How long might it take for rotors to stop turning after a reasonable successful autorotative touchdown? Account on radio from a bar occupants was something to the effect of (sorry I did not note the exact words) "the band stopping playing in response to some event, the audience joking that "the band had brought the roof down", but occupants not seeing it as a major event, *the band then re-starting playing* and then the major building collapse with smoke/dust, etc". If the rotors had been at a very low energy state at the moment of impact it seems perhaps surprising that the impact was not severe enough to trigger an immediate building collapse, or at least cause a situation where the folk inside the bar immediately saw an urgent need to get out?