One factor not really being addressed because of the onslaught of Trumpist attacks, is that this was a night-time training exercise. This means the pilot may not have had a ton of experience flying at night.
Except that it wasn't. It was an annual proficiency checkout - the pilot was an experienced and long term veteran of exactly this kind of flight, doing the kind of "training" we all have to do periodically to allow the boss to tick the box that says "This guy can still do the job he has been doing for the last X years".
Less cynically, it's supposed to be an opportunity for the instructor to note and give feedback on any minor bad habits the pilot may have fallen into.
The idea that, because it was a "training flight" we can infer an inexperienced pilot is typical of the kind of horseshit that the media generates to fill the 24-7 news cycle, in the absence of any actual new information. The 12th Aviation Battalion basically has only three categories of PAT (Priority Air Transport) flights: Carrying a VIP (typically high ranking officers, but also congressmen and top government officials); Relocation flights (going to collect a VIP, or returning to base afterwards); and Training (everything else).
The helicopter pilot was in his designated corridor, following all of the standard procedures correctly.
The problem being that it is standard procedure across the USA to allow pilots to visually separate from other traffic, even in busy Cat B airspace, and even at night.
This was an accident waiting to happen, and was due to deliberate policy decisions made by the US aviation industry as a whole (and still current).
Visual separation cannot work perfectly in any environment where it is possible for a pilot to misidentify which other aircraft is the one ATC are warning him about. It does work very well, but when it doesn't work, people die. It is a truly bad idea to use visual separation in a busy traffic environment (eg Cat B airspace); and an even worse idea to do so at night.
The USA got away with it for the last 16 years. But eventually that luck was bound to run out. Nobody directly involved did anything blameworthy - they all followed their procedures exactly as trained and documented. It is the procedure that is at fault.
Visual separation is less expensive than radar separation, both because it requires fewer ATCOs, and because it allows more aircraft movements in a given airspace. All systems balance cost against risk; and for sixteen years US ATC has been growing complacent about the risk of VS, because no serious crashes have occured, while the pressure on costs, and the pressure of work due to a shortage of ATCOs, has mounted.
VS works when there are two aircraft within visual range. If you can see another aircraft, and avoid it, then a vollision is impossible. But when there are three or more aircraft within visual range, how do you know that the one you are seeing and avoiding is the same one ATC wanted you to see and avoid? Typically US controllers distinguish aircraft by type in this scenario (and that happened here); A pilot should have no problem distinguishing an A380 from a Cessna 172. But who can tell the difference between a Bombadier CRJ and an Airbus A319 at night, just from looking it its lights?
