It is too early to conclude what happened with this aircraft, but since I have been asked about this by email, here are my own, personal and provisional, thoughts:

Mechanical failure: unlikely. The Tu-154 is a three engine aircraft and an extremely strong beast. It’s safety record is comparable to most aircraft of its time, even though it was often used in extreme conditions other aircraft types did not have to operate in. The Tu-154 had already taken enough altitude to attempt a return to base or even a water landing. The weather that day was good. Besides, the crew did not communicate any problem. Thus the disaster had to be instantaneous. Fuel problem: unlikely. Fuel problems are always a prime suspect when a crash occurs, but even if the engines had suddenly experienced problems or even a full shutdown, the pilots would have had the time to report this. Also, like any other aircraft, the Tu-154 can glide and maneuver without power. Bird strike: unlikely. I am not even sure that there have ever been a triple engine failure due to a bird strike but even if there has been, they crew could have reported it which it did not. And, again, the case of US Airways flight 1549 has shown that even a catastrophic birdstrike does not prevent a fully loaded airliner from attempting to land. Pilot error: highly unlikely. The guys flying this aircraft where extremely experienced and while human error is always possible, it mostly results in situation were it can be reported. The Tu-154 was a very complex aircraft to operate and it had its weaknesses – but these were all very well known to the Russian crews and this crew was a very experienced one. Missile: unlikely. The Tu-154 has three engines including one mounted over the top of the rear of the fuselage and a MANPAD type missile warhead does not have the kind of blast radius capable of taking out all three of them. As for bigger missiles, the Black Sea coasts of Russia is very tightly controlled by the Russian military and security services (as is the entire Black Sea) and to get that close to the city of Sochi would be risky and difficult. Sabotage/bomb: most likely simply because all other causes are even less likely. True, this was a military aircraft with, supposedly, good security. Alas, I can confirm from personal experience that if you look Russian and speak Russian like a native and if you act the right way, military security in Russia is nowhere near as good as it should be. However, if you speak with an accent or look foreign, and that includes speaking with a Caucasian accent or looking like somebody from the Caucasus, you would have a much harder time beating the controls.

For all these reasons and even though it is way too early to speculate, my of preferred hypothesis is that it was a terrorist act executed by Ukrainian operatives. I hope that I am wrong and that I will be proven wrong in the next 24 hours but at this point in time, this appears to me as the most likely scenario.

One more thing: this tragedy really breaks my heart. Not only did the entire Alexandrov Ensemble perish, but two amazing personalities were on board: Valerii Khalikolv and “Dr Liza“. There are no words to express the loss which the death of all these people represents for Russia. This is why I hope and pray that in spite of why first reaction outlined above, this is not a Ukrainian terrorist attack because if it is, the consequences will be very severe. We should know more very soon.