Using the Atmospheric Drop rules on 22, it generally takes a few turns for the opponent to get to the ground. In this case, it was specifically starting at level 9 on the Lower Atmosphere map, so 3 turns in standard atmosphere. With non-jump capable mechs, you have to have the drop-pack. OK. Take fire while inbound, take damage on the rear torso, so according to Tac Ops the drop-pack is destroyed and the damage carries through. But what then happens to the 'Mech? In our case, it was specifically at Lower Atmosphere altitude 3.