The U.S. Air Force flew 170,000 air transport sorties in 2004, plus 36,000 air refueling sorties. Some 55 percent of these were performed by reserve and National Guard crews. Transport and tanker sorties are more than double the number normally flown during peacetime. Most air force warplanes are back to peacetime sortie rates, but not the transports and tankers. If the current tempo of operations goes on for a few more years, maintenance and spare parts costs for transport aircraft will require billions of dollars in extra expense. Some aircraft will reach the end of their useful lives sooner than expected, requiring the purchase of new aircraft, or expensive overhauls. This will be difficult, as the air force leadership is more concerned with building warplanes, and provides strong resistance to Congress, and air force transport and tanker generals, for more money.