Washington (CNN) — 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.

The helicopter was approaching the Pentagon from the southwest as three planes were preparing to land at Reagan National Airport, including an American Airlines regional jet arriving from Maine, a Delta flight from Orlando

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