Plane experts query how DC collision happened in 'most controlled airspace'

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.

Airspace over the US capital sees hundreds of civilian and military flights every day, including helicopters tasked with carrying high-ranking officials and politicians between sensitive locations.

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."

However, there is a lot of restricted air space above the District of Columbia, home to the White House and US Capitol, just across the Potomac River from National Airport.

Experts agreed the intersection of civilian air traffic with regular military flights does make DC's airspace more complex.

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.

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."

Experts agreed that both aircraft would have been in direct contact with civil air traffic control.

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.

There are reports the helicopter was in contact with air traffic control and had been instructed to pass the plane via its rear, raising further questions about why they collided.

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.

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 - however, it is now known that only the crew was on board at the time of the collision.

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."

Speaking to BBC World Service's Newsday programme, Australia-based aviation expert Neil Hansford said the collision raises specific questions for US aviation authorities and will likely lead to an overhaul of procedures.

He said it wouldn't happen in Europe or Australia because there are tighter rules governing flight paths.

Another factor in the tragedy, says Mr Strickland, is the lower speed the jet was flying at because it was coming in to land.

"If any major shock happens, there isn't really any time - even if the crew survives - to do anything.

"The aircraft has got a degree of vulnerability because of its speed. You can imagine that a similar outcome would have been likely even with a much larger aircraft."