U.S. Air Force tanker crews are now flying combat missions, and some KC-135 crews have now flown over a hundred such missions. The U.S. Air Force defines a combat mission as one in which a warplane carries out a mission in the face of real, or potential, enemy opposition. A combat support mission is one in which an aircraft, usually a tanker, transport or electronics warfare plane, does a job without getting in harms way, or at least very close. However, because of the lack of aerial opposition, air refueling aircraft (KC-135 and KC-10) are now doing their refueling right in the combat zone where the warplanes operate. This has led the air force to designate these refueling missions as combat missions. The aerial refueling aircraft usually fly missions keeping them in the air for seven hours or more. The refueling aircraft, and the bombers they refuel, fly high enough to avoid any ground fire. Except for the low flying A-10s, and AC-130s, and an occasional F-16, no tankers or bombers have been hit while operating over Afghanistan or Iraq. Smart bombs have made it possible for the bombers to stay above it all. However, its still a combat zone, especially down on the ground, and missions therein are considered combat missions.