CHICAGO (AP) — With all the screening technology at US 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 US jetliner on Christmas Day got an airport pat-down, it probably wouldn't have found the explosives authorities say were hidden in his crotch.

``To have people hold up their arms and just pat them, like I'm really going to carry a bomb there,'' said industry analyst Michael Boyd, arguing that pat-downs were often of little value. ``You know where you're going to put it, and no one's going to go there.''

Most travelers at US airports never get a pat-down when they pass through security. A metal detector must be set off first and then screeners would need to find out what triggered the alarm. That often amounts to screeners just lightly tapping on a passenger's arms, legs and clothes. But even if they go ahead with a pat-down, it likely would not turn up something nonmetallic, small and well-hidden.

Unlike the frisking of suspects conducted by police, which involves officers running their hands firmly up and down the body, including sensitive areas like the groin, buttocks and breasts, the pat-downs at airports usually involve, well, patting down.

A flood of complaints by women prompted the Transportation Security Administration in 2004 to list 'dos' and 'don'ts' on pat-downs, including barring screeners from touching female passengers between their breasts. The TSA hasn't publicly released that list. But a report by the Government Accountability Office, which said federal investigators were able to smuggle liquid explosives and detonators past security at U.S. airports, appeared to prompt some changes last year in pat-down policies.

In one instance cited in the report, an investigator placed coins in his pockets to ensure he'd receive a secondary screening. But after a pat-down and use of a hand-held metal detector, the screener didn't catch the prohibited items the investigator brought through a checkpoint.