Transportation Security Administration workers do their thing. (Photo: lonelysandwich/Flickr)

Transportation Security Administration workers do their thing. (Photo: lonelysandwich/Flickr)

The trial of three men accused of aiding a 2006 terrorist plot is under way in London, and when prosecutors in the U.K. are through, maybe an angry mob of airline passengers will get a turn.

Not because they could have been killed, but because the method of death — liquid-based explosives — brought on one of the most onerous of airport security measures: the dreaded three-ounce rule, in which passengers must surrender liquids and gels (such as beverages and hair-styling product) unless they’re in containers that hold 3.4 ounces or less.

Transportation Security Administration officials had said they were “confident” the restrictions would end this year as they rolled out technology to detect explosives in liquids and gels. But the technology’s not ready, and now there’s no timeline to the restrictions’ end.

Cue the wails of despair, then answering voices of support for the rules.

For instance, there’s an editorial coming in the weekend edition of the Journal-Inquirer, a newspaper based in Manchester, Conn., saying this “minor irritation is saving lives.”

“It’s working, that tiresome and time-consuming and seemingly silly airport security drill. Turns out the conventional (heck, almost universal) wisdom was wrong … Some Scope bottles might have something dangerous in them,” the editorial concludes. “A five-minute search is well worth saving hundreds of lives.”

Ann Davis, a Boston-based spokeswoman for the TSA, made the same logical leap when I spoke with her Thursday: When asked whether the restrictions worked, she pointed out they began with a terrorist plot in which a liquid explosive would be dyed to look like a soft drink and brought aboard planes in large bottles. But the restrictions weren’t in place at the time, of course. What caught the terrorists and foiled the plot was the work of intelligence agents investigating the scheme long before anyone headed for a security checkpoint.

And that’s almost certainly what’s prevented every terrorist plot since. Not the three-ounce rule.

“I’m not aware of anyone who has been charged in that regard,” Bill Carter, a spokesman for the FBI, said Thursday when asked if the confiscating of containers violating the three-ounce rule had resulted in the prosecution of terrorist plots — or even arrests. “Off the top of my head, I’m not aware of any.”

Davis said intelligence agencies “know liquids remain a threat to aviation” and that “you can’t underestimate the restrictions as a deterrent” to those who might use liquids as a weapon of mass destruction. And on a casual basis, or as just one terrorist-dissuading layer of security among several, that’s true.

But to a person or organization truly wanting to use liquids in a terrorist threat, getting a few soft drinks’ worth of explosive onto an airplane would not be difficult. First, there’s the obvious technique of splitting the needed amount of material into smaller portions that will pass security, even if it takes more personnel to get it aboard a plane without raising alarms; then there’s getting the goods through security carried not by a passenger, but by workers at food kiosks inside most airport gates. Anyone taking a dawn flight will probably see large amounts of water, soft drinks and coffee, all in containers larger than 3.4 ounces, arrive daily, pushed through security every morning and subjected only to an X-ray. And there may be countless other ways around the rule for anyone who actually has a reason to come up with them.

No single layer of security is 100 percent effective, Davis acknowledged.

The three-ounce restriction is poster child for an ineffective layer of security, though — a burden for travelers that can’t be proven to work and could certainly be shown not to. It’s time to retire it, even without replacement technology in place, and rely on counterterrorism agents to foil liquid-explosives plots. As they’ve been doing all along.