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An autopilot (AP) is a mechanical, electrical, or hydraulic system used to guide a vehicle without assistance from a human being. Most people understand an autopilot to refer specifically to aircraft, but self-steering gear for ships, boats, space craft and missiles is often also called by this term.
The autopilot of an aircraft is sometimes referred to as "George".
Autopilot in FlightGear
We have a generic one and custom ones written for a certain aircraft, which are supposed to simulate particular autopilot types/models for a specific aircraft.
The generic one can be used for aircraft which don't have a custom autopilot implementation and seems to work reasonably well on most aircraft. You can enable it with the "Autopilot Settings" which you can open using the F11-key or in the menu bar using the "Autopilot" item.
In real life, aircraft have specifically-programmed/customized autopilot system to cater for individual aircraft characteristics. To a certain extent, Flightgear has the possibility to simulate this as well.
So it is possible to write a custom autopilot.xml to override the generic autopilot implementation. This custom autopilot configuration file can be driven by either the standard autopilot dialog, custom cockpit panel hotspots or a combination of both.
In general, it can be considered to be more complex to provide proper cockpit panel implementations than wiring up the autopilot properties to the standard autopilot GUI dialog, so if an aircraft's autopilot doesn't seem to work properly using cockpit panel hotspots, you may want to try using the standard GUI dialog instead.
Also, please note that not all aircraft/autopilot combinations provide full support for all features offered by the standard autopilot dialog. This may be due to an aircraft's completion status, but also due to technical limitations in certain aircraft/autopilot, so that only certain modes are provided. This may also apply to aircraft panels, that are seemingly offering functionality that may not yet be implemented.
In addition, JSBSim also features support for a standalone autopilot implementation, that works without any dependency to FlightGear/Nasal.
The AP Settings dialog can be found at Autopilot > Autopilot Settings.
- Wings Level: Keeps the aircraft's wings level. Some aircraft may re-use this mode to hold a set bank angle.
- Heading Bug: Tracks the heading bug (magnetic heading).
- True Heading: Tracks true heading. This mode changes to GPS/FMS Heading if the Route Manager is activated.
- NAV1 CDI Course: Tracks a VOR or ILS localizer.
- Speed with Throttle: Hold the set airspeed by adjusting throttle
- Speed with Pitch: Hold the set airspeed by adjusting pitch angle. The throttle will be unlocked and can be moved to adjust climb/descent rate.
- Mach mode tracking is possible, but the dialog does not contain a method to enable it at this time.
- The mode selected here is ignored if "Speed with Pitch" is selected as it is in conflict.
- Vertical Speed: Hold the set vertical speed in feet per minute.
- Pitch Hold: Hold a set pitch angle.
- FPA Hold: Hold a set flight path angle.
- Altitude Hold: Acquire and hold the set altitude.
- AGL Hold: Acquire and hold the set altitude above ground level (sometimes known as "radio altitude").
- NAV1 Glideslope: Tracks an ILS glideslope.
Custom Autopilot Dialogs
It is possible to provide enhanced or completely new implementations of the standard autopilot dialog to cater for autopilot/aircraft-specific features (for examples, see the b1900d or Citation Bravo in Git/HEAD).
Some aircraft in FlightGear however only provide one means of interacting with the autopilot (i.e. autopilot dialog or panel hotspots). For example, the Seneca II and the Piper Comanche autopilots can be only used from the virtual cockpit using custom panel hotspots. This is in fact increasingly the case, as new emulations of autopilot systems are implemented using the built-in scripting language Nasal which provides for much more flexible system design and functionality, compared to the relatively static way of describing autopilot systems and their relevant and modes using only the XML-configurable autopilot system. In fact, to properly emulate more complex autoflight systems that provide support for more abstract flight modes, using a combination of XML-configurable PID controllers and Nasal is likely to be the most promising approach.
See Route Manager for the main article about this subject.
The Route Manager in FlightGear is something like a very simple FMC- Flight Management Control/Computer. In FGFS you can input a list of waypoints like NAVAIDS, Fixes and Airports, which the aircraft flies along. You can also add the altitude which the aircraft should have at a certain waypoint. So it is possible to let the aircraft fly along a certain route.
At the moment, it is not possible to add velocities or other constraints.