Compared to pilots in other countries, pilots in the US have extraordinary freedom. Of course, to keep commercial airliners safe from collisions, pilots of small aircraft are excluded from certain airspace near major airports unless they have first obtained a clearance from air traffic controllers. If a pilot obtains the necessary clearance, controllers will dictate the pilot’s path and use radar to monitor the pilot’s every move.
But that still leaves many places where pilots are permitted to fly without being supervised or controlled in any way. One such area, appropriately enough, is near the Statue of Liberty. As long as the pilot stays below 1100 feet — outside the airspace used by airliners — the pilot doesn’t need a clearance, doesn’t need to have filed a flight plan, and doesn’t need to communicate with any tower or other air traffic control facility. The pilot is totally on his own.
Many non-pilots are surprised to learn that the method used to prevent collisions in such uncontrolled areas is called "see and avoid." The pilot is supposed to look out his window, "see" the other aircraft, and "avoid" them. Pilots talk about having to "keep their head on a swivel" when flying in uncontrolled airspace. Though this method of collision avoidance may sound primitive, over the years it has worked well.
There is one problem. Helicopters and airplanes don’t mix well in a "see and avoid" environment. Helicopters fly slower than airplanes. And because they have a small cross section, they are hard to spot — especially when viewed from directly behind. That puts them at risk of being rear-ended. It doesn’t help matters that helicopters tend to manuever in a fashion that most airplane pilots find to be unpredictable.
Because of all that, helicopter pilots are supposed to "avoid the flow" of airplane traffic. In other words, as best they can, they are supposed to stay out of the way. Unfortunately, when both a helicopter and airplane are headed to the same spot, or are both looking at the same feature on the ground, that can be difficult to do.
We don’t know what factors combined to result in the midair over the Hudson. But the NTSB has long recognized that when it comes to uncontrolled airspace, helicopters — especially tour helicopters — don’t mix well with airplanes.