A Pentagon-bound Army helicopter that got less than half a mile from a commercial flight landing at nearby Ronald Reagan Washington National Airport in May did not have the proper clearance from Pentagon air traffic controllers, a preliminary report from the National Transportation Safety Board said Friday. At their closest, the Black Hawk helicopter and landing Delta Air Lines regional jet were only four tenths of a mile apart and separated by just 200 feet of altitude, causing controllers in the airport tower to force the jet to abort its landing.