One thing to bear in mind- mass transit terror attacks over the past ten years have probably killed more people directly in attacks on trains (and subways/tube)

One thing to bear in mind- mass transit terror attacks over the past ten years have probably killed more people directly in attacks on trains (and subways/tube) than plane attacks. Bombs in london and madrid caused hundreds of lives lost or serious injuries, bombs in the pakistan/india region often claim thousands. The threat (as also mentioned here) is that a hijacked plane is, in reality, a massive missile and, to use the phrase, a true weapon of mass destruction. This however was resolved after 9/11 with new cockpit procedures and more alert passengers. (c.f. Bruce Schneier's blog for more)