An investigation into a mid-air collision over the Hudson River last year finds that an air traffic controller listening to three conversations at once — including a personal phone call — didn’t catch a critical pilot error before the deadly accident.

Documents released Wednesday by the National Transportation Safety Board say the controller at Teterboro Airport in New Jersey who handled the small plane that collided with a tour helicopter
couldn’t hear an incorrect radio frequency repeated by the pilot shortly before the crash. Nine people died.

The documents say the controller was on a personal phone call over his headset while monitoring simultaneous radio transmissions by the plane’s pilot and a Newark airport controller over speakers.

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