In one of the ‘most controlled’ airspaces, how could crash happen?

BBC:

The aviation world is struggling to understand how a deadly mid-air collision between a passenger plane and military helicopter was able to happen in what one expert described as “the most controlled bit of airspace in the world”.

A US Army Black Hawk helicopter with a crew of three collided with an American Airlines jet carrying 64 people seconds before the passenger aircraft was due to land at Washington National airport.

Both aircraft were sent careering into the icy Potomac River on Wednesday night.

The exact cause of the crash remains unknown.

Officials will release a preliminary report within 30 days, according to the National Transportation Safety Board (NTSB) – which is leading the investigation.

Crews were able to recover flight data recorders – known as the black boxes – from the wreckage on Thursday, according to the BBC’s US news partner CBS. The devices can help investigators pinpoint what led up to a crash.