Wreckage of the jet on river Potomac on Thursday. The two aircraft fell into the river after the collision, While the passenger plane broke into several pieces, the military helicopter unturned upside down and sank  —AP/PTI

Agencies WASHINGTON

The black boxes for a commercial flight that collided with a military helicopter in Washington DC have been located even as questions mount about staffing and other close calls at the airport that one expert described as "the most controlled bit of airspace in the world”,  where the plane was landing. There was no immediate word on the cause of the collision, but officials said flight conditions were clear as the jet arrived from Wichita, Kansas.

Officials are still investigating the cause of the accident  that killed all 67 people aboard the two aircraft. The National Transportation Safety Board (NTSB) said a preliminary report will be issued in 30 days. The flight data recorder and the cockpit voice recorder, known as the black boxes, can help offer clues to investigators about what may have gone wrong on the flight.

Short staffed Air Control

A Federal Aviation Administration report obtained by The Associated Press described staffing levels as "not normal for the time of day and volume of traffic." The federal government has struggled for years to fill certain key positions at the FAA. The type of staffing that had one air traffic control worker managing both helicopters and planes at the Reagan Washington National Airport on Wednesday night is reportedly not uncommon and did not breach guidelines.

Busy air space a challenge to navigation

The airspace around Reagan Washington National Airport can challenge the most experienced pilots, who must navigate hundreds of other commercial planes, military aircraft and restricted areas around sensitive sites. "This was a disaster waiting to happen," said Ross Aimer, a retired United Airlines captain and chief executive officer of Aero Consulting Experts.

"Those of us who have been around a long time have been yelling into a vacuum that something like this would happen because our systems are stretched to extremes."  There was no immediate word on the cause of the collision, but officials said flight conditions were clear as the jet arrived from Wichita, Kansas.

Investigators have already begun examining every aspect of the crash, including questions about why the Army Black Hawk helicopter was 100 feet above its permitted altitude and whether the air traffic control tower was properly staffed.

Not always easy to spot airliners at nigh

Pilots have long warned of a "nightmare scenario" near the airport with commercial jetliners and military helicopters crossing paths, especially at night when the bright lights of the city can make seeing oncoming aircraft more difficult.

Experts and some lawmakers said they are concerned that the airspace is about to get more congested in the wake of Congress' decision last year to ease restrictions that had limited the airport to non-stop flights within 1,250 miles (2,012 kilometres) of Washington, with few exceptions.

Lawmakers enabled airlines to launch new routes to destinations like Seattle and San Francisco. The plan fuelled intense debate about congestion versus convenience, with some legislators heralding new flights to their home states while others warned of potential tragedy. However, the flight that crashed on Wednesday was not part of the expansion.