hot wing
Investigations into the Columbia disaster are focusing on the left wing – apparently, there is telemetry indicating a serious temperature increase there, enough to raise serious concerns. A normal plane could survive the kind of minor damage that is being discussed, but the re-entry phase is so critical that any major deviation from the correct attitude can be fatal, and there is essentially nothing that anybody can do to correct it. The loss of a few tiles, even from a wingtip, would (I think) lead to the loss of the wing pretty much immediately, with a complete loss of control.
