To make a working afterburner (e.g., comes on when a certain percentage of throttle is applied), as mentioned above you need a special gauge file. I know that Rob Barendregt (do a search on his files) has made some excellent such gauges, among other things, and believe there may be a freeware one for general use somewhere. If you just want the A/B effects to display on/off, you can simply assign a lighting element in the aircraft.cfg. Here's an example:
//Types: 1=beacon,
2=strobe,
3=navigation, 4=cockpit, 5=landing
light.0 = 3, -7.75, -22.90, 1.60, fx_navred ,
light.1 = 3, -7.75, 22.90, 1.60, fx_navgre ,
light.2 = 1, -8.85, -22.55, 1.60, fx_strobe ,
light.3 = 1, -8.85, 22.55, 1.60, fx_strobe ,
light.4 = 1, 0.25, 0.00, 2.30, fx_beaconb ,
light.5 = 1, 7.60, 0.00, 6.49, fx_beaconb ,
light.6 = 1, -16.55, -9.85, 9.93, fx_beacon ,
light.7 = 3, -16.55, 9.70, 9.93, fx_navwhi ,
light.8 = 3, 25.00, 0.00, 6.50, fx_vclight ,
light.10= 2, -17.3, -8.48, 0.10, AFTERBURNER_EFFECT_NAME_HERE
light.11= 2, -17.3, 8.48, 0.10, AFTERBURNER_EFFECT_NAME_HERE
Note: To engage the effect using the latter method, use the "o" key. This does affect all the strobes however, you need to turn all lights on first then deselect whichever ones aren't required in order to have the strobes work independently of the afterburner (hope this makes sense).