With all the screening technology at U.S. airports, the last line of defense is still the human hand: the pat-down search.

But aviation experts say the pat-down is often ineffective, in part because of government rules covering where screeners can put their hands and how frequently they can frisk passengers. As a result, even if the man accused of trying to blow up a Detroit-bound U.S. jetliner on Christmas Day got an airport pat-down, it probably wouldn't have found the explosives authorities say were hidden in his crotch.