In fact Tu-154 was aged, but after solid and expensive modernization. For now it looks like a mistake of a pilot connected with really bad weather conditions.
Bad avionics. A Boeing would've been alright.
I could be wrong, but the pilot had already tried several times to land, and he was flying manually, and probably a good pilot, and there may have been a freak tree in terms of height, but I reckon he just didn't have modern enough instruments to help him avoid the crash.
No sane pilot would try a turn at this altitude, and the pilot knew very well what the altitude was because he had just aborted a landing attempt. It wasn't a turn, it was likely a stall.
These aircraft are old, but have received extensive maintenance and upgrades, so a mechanical failure isn't likely. What you have to remember, though, is that a Tu-154 does not climb as well as a modern jet. So an aborted landing is much more dangerous, as you don't have enough engine power to climb quickly.
The pilot made four landing attempts (these are the best of pilots, so I suspect he was told to), the last one being fatal. Also, I don't expect a sane pilot to try to make a turn at low altitude right after an aborted landing, so the stories about the plane being in a turn probably mean the plane was already in a stall and falling.
Also, the runway at this airport is very short for the Tu-154 even at the best of times, so the pilot would have to touch down at the very beginning of the runway. In a dense fog that is extremely difficult.
It's a sad day. We might never know what really happened on board, but I strongly suspect that the pilots did not want to proceed and that either the presidential staff or one of the generals on board told the pilots to land anyway.