'The accident occurred in very complex airspace over New York City,' the chief NTSB investigator told reporters.

Hersman explained that planes flying under a 1,000-feet altitude (300 metres) were allowed to fly visually, without instruments or radio instructions, a decades-old rule that tried to accommodate the heavy sightseeing and flying demands for the Hudson River corridor.

She said the NTSB had made dozens of recommendations over the past decades for collision avoidance steps and systems in such busy corridors. It was not clear from her comments whether any of them were heeded.

Hersman also said that collision warning systems similar to those on commercial aircraft are not practical for helicopters because they operate in such close proximity to buildings and would receive constant warning signals that would be a 'nuisance'.

The collision brought calls from federal and state legislators for the FAA to change the rules and insist on guidance from air traffic control towers for air traffic around New York City.