Aircraft maintenance is big business in the U.S. Air Force, $10 billion dollars a year's worth. And it's not just maintenance, but keeping warplanes flying much longer than originally planned. The average age of air force planes is over 20 years, and this will climb to 30 years by 2015. The extreme example of this is the B-52, which will be flying until 2040, 90 years after the first one entered service. Servicing older aircraft is uncharted territory, as this is the first time high performance jets have been used for this long. The old warbirds constantly surprise the experts by breaking down in unexpected ways. The situation was made worse by a decision in 1995 to cut back on spare parts purchases. This was to be another part of the post Cold War peace dividend. But the air force was called on to do more and more in the late 1990s. As a result, the percentage of aircraft available for action fell from 83 percent in 1991 to 73 percent at the end of the decade. In the last few years, additional billions were spent to get the spare parts inventories increased and this has stopped the slide in readiness. But there remains the unknowns of maintaining more and more aircraft that are older than their pilots.