U.S. Air Force warplanes typically stay above 10,000 feet when operating in a combat zone. This is often thought of as useful in keeping them out of range of enemy ground fire. But its also necessary to avoid friendly fire. Not that friendly troops are firing machineguns at their own aircraft. No, it's the friendly artillery that is a major danger. Mortar and 155mm howitzer shells can go as high as 10,000 feet. In order to avoid 120mm tank shells, and ricochets, as well as 25mm cannon fire, aircraft must stay above 5,000 feet. For .50 caliber machine-guns, 4,000 feet, and for other small arms, 3,000 feet altitude are required. Since American troops tend to put out most of the fire power on a modern battlefield, friendly fire is often more of a risk than enemy action. Helicopter gunships have to work closely with friendly units below, and fly through no fire zones, where the chance of getting hit by stray bullets and shells is minimized. Even so, AH-64 gunships often come back with American bullets in them. The air force A-10 aircraft are treated the same way as the AH-64s, and both types of aircraft have to avoid friendly artillery (shells and rockets) as well. The headquarters controlling the local ground troops has to coordinate the fire of artillery, and the movement of the A-10s and AH-64s. The same care must be given to transport helicopters that enter the combat zone, especially the medical evacuation UH-60s. These helicopters have to get in fast, and often on short notice, as minutes can make the difference between life and death when evacuating badly wounded troops.