Re 'nose down' resting angle. Helicopter, in more or less level attitude, pancakes onto flat roof, with fuselage (bulk of weight) in middle of roof, tail close to edge. Roof is strongest nearest edge (walls), but subjected to greatest force at weakest point, nearest (unsupported) middle. Nose penetrates further into building than tail. Thus apparent nose down angle - little or nothing to infer in terms of how/why the helicopter crashed onto it.