The Board of Inquiry into the loss of the Hercules C-130K XV206 on landing at Lashkar Gah, Afghanistan on the 24th of May last year (see here) has reported (pdf link). More documents are here.
After much effort, and extensive exploration of all sorts of other options, the Board concluded that (as was blindingly obvious from the beginning) the aircraft was destroyed by enemy action, specifically that it ran over a Soviet-type anti-tank mine on landing. Among other things, the investigators carried out an experiment involving such a weapon and an old C-130 to falsify their hypothesis. This explanation accorded entirely with the Provincial Reconstruction Team's initial report from the crash site, which was originally discarded for reasons that are not made very clear.
For that matter, two men aboard XV206 who had survived IED explosions in Iraq thought the experience was identical; the Special Investigations Branch lost no time in telling them they were wrong. (para 44, page 3 of the first doc above.) This may be accounted for by various experts' advice. Or perhaps that it was politically difficult to admit that Lashkar Gah was dangerous.
The conditions prevailing there in May, 2006 were clearly very dangerous indeed. Everyone appears to have been dubious about force protection, and especially about the Afghan police stationed in the area. An OLRT (Operational Liaison and Reconnaissance Team) from Permanent Joint HQ, which reconnoitred the place in February 2006, concluded that security on the landing zone was "insufficient to meet UK requirements". The details have been censored. However, the next sentence refers to "clearance" of the zone, which implies that mines or IEDs were the concern.
Anyway, the security force for Lashkar Gah airfield only arrived after the crash, but not before the Afghan police had had unfettered access to the crash site; no wonder they didn't find any pieces of the mine. "The Board considered that the disparity between the recommendations in the PJHQ recce and the procedures carried out on the day of the incident was a significant factor"; I bet they did. They further concluded that ground security there was inadequate and that there was no plan to search for mines, that the lack of security meant anyone could have interfered with evidence, and that the investigation was mismanaged.
Further (page 27), the situation was thought so dangerous that the crew were carrying small arms and were meant to be wearing armour, types censored. However, it seems they weren't wearing it and left their weapons behind in the plane; they didn't have anywhere to put the gear on their persons, and hadn't been issued with the fire-retardant version of CS95 uniform.
There is, it turns out, no single centre of expertise on aircraft survivability and vulnerability in the UK; although the Board of Inquiry on XV179 asked for one to be created, and in the meantime for an arrangement with the US to use their SURVIAC centre at Wright-Patterson Air Base, nothing has been done.
The upshot is that no amount of explosive-suppressant foam would have saved XV206; if you have enough wallop to carve holes in a tank, you've got more than enough to destroy an aircraft. The fuel tank punctures were of some 4 square metres; nothing is going to self-seal that. I was wrong to put so much emphasis on it at the time, although it is still a good thing to have.