The U.S. Air Force recently had to cancel ones of its Red Flag training exercises because there were not enough tankers available. The air force holds 4-5 of these exercises a year, to maintain fighter pilot skills, and sometimes to complete the training of pilots before they go overseas. The training includes aerial refueling. The problem here is that the air force fleet of KC-135 tankers are very old. An unpredictable, and growing, number of them are unavailable because of maintenance problems. Earlier this year, the air force disclosed that, on average, 20 percent of its 415 aging KC-135 tankers...