In short: good f’ing question! Here’s a nice read with a lot of answers…
For me, the most shocking aspect of the MH370 disaster is that we won’t know what fate befell those 239 souls until we find MH370′s Flight Data Recorder (FDR), aka the black box. Except for that last radar reading, we have absolutely no knowledge of the flight at all until we find that FDR. We have no clue what was said in the cockpit by the captain and first officer — though, seemingly, if something did go wrong, they didn’t even have time to send a mayday message. We have no clue if the plane hit a patch of bad weather, or whether it was hijacked. It really will be one huge mystery until the FDR is recovered — and there’s a good chance, if MH370 did crash into the ocean, that the FDR will never be recovered. In the case of Air France flight AF447, which disappeared off the coast of Brazil, it took two months to locate the wreckage, more than a year to find the Cockpit Voice Recorder (CVR), and the FDR was never found.
The FDR has an underwater locator beacon (ULB) that will ping for at least 30 days, and can be detected up to around two miles away, but when you’re talking about a search area consisting of thousands of square miles, and waters that are almost a mile deep on average, finding the black box will be no mean feat.
If I recall, the Air France plane was showing adequate airspeeds (the sensors on the wingtips were actually bad) and the plane got too slow and just fell out of the air. Also, TWA 800 over Long Island looked for sure like it had to be a missile, but in the end they concluded a short circuit sparked fuel vapors in the fuel tank.
So crazy stuff happens. And this is quickly becoming one of the craziest in commercial airline history.