Currently, there is some very old HLA support in SimGear. This is very out of date and should be ignored.
Stuart has HLA
Support using the latest OpenRTI and SimKit working on a local build, but is waiting for Mathias to officially publish his SimKit before he pushes his changes. So all of the following is local-only, but included here for information.
The Flightgear core currently supports HLA as follows.
Separately, Erik has been doing some preparatory work suitable for supporting HLA in JSBSim by adopting so called PropertyObjects to hopefully get rid of tied properties (http://sourceforge.net/p/jsbsim/mailman/message/34720784/)
== Related ==