With all the other things to target -- buildings, transit systems, to name a few -- why do Islamist militants keep returning to aircraft as a target? Defence studies professor Michael Clarke explains why.

An excerpt from the Aug. 10 Times Online commentary:

Commercial aircraft represent globalism and high technology — they shrink the world and threaten cultural conservatism. The Boeing 747 was the last of the “great machines” that characterised the 20th century: it opened up air travel to the mass market. And it was so very American; big, brash and useful. But aircraft also appear vulnerable. In truth, civil aircraft are a lot more robust than people think, but the aviation industry is selling safety almost as much as it is selling transport and passengers need constant reassurance that aircraft are operating well within their technical limits.

So destroying or hijacking aircraft has always had great symbolic value for terrorists. Since the first commercial aircraft was hijacked in 1948 — a Cathay Pacific seaplane out of Macau — there have been almost 40 significant airline hijacks. Most ended with little or no loss of life, hence the presumption among crew and passengers that it was as well to go along with a hijack if you were unfortunate enough to get caught in one. There were manuals on how to relate to hijackers, or to avoid being singled out by them; it was a routine that hijackers and airlines both came to know.

All that changed with the 9/11 atrocities. If a plane is hijacked by jihadis intent on crashing it, then simple arithmetic would come into play: there are six of them and 200 of you. Like the passengers on United 93, there is nothing to lose by resistance. Hijacking in the traditional sense is out of fashion. But destroying aircraft is not. Richard Reid, the would-be shoe bomber, was caught making a comically inept, though dangerous, attempt to blow a hole in the side of an aircraft just three months after 9/11. American Intelligence reported a plot to board aircraft in Eastern Europe where security is lax, take them over and crash them into Heathrow during their final approach. It is not clear that this plot got beyond the talking stage, though British jihadi chatter has recently speculated on the prospects of getting “thirty brothers” on board a single aircraft, to control it long enough to destroy it.

The plot revealed yesterday may have been intended to run along similar lines: to get a number of terrorists each with small amounts of explosives on board multiple aircraft bound for the US. Explosives could then be pooled in-flight and detonated to make a catastrophic hole in the aircraft’s skin or blast a way on to a locked flight deck. Such a manoeuvre is not as easy as it sounds and again the simple arithmetic would apply once the terrorists began to act suspiciously. But the symbolic prize for the jihadis of destroying yet more aircraft after 9/11, of indiscriminate British and American deaths, hitting the air bridge across the Atlantic and of panicking the aviation industry into major disruption must remain tempting.