DV, you are quite right. I have been told that super-heated air is considered to be the most likely cause of ignition. The fact that this incident in 2004 occurred and was not acted on, is reminiscent of XV196 being holed in a fuel tank by a small calibre round months before XV179 was brought down by a round penetrating the No 4 fuel tank. In the case of XV196, engineers worked out why it had not gone bang. Armed with that knowledge fuel tank protection and a change in tactics, should have been considered before the tragedy of XV179. In the case of XV230, a great deal of work has been done on the venting system, and changes to AAR procedures have been brought in. Once again, after the event. And amazingly, nobody in the CoC is shouting for fuel tank protection.