In world's 'most controlled airspace', how could crash happen?

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.

Todd Inman, part of the five-member NTSB board, said on Thursday that the agency had not yet recovered flight data recorders - known as the black boxes - but they are "confident" they will be able to retrieve them from the water.

Airspace over the District of Columbia is heavily restricted to protect both national security and the buildings that house core aspects of US government.

Commercial planes are prevented from flying over the Pentagon, the White House and other historic landmarks.

Yet the area sees a lot of air traffic, Aviation attorney Jim Brachle, who has handled numerous litigation matters related to jets and Reagan airport, told the BBC.

There is commercial traffic but also private aviation and helicopters that fly around the city, often carrying high-ranking officials and politicians between sensitive locations.

"You got these really narrow pathways in and out and you've got a lot of congestion and extra airplanes, so you're putting a lot of aircraft in a really confined space," he said.

Jeff Guzzetti, a former aircraft accident investigator for the Federal Aviation Administration (FAA) and the NTSB, told BBC Newshour that it seems the helicopter was in communication with the control tower before the accident. The air traffic controller pointed out the American Airlines flight to the helicopter, he said.

"The helicopter pilot indicated he had the airplane in sight and was going to visually separate from it – and then the accident happened shortly thereafter - so there's going to be a lot of questions about exactly what did the helicopter pilot see?" Mr Guzzetti said.

Mr Inman with the NTSB said that DC is also a "unique environment" because of helicopters and specific zones they're allowed to fly.

"If you look at DC, you see a lot of helicopters going down into this area so there's a very well-defined system in that regard," he said.

Mr Inman could not provide any specifics on altitude of the helicopter before it collided with the American Airlines flight.

Mr Brachle, who has handled numerous litigation matters related to jets and Reagan airport, told the BBC the question that remains is how two aircraft ended up in the same airspace.

"What's really unique about Reagan is right there on the river. There's also a helicopter route that crosses right through that final approach and that's at or below 200 feet," he said.

Brachle said the routes for both helicopters and the approach for aircraft intersect.

"You're putting potentially two different aircraft in a really small space with hardly any separation," he said. "If you get one that's maybe a little too low, one that's a little too high, you end up being in the same spot."

Aviation consultant Philip Butterworth-Hayes said the incident occurred at the "nexus of different aviation systems", including civilian and military systems, as well as procedures specific to the airport.

"You are at the border of three or four aviation systems here - and it's at those borders where most accidents tend to happen," he added.

But UK-based aviation expert John Strickland said the amount of commercial air traffic in the area cannot fully explain why the deadly collision was able to happen.

As well as Washington National close to the city centre, he notes, there is the international gateway, Washington Dulles, and also Baltimore Airport a little further away.

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"There has to be management of traffic flows to keep separation. It's much like we have in London where you have to manage traffic flows between Heathrow, Stansted, Gatwick and London City.

"So DC is not different in that sense to London or New York… it's not totally unusual."

Mr Butterworth-Hayes continued: "This is the most controlled bit of airspace in the world. You have both US government and civilian systems - Ronald Reagan airport is even owned by the government, it's one of the very, very few like that.

"This really is the most secure - and should be the safest - airspace in the world, given the number of security and civilian safety organisations working in that area."

The last fatal crash involving a commercial plane in the US was in February 2009. Officials and experts alike have stressed that this type of incident is incredibly rare due to tight safety restrictions on all types of flights.

Footage obtained from an air traffic control source by CBS News, the BBC's US news partner, showed the two aircraft which appear to have been involved in the crash clearly visible on radar systems accessible to controllers.

Audio sourced by BBC News appears to confirm the helicopter was in contact with air traffic control on the ground at the airport.

The helicopter was asked if it had the passenger plane "in sight" and to "pass behind" it. In the audio that follows, controllers appear to realise there has been a collision and can be heard directing other planes in the air to neighbouring airports.

Mr Butterworth-Hayes said an in-air collision like this requires a number of things going wrong.

He said that in order to fly in civilian airspace, the military helicopter would have needed to be fitted with a transponder alerting surrounding aircraft to its position.

That means both aircraft should have been able to see each other, he says, plus there would have been instructions from air traffic control and an aircraft protection safety device that operate separately from each other.

"On this occasion, you have these two different systems and both should have been able to keep these aircraft separate."

The Black Hawk helicopter was part of B Company, 12th Aviation Battalion. It left Fort Belvoir, a military base in Virginia, and was taking part in a training exercise.

US Secretary of Defence Pete Hegseth has said the helicopter's crew were "fairly experienced" and taking part in an annual night flight training session.

Speaking to CNN, Cedric Leighton - a retired US Air Force colonel - said it was normal for that type of military aircraft to be training at night in the area, particularly to make sure pilots are proficient with using instruments needed to fly in the dark.

He said one of the unit's duties is to transport high-ranking personnel around the capital - though none were on board at the time of the crash as it was a training flight.

The unit's pilots are expected to be proficient at flying in DC's busy airspace and "train in order to avoid incidents like this", he added.

Mr Butterworth-Hayes said only experienced pilots would be able to train in such a busy section of airspace.

"Whether it's training for new systems or equipment, we need to know what systems the pilot had turned on in the helicopter and whether they had all the safety systems on board, or whether they were trying a new procedure or new route."