Flight Management System — Page 283, Lesson 333

Let’s pick up right where the flight crew’s hands meet the machine: the Control Display Unit, or CDU, of the Flight Management System. This is the keyboard and screen through which you talk to the FMS, and every key on it has a precise, professional function. I’ll walk you through each one, starting with the function keys that pull up different pages of data. First, the DIR/INTC key — that stands for direct / intercept. This gives you the data capability to proceed direct to any point you desire, or to intercept any leg that is presently in the flight plan. So if ATC clears you straight to a waypoint, or you need to join a route leg, this is the key that opens that page. Next, the FIX key — fix information. This displays range and bearing data from your present position to an entered fix. It facilitates the creation of fixes for use in flight planning. So you can define a reference point and see how far and in what direction it lies from where you are now. Then the PROG key — flight progress. This displays current flight status information such as ETA, fuel remaining at waypoint, navigation radio tuning status, wind, and path errors. This is your in-flight status page — where you check how you’re tracking against the plan. Now, the Line Select Keys, abbreviated LSK. Entry of data from the scratch pad into the selected line and field is accomplished by using the LSKs. There are twelve LSKs on the CDU panel — six each to the left and right of the CRT display. Data entries are permitted only on lines adjacent to the LSKs. Data can also be duplicated into the blank scratch pad by pressing the LSK adjacent to the desired data line. So these are the buttons that move information between the scratch pad and the display lines. The EXEC key — execute. This is used to incorporate data displayed on the CDU as part of the active flight plan. The EXEC key is operable when its annunciator bar is illuminated. The key is used for activating the flight plan, changing the active flight plan, changing the active guidance mode, or inserting data which will affect the active flight plan, guidance mode, or data base. Illumination of the white annunciator bar indicates that a valid set of data is on display and may be made active for guidance of the aircraft. So when that white bar lights up, you have a valid entry waiting — pressing EXEC makes it live. The Change Sign Key, marked + / -. This changes data in the scratch pad from positive to negative and back again. It may also be used to insert a hyphen for specialized data entries. Then the Page Select Keys — NEXT PAGE and PREV PAGE. When multiple-page displays are shown, pressing the NEXT PAGE key advances the display to the next higher page number. Pressing PREV PAGE backs up the display to the next lower page number. Page access wraps around — so from the last page, NEXT PAGE takes you back to the first. The CLR key — Clear Key. The scratch pad’s contents can be edited or cleared by pressing the CLR key. When an entry is present in the scratch pad, a brief depression of the CLR key will clear the last character in the entry. If the CLR key is held down for more than one second, the entire entry in the scratch pad will be cleared. The CLR key is also used to clear advisory and alerting messages from the scratch pad. Finally, the DEL key — Delete Key. This is used to remove data from a display — and thus from a flight plan — after it has been line selected and accepted into a data field. If the scratch pad is empty, depression of the DEL key writes “DELETE” into the scratch pad. The delete process is then completed by line-selecting — pressing the LSK — the data item to be removed. If the deletion is a valid one, the data field reverts to its default value — which could be box prompts, dashes, or a system-generated value. The system prevents invalid use of the DEL key. So the full picture: the function keys call up pages, the LSKs move data between scratch pad and lines, EXEC makes it active, CLR edits the scratch pad, DEL removes accepted data, and the sign and page keys handle sign changes and page navigation. Every key has a specific, disciplined role — and that discipline is exactly what you’ll rely on in the cockpit.