchpad display.  Once the pilot has checked the information is correct, he can enter the data into the computer store by pressing the appropriate dedicated key typically labelled "Load" or "Enter". 

Figure shows an FMS Control/Display Unit (CDU).

During a normal flight, the FMS sends navigation data to the EFIS, which can then display a route map on the EHSI.  If the flight plan is altered by the flight crew en-route, then the EHSI map will change automatically.