The Washington Post broke this story last night. Ashley Halsey's
report has the 12-second figure, based on the combined closing speed of the incoming and departing US Airways Express commuter jets. His story doesn't clarify where the second departing aircraft was; how could you have two planes taking off from National in a position where both could collide with a third?
The Post's site also has about
four minutes of ATC audio, in which everybody sounds pretty calm considering the description in the story. The departing flight is Republic Airlines 3467 (today's newly-learned tidbit: "Brickyard" is Republic's call sign) and the arriving flight is Republic 3329. That audio makes it sound like a third Republic flight, 3395, was about to take off instead of being in the air, but I'm not sure about the last detail.
Update: I shouldn't have been sure; it was Republic 3071 that had taken off before 3467, as this
graphic on the Post's site clarifies.
Here's
FlightAware's track of 3329 from July 31; note the curlicue routing at the very end.
I should note that as I was reading the Post's story on a phone last night, we had the radio tuned to the Nationals game--and one of the Nats' most prominent sponsors on the air is the
National Air Traffic Controllers Association. Their ads seemed a little awkward then.