The X-Plane autopilot knows three levels of operation:
- Flight director off, no cues are generated
- Flight director on, steering cues are generated, no servos are actuated
- Flight director and autopilot on, servos act on steering cues
This is represented in the datarefs sim/cockpit2/autopilot/flight_director_mode and sim/cockpit2/autopilot/flight_director2_mode for the pilot-side and co-pilot side units.
Flight director switches
Depending on the specific aircraft, it might be necessary to turn off the steering cue display with a switch that does not switch off the autopilot computer. On some aircraft, it is possible to hide the flight director bars even if the autopilot is flying the aircraft. To facilitate that, the new datarefs sim/cockpit2/autopilot/flight_director_command_bars_pilot and sim/cockpit2/autopilot/flight_director_command_bars_copilot were added. These allow for turning off the command bars but let the autopilot keep flying the aircraft if desired.
Flight director master lights
When a flight director is generating cues, no matter if its bars are displayed or not, it can act as the master (generating the cues from its on-side altimeter, airspeed, attitude and heading instruments). If a flight director is on but not the master, it will copy the steering cues from the other flight director. During certain phases of flight, it is possible for both flight directors to act independently and both be considered master, see the article on auto lands for more information.
The datarefs sim/cockpit2/autopilot/flight_director_master_pilot and sim/cockpit2/autopilot/flight_director_master_copilot can be used to light up lights associated with the flight director switches.
Auto-throttle arming
Traditionally, X-Plane has always considered its auto-throttle to be “armed”. That is, when asked to provide reference N1 or EPR, or speed hold, it would always come on in response to a command like sim/autopilot/autothrottle_n1epr.
It is now possible to actually dis-arm the autothrottle so that commands for speed hold or thrust reference are ignored. This is reflected by the dataref sim/cockpit2/autopilot/autothrottle_enabled reading -1 when the system is disarmed, also the new dataref sim/cockpit2/autopilot/autothrottle_arm can be used which will read 0 when the system is dis-armed and 1 if the system is armed or active.
For compatibility with older aircraft that expect X-Plane’s autothrottle to be always available, X-plane defaults to loading aircraft with the system armed. Authors are advised to set the new datarefs as desired when their plugin is loaded if they want to take advantage of the new behaviour.