As with any airplane accident or disaster, it can take a while to know what really happened. That is certainly the case with the apparent collision a few hours ago between a small airplane and a helicopter off lower Manhattan. What follows is just some orienting info to put in context today's unfolding news -- and, below, a request to any current-pilot reader with access to a scanner.

It appears that a small Piper airplane (Arrow or Cherokee, initial reports differ -- doesn't matter for our purposes) hit a sightseeing helicopter over the Hudson River, sending both craft and their occupants into the river. The airplane had reportedly taken off from Teterboro airport in New Jersey, not far away. Here is what the relevant section of the New York "Terminal Area Chart" would look like for the airplane pilot planning a VFR -- "Visual Flight Rules" -- trip on this route:



Teterboro airport is the blue elongated-X shaped mark in the upper left corner. The reported crash site would be near the center bottom. The helicopter chart for the same area would look like this (both of these are way more legible in real life):


Why would an airplane and a helicopter be in the same area, and neither of them actively directed by air traffic controllers? Because there is a "VFR Flyway" over the Hudson that lets aircraft travel through on their own guidance, and providing their own look-out for other traffic, if they stay below a certain altitude. (Above that altitude is controlled "Class B" airspace for Kennedy, Newark, and LaGuardia airports.) The exact altitudes differ, but typically in this area the planes would stay at around 1000 feet to make it through. That's relatively low for an airplane -- it's often the elevation above ground level at which you fly the "pattern" in preparation for landing at an airport -- but more normal for a helicopter.

Because the New York VFR flyways, and their counterparts in other big cities, are very busy, there are all sorts of specific instructions for flying there. Usually there's one radio frequency that planes flying this route are all supposed to monitor, and on which they announce their positions. The last time I flew along the Hudson, it was 123.05, but it might have changed. Usually you're supposed to turn all the plane's exterior lights on, to make it as noticeable as possible -- and to keep to a limited speed, and observe other procedures designed to keep traffic moving in one direction away from opposing traffic. All these procedures, safety tips, and operational details are spelled out on the back of the New York "Terminal Area Chart," but since I don't have one any more, I can't show them. Any pilot-reader who can do a scan of the "VFR Flyway" procedures for the Hudson River flyway, please send it in.

For reasons still unknown, one craft or the other might not have been following those rules  -- or one of them might have ended up in the "blind spot" from the other pilot's cockpit (it happens with aircraft as it does with cars). Pilots of sightseeing helicopters are presumably very familiar with this area and the associated procedures, so a starting assumption is that the airplane was doing something unusual -- for example, flying unusually low. But that's pure hypothesis.

Nearly three years ago, the pitcher Cory Lidle also crashed a small plane over Manhattan, but that was in different circumstances. (Here and here with other links.) That was a single-plane accident, not a collision -- and it happened over the East River flyway, which has a very different function than the flyway over the Hudson on the west. The East River flyway comes to an air-space dead-end when it run into LaGuardia's controlled airspace. The function of that flyway has largely been to give helicopters and seaplanes a way to get out of the Manhattan area. The Hudson river route, by contrast, is an actual throughway for planes traveling north/south past New York, in addition to being a favored sightseeing route. Here is an account from someone who flew there recently.  I've flown the Hudson route many times -- always feeling as if I had to be very alert, but never feeling that another plane was dangerously near -- but never even thought of trying the East River.

Condolences to all affected. More information as it is available.