The U.S. Air Force may buy as many as 18,000 iPad 2s. This would be one of the military’s biggest orders of computer tablets, accelerating Apple Inc. (AAPL)’s inroads into the federal government.According to a notice posted on the Federal Business Opportunities website, The service’s Air Mobility Command plans to issue a request for proposals to buy between 63 and 18,000 iPad 2s or similar devices to lighten the load of flight crews.