The vast majority of summer air travelers have already purchased for tickets, so paying for checked bags is not an issue. But for everyone at American after June 15, the checked bag fees will kick in, and also for United in the middle of August. After those fees kick in, the size of the carry on will matter more.
Now that the airlines are charging, or soon will be, for checked bags, passengers will stuff as much as they can in their carry on, right?
"Yeah pretty much everything I have to get in there is a carry-on," said Sean Walker, traveler from Oswego.
"I have to put more in the carry on, so I wouldn't have to pay for extra baggage," said Jamar Hill, passenger from New Orleans.
Fat carry-ons could lead to swollen overhead bins and the possibility of longer, slower lines. That hasn't happened, at least not yet because the check bag fees have just started. American has employees keep an eye on the carry ons. No problems yet, they say. But if a carry on appears to be too big?
"In my case, I would direct them to the counter where they would be told to consolidate. Or if it is too heavy, they are going to have to take something out and possibly buy another bag," said Mia Helm, American Airlines employee.
Carry-ons are supposed to measure no longer than 22"x14"x9, but it hasn't been rigidly enforced. But as fuel costs continue ballooning, costs of flying will go up.
"My wife suggested another way of doing it - ship your bag. Ship a bag to your destination ahead of time, save the money that way and don't pay the $25," said Mike Passaglia of Huntley.
That option opens up an array of possibilities as well as problems. But the science of policing the size of the carry on bag, it is fair to say at this point, is still very much a work in progress.
Checked bags are now a moneymaker for US Airways, American Airlines and United Airlines, and officials say they're going to keep a closer watch on how much you take on board as they begin their new baggage fees.
"We're planning on having extra staff where possible, especially at peak times at busy airports," US Airways spokesman Morgan Durrant said.
The airlines point out that the carry-on policy came from the Federal Aviation Administration, not the industry, and they have an obligation to keep people from sneaking bulky bags onto planes to avoid fees. Not only is it unfair to the honest, fee-paying traveler, they say, but it would also overload the overhead bins and force gate crews to delay takeoffs while they checked excess bags.
But the added scrutiny means something else as well. Every passenger -- even those not used to checking bags -- is going to have to start following the rules.
People who are accustomed to boarding with a large roller case might find themselves handing $15 to an airline employee before they get through security.
"It's up to us to get to them before they get too far in line and say 'Ma'am or sir, you need to check that bag,"' said Mark Dupont, American's senior vice president of airport services planning.
AMR Corp.'s American Airlines, the nation's largest carrier, was the first to announce fees for a single checked bag. It started charging $15 each way for the first bag on tickets purchased June 15 or later.
US Airways Group Inc. will match that fee for tickets booked on or after July 9. UAL Corp.'s United Airlines will follow later in the summer with the same bag fee for domestic flights as of Aug. 18.
The three airlines say they are responding to tremendous pressure to cover sky-high fuel prices that have erased profits and sunk stock values. The Air Transport Association says fuel costs will hammer the industry this year, contributing to about $13 billion in losses.
Most airlines plan to cut back on available seats and routes in hopes of eventually boosting demand and keeping fares high. American, US Airways and United also will shed thousands of jobs to cut costs.
US Airways, which does not have an extensive network of lucrative international flights, says it will look for additional ways to raise cash. It is already planning to increase service fees on tickets, charge for items like soda and for popular seats in coach. Along with the bag charges, the fees are expected to bring US Airways an additional $300 million to $400 million annually.
"They're in such trouble," said Honor Guthrie, 45, an American Airlines passenger on her way back to Chicago from Phoenix. Guthrie said she understood that fuel costs are killing the industry.
Next time she travels, Guthrie said she will likely take something smaller than the large black roller case that she brought on the Phoenix trip.
"I'll wear the same pants a couple of days, same shirt, same skirt. I'll probably do some laundry wherever I go," she said.
Dan Weisberg, 51, also sympathized with the airlines. Weisberg, a businessman who travels a few times a month, added that he is happy the airlines are devoting extra employees to policing carry-ons.
"This is going to create bedlam in the overhead compartments," Weisberg said.
American, United and US Airways passengers can pay the new bag fees curbside, or at airport ticket counters and kiosks. US Airways and United are working on ways to allow passengers to pay the fees online as well.
Each airline will charge passengers in slightly different ways. United and US Airways will allow passengers to use cash at curbside, credit cards at the kiosk and cash, credit cards and checks at the ticket counter. United will not allow passengers to pay bag fees curbside at some airports.
American passengers can use credit cards to pay their bag fees curbside and at airport kiosks, and they can use credit cards, cash or check to pay at the ticket counter.
If a passenger is stopped from boarding with an oversize carry-on, American will charge that passenger the $15 fee to check it at the gate. United has not decided how it will handle passengers with oversize carry-ons, and US Airways says its passengers can check those bags at the gate for free.
Meanwhile, other carriers are watching how passengers react to the new bag fees.
Continental Airlines Inc. Chairman and Chief Executive Lawrence Kellner, for example, hinted that he might add a bag charge if he sees that customers are not scared off.
US Airways flight attendant and union leader Mike Flores said the new bag fees are going to frustrate a lot of people, and he is not excited about being the one to sort out the confusion.
Passengers are not going to be happy when they see people checking their bags at the gate for free, he said.
"They really haven't thought this thing out," Flores said. Passengers who paid the fee are going to say "'Wait a minute, I paid $15 to have my bag checked, and this guy is getting them checked for free."'
But the new fees may finally force a lot of people to start following the federal carry-on guidelines, said Laura Glading, president of the Association of Professional Flight Attendants.
Glading said passengers have gotten used to bringing in large bags in part because airlines rarely have enough staff to watch them. In her 30 years at American Airlines, Glading said she has seen passengers bring on tubas, paintings and wedding gowns.
"We're very reluctant to be separated from our bags," Glading said. "I imagine that that reluctance for passengers will grow into sheer horror now that they not only have their bag taken away, but they now have to pay for that."
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The Associated Press contributed to this report.