My theory is double engine failure, how that would occur on a modern 787 I have no idea (I’ve seen speculation of fuel problems as a potential cause). In the video clip of it flying closely overhead I do not hear any engines just what is likely the buzzing of the RAT. The engines would have been roaring at full thrust had they been operational.
Double engine failure is very rare. The only other plausible theory I’ve seen is a single engine failure where the pilot or copilot then accidentally shut down the remaining engine instead of the failing one. They wouldn’t have had time to start it back up.
It’s a huge tragedy, I feel for those people slowly dropping out of the sky. Not a way I’d want to go.
And if it was a double engine failure, it occurred during an extremely narrow window of time where recovery would have been impossible. A little sooner and they could have aborted the takeoff. A little later and they could have gotten enough altitude to perform an emergency landing. If they had only lost one engine they should have been able to complete takeoff and perform an emergency landing.
And then somehow a passenger not only survived, but walked out of that crash? There’s so many things that are against the odds here. The black box should reveal everything. I feel so sorry for everyone onboard and on the ground that was impacted.