Russia's Interstate Aviation Committee (MAK) has been emphasising the co-operation between Russian and Polish investigators during the high-profile inquiry into the 10 April accident, which killed Polish president Lech Kaczynski, and has been issuing frequent updates in a bid to stifle any doubts over the openness of the process.
The flight from Warsaw, originally scheduled for 06:30, had its departure time revised to 07:00 and subsequently took off late at 07:27. Four crew members - a pilot, co-pilot, navigator and flight engineer - were on board, although MAK points out that the aircraft's manuals were designed to support a three-person crew, without a navigator.
While the crew had weather information for the departure and diversion airports, as well as the flight route, they had no forecast - or other aeronautical data, including NOTAMs - for Smolensk North Airport.
The aircraft was technically sound and it had 19t of fuel, enough to operate the service given the alternate airports selected. Analysis of the fuel shows that it met quality standards and the aircraft's take-off weight and balance were within limits.
Russian authorities had conducted an assessment of Smolensk on 16 March, to check that the airport was capable of handling Tu-154 and Tu-134 aircraft. On 25 March a subsequent flight test of the airport's equipment showed that the approach radar, the NDB beacon and its markers, as well as the airfield lighting and radio systems met criteria for an approach on an easterly magnetic heading of 259°.
MAK says that Smolensk could not support an approach using the aircraft's flight director.
Five days before the accident the airport was declared ready to receive both the Tu-154 and the Yakovlev Yak-40 used by the Polish presidential air wing, down to a minimums of 100m (330ft) height and 1,000m visibility.
After departing Warsaw on 10 April the Tu-154 cruised over Belarus at an altitude of around 10,000m. During the en route phase of flight the crew communicated with both Minsk and Moscow area control centres in English, but switched to Russian when the aircraft came under the responsibility of Smolensk controllers.
MAK says it has completed its interpretation of the crew members' conversation, and clearly identified their voices. "The work was complicated by high levels of noise, including that due to the open cockpit door," it states, adding that the investigators have had to use special analysis techniques to filter out the extraneous sound.
But it points out: "It was discovered that, in the cockpit, were people who were not members of the crew."
One of the voices has been "accurately identified", it states, but the source of the other voices is subject to further analysis by Polish specialists. "This is important for the inquiry," adds MAK, but stresses that it is not disclosing names. Several senior military and political figures were among the delegation on board the aircraft.
Minsk and Smolensk controllers, as well as the crew of a Polish state Yak-40 flying ahead of the presidential Tu-154, "repeatedly informed" the Tu-154 crew about the weather at Smolensk. The Yak-40 landed at Smolensk at 09:15, about 1h 30min before the accident.
Some 27min before the crash, the Tu-154 descended to a height of 7,500m and Minsk controllers informed that Smolensk was experiencing fog with visibility down to just 400m. Having been handed off to Smolensk the crew twice received similar information, the 400m visibility being far below the minimum criteria for approach.
Sixteen minutes before the accident, the Yak-40 crew relayed the 400m figure and added that the vertical visibility was just 50m. Another aircraft, an Ilyushin Il-76, had made two approaches to Smolensk before aborting its approach and diverting to an alternate airport. The visibility continued to decline and, four minutes before the crash, the Yak-40 crew told the Tu-154 pilots that it had fallen to just 200m.
The crew opted initially to test the approach down to the 100m decision height. MAK says the controller, during the Tu-154's turn to base leg, informed the pilots that they would need to prepare for a go-around at 100m.
During the approach the Tu-154 had its autopilot engaged for longitudinal and lateral control, and its autothrottle was active.
The aircraft's terrain awareness and warning system had signalled 'terrain ahead', before instructing the pilots to 'pull up' around 18s before the accident. Just 5s before the impact the longitudinal autopilot channel was disengaged, followed immediately by the autothrottle, in preparation for a go-around.
While the runway lay at 258m above sea level, the aircraft by this point had descended into a ravine located about 1km from the threshold. It was flying some 15m below the level of the runway elevation and - at 1,100m from the threshold, and 40m left of the centreline - struck a tree at a height of less than 11m.
The aircraft continued for another 260m, and drifted 80m left of the centreline, before colliding with another tree, whose trunk was 30-40cm in diameter.
This impact badly damaged the jet, shearing off the tip of the left wing. Crippled, the Tu-154 rolled to the left. Within 220m it had rolled 90° and was fully inverted within a further 120m. MAK says around 5-6s elapsed between the initial structural damage and the complete destruction of the aircraft just after 10:41.
Forensic examination shows the occupants of the Tu-154 were subjected to forces in the region of 100g, says MAK, and survival was "impossible".
The airport's lighting had been checked earlier in the day, prior to the jet's arrival, and found to be operational. There were no problems with the Tu-154's engines.
Further investigation, says MAK, will include additional analysis of the flight-management system - the aircraft was fitted with satellite-based navigation equipment - and the terrain warning system, as well as a detailed assessment of the crew and operating procedures, and an examination of the flight under the weather conditions present on the day.
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