Flight Management System — Page 283, Lesson 333

Let’s get straight into the Flight Management System’s control display unit—the CDU. This is your primary interface with the FMS, and every key on it has a specific, non-negotiable job. I’ll walk you through each one, starting with the function keys. First, DIR/INTC—that stands for direct/intercept. This key 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 segment, this is the key that sets up that command. Next, FIX—fix information. This key 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 geographical point, and the system will show you how far and in what direction it is from where you are right now. Then we have PROG—flight progress. This displays current flight status information: your ETA—estimated time of arrival—fuel remaining at a waypoint, navigation radio tuning status, wind, and path errors. It’s your in-flight status page, giving you a snapshot of how the flight is progressing against the plan. Now, the Line Select Keys, abbreviated LSK. These are the workhorses for data entry. 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. You can also duplicate data into the blank scratch pad by pressing the LSK adjacent to the desired data line. So, to move a value from the scratch pad into a field, you press the LSK next to that field. The EXEC key—execute—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. That white annunciator bar lighting up indicates that a valid set of data is on display and may be made active for guidance of the aircraft. You use this key 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. So, it’s the final confirmation that the changes you’ve made are now live for the aircraft to follow. The Change Sign Key, marked + / -, 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. Now the Page Select Keys. 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. And importantly, page access wraps around—so from the last page, pressing NEXT takes you back to the first. The CLR key—Clear Key—is for editing the scratch pad. 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 you hold the CLR key 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—using the LSK—the data item to be removed. If the deletion is a valid one, the data field reverts to its default value: box prompts, dashes, or a system-generated value. The system prevents invalid use of the DEL key. So, to tie it together: you use the function keys to call up pages, the LSKs to move data in and out of fields, the scratch pad as your temporary buffer, and EXEC to make it all active. The DEL and CLR keys manage what’s in the scratch pad and what’s in the flight plan. Every key has a precise role, and understanding each one is foundational to operating the FMS correctly.