While the B-2 stealth bomber was the most effective warplane used during the 1999 Kosovo bombing, it is difficult to keep operational. The radar deceiving skin of the aircraft requires a lot of maintenance. For each hour the B-2 is in the air, 53 man hours of maintenance are required. More worrisome is the difficulty in keeping the aircraft ready for combat. The air force likes to have 60 percent of its aircraft ready to fight, but for the B-2 only 33 percent are available.