To make it easier to create addons there is since FlightGear 2017.3 a new way to create addons. In essence FlightGear will load an overlay XML into the property tree and start a well known Nasal function.
We now have a simple way to add addons to FlightGear without the need to mess around with
Installing and using an addon
Download and copy the addon to a directory on your computer.
If you use the launcher, select the Add-ons page from the icon bar on the left, then find the section Add-on Module folders and click the Add(+) button. Select the folder where you put the addon. Once the addon is shown in the list, make sure its check box is selected.
Use the command line switch
Creating an addon
A leading slash (
/) in this section indicates the base directory of the directory structure of the addon.
An addon may be installed in a directory anywhere on your hard disk and need at least two files:
/addon-config.xml- A standard PropertyList XML file to be used to populate or modify the property tree. (Same as to be used in
/addon-main.nas- The Nasal hook for the logic. This file needs a function called
main()which will be called from the global addon initializer (
Additional common files
/addon-metadata.xml- A PropertyList XML file with metadata about the addon it.
/addon-menubar-items.xml- A PropertyList XML file describing menus to be added to the FlightGear menu bar.
/gui/dialogs/<my-foobar-dialog>.xml- PropertyList XML files to create custom dialogs.
Good to know
The new addon mechanism lets you add a
addon-config.xml to override the settings in
defaults.xml and other files.
That will allow an addon to
- Override key bindings (as in the spoken ATC addon)
- Add or override autopilots and property rules
- Introduce XML state machines
Unless your really want to add/change/remove those at runtime, this should cater for most use cases.
On initialization fgfs takes care of:
- Creating a property under
/path/to/some/addon/addon-config.xmlinto the property tree (same as
/path/to/some/addonto the list of allowed directories (same as
- Creating a property under
There is a C++ API on FlightGear's side that handle some on the addon related tasks manly through the
The Nasal add-on API lives in the 'addons' namespace and can for example do queries to
AddonManager() and read data from
addons.Addon objects. It can for example compare addon versions if there is dependencies.
ATC chatter was removed in 2015, see fgdata commit: 81607f734e13add9be02816ddaec305d05bc4e47
And the devel list messages referenced in the commit log. the other relevant commit is this: b60736ba7add2a7cd39af3d8a974d5be3ea46e8b It would not be very difficult to restore this functionality, or even generalize/improve it significantly (which was the scope of the original discussion on the devel list)
The restored functionality could be distributed as a tarball that is extracted into $FG_ROOT - alternatively, into $FG_HOME, because Nasal files there are treated as overlays, which basically means that you can install user-specific extensions there without having to tamper with $FG_ROOT, it would only take very minor changes to turn the chatter feature into a corresponding "addon" - not unlike fgcamera ...
We should absolutely stop telling anyone to edit preferences.xml in FG_ROOT; any documentation or advice which says to should be changes ASAP. 
List of Addons
You can find the official repository at flightgear/fgaddon/trunk/Addons
- ATC Chatter (ported by Torsten)
- Cargo Towing Addon 
- cockpit-view (work in progress by wkitty42)
- Earthview#Customization - High resolution customization
- FaceTrackNoIR (ported by HHS) - An addon to interface this head tracker with FlightGear
- FGCamera - (manual) - Wiki Page
- Ground Services (ported by ThomasS)
- Landing Rate addon 
- mdanilov hax!: landing evaluation and aircraft development tools, TerraSync toggler
- Model Cockpit View
- Oscilloscope addon - Allows displaying a property of Nasal function over time
- PAR instrument - Precision Approach Radar and Ground Controlled Approach
- Red Griffin ATC - Speaking Air Traffic Controller
- Spoken ATC (ported by Torsten)
- Spoken GCA - An offline ground controlled approach (GCA) addon
- YASim Development Tools (by jsb)
- VFRFlight integration - (manual)
- KML Exporter (Google Earth) - (manual)
- VFR Flying Helper - (manual)
- LinuxTrack Head Tracker integration - (manual)
- Additional Heli HUD's - (manual) - encapsulation of HeliHUD package as an addon
- Tiny HUD for mouse flying in FlightGear - Visual feedback for mouse flying, to make up for mouse's lack of self-centering
- LittleNavMap integration - (manual)
Addons which are in the development stage/unfinished but can be used as a quick view of addon functionality
- FlightGear Missions addon - Add-on for missions/adventures code
Hooking into features using legacy OpenGL code
In 09/2018, James suggested that legacy features using raw OpenGL calls (e.g. HUD/2D panels) could be easily replaced via scripted Canvas/Nasal solutions at the mere cost of providing a mechanism to hook into the legacy code implementing these features (namely, the HUD/instrumentation subsystems)  
Catalog & Package Manager support
if this works with more complex, pre-existing addons such as Bombable, where the file layout is a replica of the old FGData layout, these types of mature addons might be better as SourceForge FlightGear sub-projects rather than being copied into FGAddon. Maybe the config file and nasal script could be used to automate the installation process ? The Bombable addon is one of the most popular addons out there, and a large number of aircraft in FGAddon have bombable support, so it is worth not forgetting about. Especially if the addon system one day becomes automated through a catalog.xml type system
- FlightGear configuration via XML
- FlightGear configuration via XML#preferences.xml
- Property tree
- Properties persistent between sessions
- PropertyList XML File
- $FG_ROOT/Docs/README.gui - Details on how to add menus and custom dialogs.
- flightgear/fgaddon/trunk/Addons/Skeleton - Skeleton addon to be used as a template.