Autopilot NAV modes: source selector and engagement
X-Plane’s autopilot always has and always had one channel for a tracker/coupler function, that works on both low-sensitivity sources like VORs and high-sensitivity like LOCalizers. With the HSI source selector, either a VOR/LOC receiver or a GPS/FMS can be selected to feed lateral deviation (what is indicated by a CDI) to the autopilot, whilst the desired track is fed to the autopilot by a number of different sources that can be configured. Thus, there’s only one NAV mode to take care of tracking the GPS flight plan, or intercepting and tracking a VOR radial or localizer.
Depending on the source, the nav mode can be armed before it actually engages, which is done for example in dual-mode intercepting. Dual-mode intercepting allows the autopilot to follow a heading, until a selected VOR radial or localizer course is intercepted, at which point the tracker/coupler takes over from the heading mode. In order to engage, the NAV mode needs either a both falling and less-than-full deflection on an angular offset (VOR, LOC), or an absolute XTK (GPS/FMS). Thus, NAV mode is never engaged directly, but armed (using a command, dataref or 2d panel button).
The new separate Autopilot NAV mode: GPSS
X-Plane 11.10 now supports a separate autopilot nav mode that works only with GPS steering cues: GPSS. GPSS tracks the GPS or FMS flightplan, with all the benefits you are used to from GPSS: turn anticipation and automatic tracking of arc segments, holdings and procedure turns. However, it works on a separate channel that is hardwired to the GPS/FMS in the plane including for front course (DTK) information.
This mode cannot be armed. If a valid GPS steering cue is available (a waypoint is active) it engages immediately. Otherwise, it just refuses to engage.
What is important is that the GPSS mode is treated like a heading mode when it comes to dual-mode intercepts. Thus, it allows you to arm localizer interception, while tracking a GPS flightplan that for example takes you through an RNAV transition onto the ILS approach. In this case, GPSS is active, the HSI selector can be turned to a VOR/LOC receiver, and then NAV or APPR mode can be armed. The autopilot will follow the GPS flightplan until it intercepts the localizer, at which point tracking will switch from GPSS to NAV (VOR or LOC).
Various X-Plane add-on aircraft have achieved similar functionality through various kinds of plugin trickery in the past. This new mode is now available without plugins.
Datarefs and commands
The new command sim/autopilot/gpss toggles the GPSS mode on or off. Note that the mode will refuse to engage when no waypoint is active in the GPS/FMS. The sim/cockpit/autopilot/autopilot_state flag field now has the flag 2^19=524288 to indicate whether the mode is active, similarly, writing that value toggles the mode. sim/cockpit2/autopilot/heading_mode will read 13 if GPSS is active. Finally, sim/cockpit2/autopilot/gpss_status will return 0 for off and 2 for engaged for the new mode.