The U.S. Air Force's hard working fleet of 544 KC-135 aerial refueling aircraft are continuing to show their age. Currently, some 40 percent of the KC-135s are out of action to deal with the growing maintenance problems. The aircraft, a military version of the Boeing 707 airliner, are over 40 years old. In theory, an aircraft can be continually rebuilt and kept in flying conditions. But the air force, and some commercial air lines, are finding that the maintenance problems multiply as the aircraft get older and, after a while, the downtime for maintenance is more trouble than it's worth. For example, the hundred oldest KC-135s spend an average of 80 days a year getting major repairs. The current proposal to buy new Boeing 767 aircraft to replace the KC-135s is in trouble because it will cost more money. Overhauling and maintaining the current KC-135 fleet would be about $10 billion cheaper than buying new 767s. Staying with the old aircraft would be pushing air force maintenance people into uncharted territory. But that's already happening with the 90 B-52 bombers, which are as old as the KC-135s. Ancient weapons are nothing new. In the 18th and 19th century, the British Royal Navy often kept warships in use (with regular overhauls and lots of maintenance) for over a century. The problem here is that the KC-135s (and B-52s) get the job done. And with the growing use of smart bombs, fewer bomber sorties are needed, meaning fewer KC-135 sorties are required to keep the bombers flying. There is risk in keeping 40 year old aircraft flying, but the safety record of the KC-135 and B-52 remains excellent. Newer isn't always better, but it is often more expensive.