An .xml gauge is needed in order to have a working afterburner. Look for freeware gauges made by Rob Barendregt and follow the instructions. The .xml file is placed within your main Gauges folder, and a reference line is then added to the aircraft's panel.cfg so the gauge reads it (this would be covered in the installation readme). This adds the effect to any aircraft you wish. The trick to getting it to function at a certain percentage (e.g., without having to click a switch) is to assign the STROBES function to the afterburner. This is accomplished by modifying the [Lights] section in your aircraft.cfg. The result being that afterburners will engage when strobes are switched on, so the type of light previously assigned to the strobe (2=strobe) should be changed to a different light type, e.g., 1 or 3. An example of lighting types in the aircraft.cfg is here:

[LIGHTS]       
//Types: 1=beacon, 2=strobe, 3=navigation, 4=cockpit, 5=landing, 6=taxi, 7=recognition, 8=wing, 9=logo, 10=cabin

Hope this is of some help.

It would be really great if more designers used the "shift+F4" afterburner combination. It give a kick when it is activated and lets you run to max throttle befor hitting the burners.