Currently the predominant theory is that the mess at the Smolensk's Severny airbase was so massive that the incoming plane nearly crashed with another plane (Russian Ił) that also approached the landing strip in the same dense fog that covered the airport. The previous plane tried to land shortly before the arrival of presidential plane, but nearly crashed and flew off.
It is now discussed whether on its second attempt the Russian plane could enter into colliding path with approaching presidential plane. Traffic controllers were fleeing from the tower when police arrived, allegedly they were catched running away.
The airport's runway lights got the bulbs replaced, but some hours after the tragic crash. Independent Russian bloggers took pictures of soldiers replacing these bulbs shortly afterwards. Different measure units could also pose problems: in Russian aviation the altitude is given in meters, in Europe- in feet. The altitude is given from the earth's bottom, and in Europe- the zero point is the level of the sea. Pressure is measured differently, and it is uses to callibrate altimeter.
Polish pilots could not callibrate their altimeter as the Russian controllers could not say which of two measuring systems they use- they knew of only one. At least 3 different measuring systems exist, that could lead to the catastrophic end. Independent investigation already revealed systemic faults in measuring units that may cause also future death toll as a result of misunderstandings.
Given the amount of questions and the probability that Russian side is trying to cover up its alleged mess and faults, Polish bloggers and independent organisations are repeating their plea for an international investigators' team to explore the case, that already revealed staggering approach to aviation safety in Polish air forces, being the 4th crash in a series of deadly accidents under similar circumstances occuring in recent months.
If the presidential plane would be a commercial airliner and not a military one, it would not be possible to even try to land given the weather circumstances and lack of instrumental landing system except for NDB beacon at that airport. Even the TAWS system that the plane was equipped with, was probably switched off, as the map for the obscure Smolensk airport was not included in the commercial software that the system was operating with.
Mr Jacek Trznadel gathered 20 000 signatures for his open letter to Polish prime minister, moderately conservative historian Donald Tusk. Polish and Russian goverments have not yet released the records of the pilots' conversation in the last minutes of the ultimate flight of presidential Tu-154 3 weeks ago.
Literature:
http://ksymenes.salon24.pl/177258,nawet-tvn24-coraz-bardziej-watpi-w-sprawie-smolenska
http://kataryna.salon24.pl/175946,katastrofa-smolenska-ciekawa-hipoteza