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Preparing the public for yet another big change in air travel, the government asked passengers Thursday to stop packing certain kinds of items in checked bags, including, for example, Christmas fruitcakes.

High-tech bomb-detection devices might mistake them for explosives.

Just in time for the holidays–and with the Dec. 31 deadline to screen 100 percent of checked baggage–aviation security officials want travelers to know that things like chocolate bars, wheels of cheese and books can trigger a hand-inspection of luggage.

So security officials also advise that fliers stop locking their luggage to prevent screeners from opening bags with lock cutters in the event that baggage triggers alarms on the explosives-detection machines. Some of the bomb-screening equipment produces a 30 percent false-alarm rate, requiring further inspections on thousands of bags at large airports before the luggage is certified as safe to go on aircraft.

Many travelers at O’Hare International Airport on Thursday said they thought the recommendation to leave luggage unlocked crossed the line.

“It’s kind of asking a lot for us to trust the airline people,” said Sarah Bullard, 19, who arrived on a flight from Idaho.

The security lines were already so long Thursday morning that Bullard almost missed her flight. Any additional rules would lengthen the lines and create more headaches for travelers, she said.

“They complicate everything,” Bullard said.

The Transportation Security Administration also suggested that personal items that passengers would rather not have touched–say, a toothbrush–should be packed in clear plastic bags.

“We can’t force people to leave their suitcases unlocked, but we are trying to keep the new security procedures in the realm of workability,” said Brian Doyle, a spokesman for the security agency. “The bottom line is that the baggage screeners have to do their jobs.”

Doyle said the agency will soon begin providing travelers with red throwaway padlock-like seals that screeners can snip open if a search is necessary, but he said the seals probably won’t be distributed in time for the Christmas and New Year’s holidays.

In the meantime, the agency is recommending that passengers buy plastic cables or zip ties at hardware stores as an alternative to suitcase locks. Such clasps can be cut off easily by screeners needing to look inside baggage, officials said, and they provide some deterrence against tampering.

Video surveillance cameras are also being placed in airline baggage-sorting areas to control theft, but they have not been installed yet at many airports.

If security personnel search a checked bag out of the owner’s presence, a written notice will be inserted in the bag to inform the owner, officials said. The note will include a toll-free phone number (866-289-9673) for travelers to make damage claims and report lost or stolen items, but the security agency has not accepted blanket liability. Complaints will be handled case by case, Doyle said.

The recommended open-bag policy comes amid the arrests of hundreds of airport workers across the U.S. who concealed criminal backgrounds to get their jobs, as well as professional theft rings that scout the carousels in airport baggage-claim areas looking for unattended and unlocked baggage.

Twenty-five workers at O’Hare International and Midway Airports were charged last week with concealing criminal records or using fake IDs, and security clearances for another 553 employees were revoked.

In addition, six workers at Miami International Airport were indicted last week after being accused of pillaging items from checked baggage.

The Department of Transportation has received 178 complaints this year through Oct. 31 from passengers saying items were stolen from their checked luggage, and 10 instances in which the entire bag was taken, said department spokesman Bill Mosley. The numbers do not include passenger baggage complaints filed with the airlines.

Airline and airport officials said they had not received advanced notification of the security agency’s new baggage guidelines.

“A passenger at O’Hare asked me today if she could bring a roast beef in her suitcase and I told her `yes,'” said Monique Bond, spokeswoman for the Chicago Department of Aviation. “That was the rule before today.”

But on Thursday, James Loy, chief of the security agency, asked passengers to refrain from packing any food or beverages in checked bags and to pack shoes and boots on top of other contents.

Loy appeared in Florida during a demonstration of Jacksonville International Airport’s new baggage-screening system, which is much more efficient than screening bags at ticket counters but is largely conducted behind the scenes while bags move along conveyor belts.

At O’Hare and Midway Airports, most of the bag screening will be done in terminal lobbies with the passengers present, officials said. But the possibility exists that bags could be opened for inspection in baggage-sorting rooms or inside aircraft cargo bays, requiring screeners to force open locked bags.

The security agency’s primary bomb-detection system uses computerized tomography to search for explosives on the basis of a material’s atomic weight and density. But shoes, jars of peanut butter, fruitcake and other items in suitcases often cause false positives, officials said.

The explosives-detection systems are equipped with an on-screen function that allows the machine’s operator to resolve false alarms without opening bags. But in its rush to hire more than 22,000 baggage screeners to meet the year-end deadline, the security agency has not had time to train a large pool of workers to go beyond the initial screening procedures.

“It requires screeners to have two sets of skills, but the [Transportation Security Administration] doesn’t have adequate numbers of trained staff yet to do both,” said Stephen Van Beek, senior vice president of policy at Airports Council International of North America, the leading airport trade group.

The other electronic method of screening checked baggage requires screeners to swab gauze pads over the outside of luggage, then insert the pad into a machine that can detect traces of explosive materials. Officials said the interiors of some baggage will be tested as an extra precaution.

Van Beek said he is concerned about the unintended consequences that the baggage-screening policy will have on passenger behavior. He said more passengers may decide not to check luggage, resulting in many more carry-on items being scanned at passenger checkpoints by less-sophisticated X-ray machines and metal detectors instead of sensitive explosives-detection devices.

Meanwhile, the major airlines contend the security agency has failed to provide passengers with adequate recourse if their property is damaged or stolen in screening.

“When airlines mishandle baggage … a written claim is taken on the spot,” said Michael Wascom, spokesman for the Air Transport Association, which represents the major air carriers. “Who’s to say that anyone [at the security agency] is doing anything when you call an anonymous person at the end of an 800 number that isn’t even manned 24 hours per day? … The absence of consumer information is breathtaking.”

Tips for packing

The Transportation Security Administration has the following suggestions to avoid problems with checked baggage:

– Put personal items like toothbrushes in clear plastic bags so they are not handled.

– Pack footwear on top of other luggage contents.

– Secure your luggage with plastic cables or zip ties rather than suitcase locks.

– Avoid food or drink items.

– Avoid stacking books or documents.

– Don’t pack film; screening equipment will damage it.

– Avoid overpacking your bag so it may be easily closed if a screener must open it.

– Wrapped gifts may need to be opened.