I think it is worth trying to get the ATC radio recordings for this one from the different recordings to understand some key issues. (you can find them on You Tube). I have been trying to follow information as it has dribbled out. 1. The ATC to hele radio traffic was entirely focussed on the hele formally requesting and ATC accepting visual separation. From the way the phrases were spoken it sounded a bit like a mantra. That rather supported the suggestion above that all involved were clinging onto "visual separation". Effectively, there seemed to be no alternative to VFR with visual separation. 2. The hele and aircraft involved were on different frequencies although talking to the same controller. Some SA was likely lost as a result. 3. Runway 1 was in use at DCA (presumably broadcast on the ATIS). It was apparently practice to divert some arrivals onto Rwy 33 to allow more departures on Rwy 1. This is what happened to the aircraft involved. On the IFR approach for Rwy 1, ATC asked the aircraft to accept a fairly late change to a circling VFR approach to Rwy 33. 3. From the latest NTSB broadcast that I heard the information received at the hele (recorded on the hele CVR) indicated that traffic information (about the arriving aircraft) failed to communicate to the hele pilots the changed runway (it was transmitted by ATC and recorded on the ATC recording but got lost
by the time it "arrived" at the hele). I think that may be quite key. So, we are left with a low level hele with its crew using NVG looking for an aircraft approaching to the wrong runway. If you look at the charts for DCA, some of the hele routes are so close to the aircraft operating to/from DCA. Sadly the route the hele was on was effectively at the threshold of Rwy 33. NVG-wearing pilots over a densely lit city at very low level looking to the approach of the wrong runway (Rwy 1) for an aircraft on finals for Rwy 33 (with no way of distinguishing betwen aircraft at night) seems like a classic swiss cheese event. All preliminary and subject to NTSB report, of course