What do the green lights next to the mode selection buttons on the Flight Control Panel indicate?

The green lights next to the mode selection buttons on the Flight Control Panel indicate that both Flight Control Computers (FCCs) have received the request for the selected mode, and that the conditions are met for that mode to function correctly. This signaling shows that the system is ready and that the chosen mode can be activated with appropriate parameters.

In a well-functioning flight control system, the indication of green lights serves as reassurance to the flight crew that their input to select a specific mode is valid and recognized by the redundant systems in place, which are critical for maintaining safe flight operations. Therefore, the presence of these green lights is crucial for confirming that the system is primed and operational according to the crew's selection.