If you've ever flown on an airline you've seen them. They travel like refugees fleeing a foreign land, their belongings strapped to their backs: purses the size of steamer trunks, garment bags stuffed like giant raviolis, backpacks that would stagger a Marine. They are the carry-on travelers.

No more. Well, maybe no more.

The nation's major airlines have declared war on passengers who insist on bringing all their luggage on the plane with them. The recent boom in air travel, fueled in part by cut-rate airfares, means that airlines have found themselves dealing with full flights, overcrowded airports and passengers refusing to check bags for fear they'll never see them again.

The result is a carry-on overload that has not only delayed many flights but also angered many latecomers who have discovered there's no room in the bin.

The assault on carry-on bags is being led by United Airlines, which this month began strictly enforcing its nationwide "Know Your Limit" campaign. The campaign, begun in May, is designed to cut down on the number and size of carry-on items. From now on, United is restricting all passengers to two, each no more than 45 inches in height, width and depth combined. That includes briefcases, computers, cameras and large purses.

The first test for the new, tougher carry-on limits will come this week as passengers depart for the long Thanksgiving Day holiday, the busiest weekend of the year for the airlines.

But if a recent sampling of flights out of Washington's Reagan National Airport and Baltimore-Washington International Airport are any indication, the test may prove a tough lesson for everyone involved.

"We're not backing off," insists Christopher Bowers, United's senior vice president in charge of North American operations, who says airlines have to be able to manage luggage better in a travel world where being on time has become sacred. Where it can, United plans to put a metal template over the front of airport security scanners to limit bags to no more than 14 inches wide and 9 inches deep. If bags don't fit, they have to be checked.

The new rules will apply to everyone, first class or steerage, Bowers said. In fact, they will even affect Bowers, who admits that his own carry-on garment bag is now too big and will have to be checked.

Even the nation's luggage makers are getting into the act. Working with the airlines, they are designing smaller carry-on bags to meet the new standards. Others are printing thousands of special marketing tags -- hopefully in time for Christmas shopping -- to tell shoppers which bags meet the tighter restrictions.

And in the process, at least one bag maker suggests the ubiquitous carry-on garment bag may be on its way out. "At this point, it's basically toast," said Skip Kotkins, president of Skyway Luggage Co., one of the nation's largest luggage manufacturers.

Kotkins said his company plans to put "renewed emphasis on the checkable garment bag as a really good way to carry suits or dresses; the carry-on garment bag was always a little tight for holding suits."

Meanwhile, Samsonite Corp., the nation's largest luggage maker, is upgrading and promoting its renowned hard-sided suitcases as the luggage that won't be easily crushed in its new home in the belly of the plane.

Although all but one major carrier -- Continental Airlines -- insist they are cracking down on carry-on violators, enforcement appears to vary from flight to flight, even at United. It all depends on whether the flight is full: The more crowded the flight, the stricter the gate attendants.

One thing is certain. The earlier passengers get on a plane, the better the chances of getting their luggage on board with them. With passengers normally boarded by row numbers from the rear of the plane, those sitting in the back are more likely to catch a break with gate agents and flight attendants. As the overhead bins fill up, the airline is apt to get stricter with the remaining passengers waiting at the gate about what can be carried aboard.

That's what happened recently on US Airways Flight 1744 to Hartford, Conn./Springfield, Mass., scheduled to depart from National at 5:39 p.m. At 5:23, the gate attendant announced to the remaining passengers waiting to board that the flight attendants had just called to say there was no more room in the bins for carry-on bags. "You will have to check all bags," the gate attendant announced, other than those that could fit underneath the seat.

"We usually tell the crew, when you start running out of room, call us and then we'll start checking the {carry-on} bags," said Lillian Edwards, an American Airlines gate agent at National.

That's one of the chief reasons airlines allow passengers with high frequent-flier miles to board first; the first passengers aboard get the best treatment, according to one airline official.

The first-come, best-served policy is most noticeable at Southwest Airlines, where passengers are given a number for boarding in the order in which they arrive at the ticket gate. At a recent Southwest flight out of BWI, for example, the agent in the foam rubber chicken hat reduced the number of carry-on bags to one when passenger No. 90 arrived.

Most major airlines have warning signs and some sort of bin or metal frame in which to test whether a bag meets their carry-on specifications. The signs range from United's "Reasonable Carry-On" campaign and Delta's invitation to "Put Your Luggage to the Size-Wise Test" to American's "Am I Too Big?" and Southwest's "Don't Be a Bin Hog."

United and American airlines as well as the Delta and US Airways shuttles also have installed silo-like carousels at their gates at National Airport to allow attendants to dump oversized bags quickly to planeside below.

