Thanks to FG dev Thorsten we now have great looking afterburner plumes in FG. I've added this to the newest DavePack, version 5.2. I also uploaded to the hangar one model that has been modified to use it, the F-4C version 1.0. Here's an earlier test with the F-105 (not uploaded yet).
The DavePack version is a one-size-fits-all plume, using a new 12 sided bounding cylinder to keep the plume within the nozzle. The plume is added to the model in the usual way, in the model's XML wrapper file. You just specify the XYZ locations and any tilt (i.e. pitch-deg).
I'm running this in FG 3.5. I don't know if will run in earlier versions. I hope it doesn't crash earlier versions. If it doesn't work for you just comment out the new afterburners in the XML file and uncomment the old ones.
UPDATE: Added two other plumes, a smaller one for smaller engines, and a new full sized plume with a different shape to make it easier to fit it into the nozzle hole. DavePack updated to version 5.3.
UPDATE, PART DEUX: Since this effect is so new I've added code that will use the old-style burners on FlightGear 3.4.x or earlier. You'll need 3.5.x or newer to see the new burners. DavePack updated to version 5.4. So far only the F-4N has the conditional burner code.
UPDATE, PART TROIS: The F-4C, F-4D, and F-4E now have the conditional burners.