ISIS may be the most violent and sadistic Islamic terrorists on the planet right now, but basic smarts is not in the cards for them. The range of a heavy machine gun, especially against a jet flying at speed is just no match; a waste of ammunition. It also provides a bright, shiny signal to the pilot as to where you are placed with your weapons. See what happens in the video on the next page:
It is a Russian made plane , a Mig-21 but it is flown and operated by the Syrian Air Force. The Plane is at least 50 years old but very fast , maneuverable and tough.