The exception to the signage and the size bins is US Airways. The Arlington-based carrier took away its signs and size bins about two years ago when the company's new management decided they weren't working and, in fact, were counterproductive.

"The sizer boxes didn't serve their purpose," said a US Airways official who asked not be identified. "If you sat in the gate area and watched one of our agents suggest a passenger's bag was too big you often saw people trying to stuff a garment bag into the box to prove it fit." The boxes, he said, "did not bring us value."

Instead of sizer boxes, US Airways has a regular measuring tape at each gate to help agents prove to passengers their bags are too big. The decision to remove the boxes was not universally applauded by US Airways gate agents, according to recent interviews at National Airport.

"We used to have size boxes and that really helped," said one gate agent as several of his co-workers nodded in agreement. They said the confrontations between agents and passengers over carry-on bags have been getting worse as flights become fuller and the airports get more crowded.

"Passengers have pushed agents and they have cursed agents" when they were told they had to check their bags, said one of the US Airways agents at National who asked that his name not be used. Business travelers seem to be among the worst offenders at National, they said, often rushing to the gate at the last minute with their bags in their hands, demanding to get on the plane.

"People get really mad, so you have to very politely ask them to check their luggage, make it sound like it's a service you're doing for them," said Leanne Nebinger, a US Airways gate attendant at National.

To avoid confrontation with belligerent passengers, gate attendants often will let travelers carry their bags to the plane, where they can see for themselves there's no room.

It's not uncommon for passengers to try the carry-on shuffle to get around the restrictions. The shuffle goes like this: As a passenger approaches the gate, he switches his bag to the hand away from the agent and twists his body sideways to cover as much of the bag as possible. But the shuffle only works until he gets to the end of the jetway, where bags frequently are taken by a baggage agent, tagged and checked with the rest of the oversized luggage.

United's Bowers ties the new carry-on policy to the airline's need to maintain its on-time performance in today's overcrowded skies. He called on-time performance "the key measure of reliability" and said it is getting harder and harder to maintain United's complex schedules trying to stuff a plane with a full load of passengers in airports designed to handle planes that are only 60 percent to 70 percent full.

As a result, Bowers insists the new carry-on policy is needed. Since the airline began its carry-on education campaign, United has "experienced a sharp reduction in the overall volume of carry-on baggage," Bowers said. He wouldn't quantify the reduction.

He also acknowledged that many passengers don't want to check their bags for fear the airline will lose them or that it will take longer to retrieve luggage than it took to complete the flight. "If we are going to expect customers to check their bags instead of dragging them on board, we recognize that we need to improve baggage check-in, handling and delivery," Bowers said. He said United is investing more than $10 million to improve its baggage handling. "We are buying ground equipment, improving our automated {sorting} systems and adding manpower and front-line supervision."

The need to improve baggage handling is the key to reducing carry-ons, said Anne DeCicco, president of the Luggage and Leather Goods Manufacturers of America. "People do not carry bags on because it's convenient; they do it because they're afraid bags will be damaged, lost or won't arrive at their destination on time."

That's why District resident Carol Light was carrying a full load -- a stuffed carry-on, a swollen backpack and an oversized purse -- on her way to Tampa. "On my very first business trip, I went west to Kong Kong, my luggage went east to Hong Kong, and it took two days for my clothes to catch up with me. It was awful."

Still, it's people like Light who make other passengers angry. On Light's flight another passenger, who declined to be named, looked at her load and said curtly, "It's my pet peeve. It slows up travel and delays flights. I wish they would restrict carry-ons to a purse and a backpack." All this passenger was carrying was a book.

Passengers aren't the only ones grumbling about the new carry-on rules. Retailers have been burned by the new standards.

Steve Cohen, president of Lane's Luggage in downtown Washington, said he has had to refund hundreds of dollars to two customers who bought what the store told them were carry-ons, only to discover that they were no longer accepted by the airlines they flew. Now, Cohen said, customers are told that Lane's "cannot guarantee that any carry-on we sell will be allowed to be carried on to every airline all the time. We would rather not sell a case than have it used and then come back to us" for a refund. "Even experienced travelers are going to be stopped dead in their tracks," Cohen said. That concerns frequent traveler Tammy Stroud, a Denver software consultant who would rather switch airlines than fight the new rules after a United gate attendant observed that her purse was too large and might count as one of her carry-on bags. In addition to the purse, Stroud had a carry-on and a briefcase, both exceeding United's limit.

