well it said: просел и затем упал
So it fell down and then fell down??? I think it is more like "stopped mid-air"
Probably the thought was something like: Suddenly the airplane appeared to stop mid-air and then fall literally straight down.
Ok, maybe my translation wasn't good, after all. I'll try to explain.
'самолёт как будто просел и затем упал буквально плашмя' means 'the plane kinda jerked (made a sudden movement) downwards and then fell down flat.
By the way, sperk, your translation is incorrect.
'И вдруг по выражению специалистов, видавших момент аварии' means 'and suddenly, in the words of the experts who saw the moment of the crash'
So the full translation is:
'And suddenly, in the words of the experts who saw the moment of the crash, the plane kinda jerked (made a sudden movement) downwards and then fell down flat.