If you haven't been living under a rock, you will by now have seen yesterday's dramatic mid-air collision video.
No one in the media has made anything of it so far (though I suspect the NTSB has it very much in mind), but the two airplanes were flying in formation when they collided. That they were was obvious to me from the first few seconds of the video, but I had to Google several sources before I found one that said so; finally I found this Twin Cities.com article, which mentions it in the third paragraph.
Why is that fact important? Because Rule # 1 of flying formation is Don't Hit Lead! As a matter of fact, the first rule of flying in general, a pilot's primary responsibility, is to see and avoid other aircraft.
At the 40-second point of the above video you can clearly see the lead aircraft through the open door of the trailing aircraft. It's to the right, ahead and a little below. As the video progresses you suddenly realize that the trailing pilot is rapidly closing on lead from above and behind; unbelievably he doesn't turn away but continues straight on, making no effort to avoid the collision.
It's clear to me the trailing pilot became distracted and lost sight of lead, but that's no excuse: when you're flying formation it's your job to keep lead in sight at all times, just as it's your job as a driver to see and avoid other cars, lightpoles, and pedestrians. I can guarantee you the NTSB, which is already investigating the accident, is focusing on why the trailing pilot flew into the lead aircraft, and that the final report will cite pilot error. There will be no error on the lead pilot's part: clearly he was flying according to plan, and unless he had X-ray vision there's no way he could have seen, through the roof of his cabin, his wingman closing from above and behind. No, the fault will be 100% on the pilot of the number two plane.
I wonder if, during the last few seconds before the collision, any of the skydivers in the second plane tried to warn the pilot of what was about to happen, but that is neither here nor there. I also wonder if either pilot had military flying experience. You learn to fly formation in the military. I don't know what kind of formation training, if any, civilian pilots get, or if they have to demonstrate proficiency in formation flight before they go out and do it.
Here's all the FAA guidance I can find, taken from Federal Aviation Regulations, Section 91.111: “Operating near other aircraft. (a) No person may operate an aircraft so close to another aircraft as to create a collision hazard. (b) No person may operate an aircraft in formation flight except by arrangement with the pilot in command of each aircraft in the formation. (c) No person may operate an aircraft, carrying passengers for hire [*], in formation flight.” That's it. Nothing about demonstrating proficiency or maintaining currency. As far as I can tell, any two bozos with private pilot certificates can go out and try to play Blue Angels.
I have some experience flying formation ... oh, okay, a lot ... and I'm here to tell you it's not something you approach casually. Formation can get dangerous in a hurry and you need to have your shit together.
* I'll assume that skydivers paying to be taken aloft in a jump plane are somehow exempted from that "passengers for hire" category mentioned in the FARs.