"I've had United lose my luggage twice in one year, and waited at the carousel for 45 minutes. I don't have time to hassle," Stroud said. "I'd rather change airlines than check my bag." IF YOU REALLY WANT TO CARRY ON YOUR BAGS: * Call the airline and get the specific carry-on rules, including dimensions of luggage and whether purses and computers qualify as carry-ons. * Try to be one of the first to board the plane. Don't assume that just because airlines allows two carry-ons, you'll be able to take two onboard. If the flight is full, you may be limited to only one carry-on, especially if you arrive late and all overhead bins are full. * Don't assume you're home free just because the gate attendant lets you down the jetway with your bags. Often, attendants will catch you as you enter the plane -- or at your seat, when you find you can't stuff your bag in the already crowded overhead bin. Anything that can't be fit under your seat will have to be checked. A LOOK AT . . . Airline Carry-On Policies AMERICAN AIRLINES Policy: Limit of two carry-ons; each bag is to be no bigger than 45 inches in combined height, length and width Not Counted as Carry-On: Handbags, coats, canes, walkers or other assistive devices Counted as Carry-On: Cameras, briefcases Special Treatment: First class and business-class passengers can carry on more items within reason because there's more space Also of Note: Strollers checked at gate and returned at jet bridge on arrival CONTINENTAL Policy: No specific limits -- depends on the flight and space availability; airline is enlarging carry-on space in overhead bins on its planes DELTA Policy: Limit of two carry-ons; each must weigh less than 40 pounds and be no bigger than 24 inches long, 16 inches high and 10 inches wide Not Counted as Carry-On: Handbags, coats, food for onboard consumption, crutches, canes & other assistive devices, umbrellas, reasonable amount of reading material, strollers and car seats (if used in flight) Counted as Carry-On: Computers, briefcases, duty-free purchases Also of Note: Car seats not used in flight must be checked. Violins and other delicate items that don't fit the measurements must be checked. NORTHWEST Policy: Limit of one carry-on with maximum dimensions of 22 inches by 14 inches by 9 inches plus one other "special item," which may include a briefcase, laptop computer, large purse, camera, infant car seat, diaper bag or stroller Not Counted as Carry-On: Small handbags, coats, umbrellas, canes, walkers, crutches, collapsible manual wheelchairs, reading material Special Treatment: Passengers flying first class, world business class or members of International Gold Elite or WoldPerks Gold allowed one extra carry-on SOUTHWEST Policy: Limit of two items, each measuring no more than 16 inches by 10 inches by 24 inches Not Counted as Carry-On: Handbags of reasonable size, small cameras, coats, food for onboard consumption, child safety seats (if used in flight or space available) and assistive devices for disabled Counted as Carry-On: Briefcases, shopping bags, backpacks, computers and if space available: portfolio cases, delicate stowable items of reasonable size (e.g. musical instruments), papoose-like baby carriers with hard nonfolding frames and FAA approved child safety seats Also of Note: On heavily booked flights, carry-ons may be limited to one per person TWA Policy: Limit of two items, with each having maximum dimensions of 10 inches wide, 16 inches high and 24 inches long Not Counted as Carry-On: Coats and umbrellas, canes, walkers and cameras Counted as Carry-On: Handbags, computers, briefcases, baby strollers Special Treatment: On international flights, one extra carry-on is allowed for business class travelers; on domestic flights, same rules apply to all passengers UNITED Policy: Limit of two items, each no more than 45 linear inches (length plus width and height); At some airports, there will be a 9 inch by 14 inch "template" at security checkpoints. If luggage doesn't fit, bag must be checked Not Counted as Carry-On: Small purses (less 25 linear inches), coats, child safety seats used in flight, canes, walkers & other assistive devices Counted as Carry-On: Briefcases, computers, large purses, cameras and strollers (can be checked at gate) Special Treatment: Same rules for everybody US AIRWAYS Policy: No more than two bags are permitted. Maximum for overhead bin is 24 inches by 16 inches by 10 inches; underneath seat maxiumum is 21 inches by 16 inches by 8 inches. On aircraft with valet bag bins, maximum is 45 inches by 23.5 inches by 4 inches. Not Counted as Carry-On: Handbags or camera bags (not exceeding 18 inches by 12 inches by 4 inches), coats, umbrellas, reading material canes, child safety seats, strollers if they fit Counted as Carry-On: Briefcases and laptops Special Treatment: Same rules for everybody, including passengers on the US Airways Shuttle and Metrojet CAPTION: A sign at the Delta Air Lines counter at Reagan National Airport arns passengers about restrictions on the size of their carry-on luggage. ec CAPTION: At National, US Airways gate attendant Leanne Nebinger places a check-in tag on a piece of luggage that can't be carried on the plane for passenger Lynn Moneyron. ec