That's Why!!

Yesterday, IIRC, I read a short report that the AA flight was directed to change runways 'at the last second' to runway 33 from its original heading of runway 1. (Runway numbers are compass heading divided by 10).

If you've never spent time in DC (I spent a decade some of those ago), then you need to know what National looks like. This is it. The main runway 1 is 7169 feet and the secondary 33 is 5204. I just now found the CNN report with transcripts, and yes, ATC did change runway for the AA plane.

Time to see what this all tells us. First, the AA plane was scheduled to land on 1, which threshold is farthest south on the facility, and, more importantly, on the VA side of the river opposite to the helicopter lanes. 33 has threshold to the southeast, and most importantly, the approach is over the river and DC and crosses the river very, very close to said threshold. Which means landing on 33 puts you very close to the ground as you overfly the east side of the river. Which, as it happens is right where the chopper lane(s) are. Oops. If the AA plane had stayed on its intended landing, it would have been much higher off the ground when it would intesect (in 3D space) the chopper; if it ever would. Nothing would happen, even with the chopper 100 feet (by some reports) above its lane.

The CRJ is about half the size of mid-sized 737 and about the size of the original DC-9. According to the wiki data, the CRJ700 has 5,040 foot landing spec; 33 is a tight squeeze. I'm no pilot, but seen enough "Air Disaster" episodes that ain't much margin. The CNN transcript gives us the reason (I speculate) why ATC changed runway: wind was at 320 with 25 (knots, normally) gust. That's a bit cross for landing on 1, but near a dead headwind for 33. Safer to land into the wind. The CRJ isn't the heaviest plane in the world, so not landing in crosswind is always better.

According to the innterTubes the CRJ700 is spec'ed to 27 cross!! Now, wind at 320 isn't direct cross and 25 gust is below 27, but switching to 33 makes a lot of sense.

QED

My SWAG is that the chopper pilot didn't register the runway change and thought the AA plane was headed for 1 or mistook another plane for the AA. The latter is what the transcript implies.