First I saw of this was about 20 minutes ago when I got in from a trip. Saw the video. Listened to some guy on CNN talking about the possibility of a high speed stall..which doesn't make much sense. That might have made sense if he was turning around one of the pylons tightly and pulled too hard..but the video I was shows him coming in nearly vertical from above the crowd. So to me..that means he pulled up (the traditional exit maneuver in the event of an emergency - as someone alluded to..trading airspeed for altitude) and then lost control at the apex of the pull up maybe.
Purely speculation of course..and I'd guess two things..either a mechanical problem (broken elevator cable or something similar) or perhaps a physiological problem. You can declare an emergency if you are having a stroke or heart attack for sure.
I don't know what to make of the pilot's age (reported as 74 or something like that?)..but air racing is some pretty seriously strenuous stuff. Hot, high-Gs, dehydrating, and then throw in a lot of excitement. To say it shouldn't be a possible consideration would be dismissing it just to be polite.
I'm sure we'll get more details as they come to light. Terrible tragedy for the pilot, his family, the victims, and their families.
