If the system is working properly, airline passengers who need a wheelchair to negotiate lengthy airport terminals should find a chair and an escort waiting when their flight lands. But sometimes the system stumbles, as appears to be the case involving one beleaguered Washington-area family who had to wait so long for help they missed their flight home.

By law, U.S. airlines must make available some form of wheelchair or electric cart service at no additional charge to passengers, and many infirm, disabled or elderly travelers have come to rely on the assistance for boarding, unloading or making connecting flights. So did Jerome and Mary Jane Kossar of Fairfax City, who say they reserved an airport wheelchair for his 89-year-old mother Sonia when the three took a vacation to Mexico last month. Twice when they needed the chair to make a tight connection, it was missing.

Mary Jane Kossar has mailed a letter of complaint to United, the offending airline, and forwarded a copy to this paper as a way of alerting other travelers of the potential problem. Her hope is the story of her family's unpleasant experience will spur United and other airlines to improve their service. If you find yourself in the Kossars' predicament, to whom do you turn for help?

Procedures for providing wheelchair service are, to say the least, confusing because they vary by airline and airport. So it is no wonder in the hectic flurry of air travel that things do go awry occasionally. "I'd say we get a gentle stream of these complaints," says Peter Shaw-Lawrence, executive director of the New York-based Society for the Advancement of Travel for the Handicapped. He estimates that about 10 percent of travelers who request wheelchair assistance run into difficulties.

The Kossars became part of these statistics on a round-trip flight from Washington National to the Mexican resort of Cancun, which required a change of planes each way in Chicago. Months in advance, the family had requested a wheelchair, says Mary Jane Kossar, and no trouble was expected because they had flown United on a previous trip and the carrier had provided excellent service.

On the outward-bound leg of last month's journey, however, the chair they expected was not waiting at the Chicago terminal -- although the Kossars say they repeated their request to an attendant on the flight to Chicago -- and they were delayed in Chicago 15 minutes while they arranged for one. As a result, they just barely made their connecting flight to Cancun. Nor was there a chair waiting at Cancun, and one had to be rounded up on the spur of the moment. And again on the homeward leg, there was no chair waiting in Chicago, and this time 10 minutes evaporated before they were able to find one.

Those lost minutes proved crucial, says Kossar, because the three missed their Washington-bound flight "by seconds." Unfortunately, it was the last flight that evening for Washington National, where the Kossars' daughter was waiting to pick them up. Instead, they were put on a plane for Washington Dulles, scheduled to land about 90 minutes after their original scheduled arrival time in Washington.

Because of the hassle, says Kossar, "My mother-in-law was tired, and I was very concerned about her." The Kossars were also upset by what they perceived as the rude and uncaring treatment they received from the flight attendants and airline ground crew they turned to for help. "We only wanted assistance for a frail old woman," Kossar said in her complaint letter to United.

As this column is written, United has not yet replied to the complaint. However, Joe Hopkins, a United spokesman, says the airline's policy is to have wheelchairs and trained escorts awaiting flight arrivals when requested. But in a busy airport such as Chicago, where hundreds of passengers may want assistance in one day, sometimes not enough chairs and escorts are available at the moment they are needed, and delays can occur.

In response to the complaint of rudeness, he says, "Our flight attendants have the responsibility to be courteous."

By requesting a wheelchair in advance, the Kossars took the proper step that should have assured them one. But the system failed them, perhaps because it has an inherent potential for foul-ups. Each airline has established its own procedure for providing wheelchairs, and each airline's procedure varies by airport. Not surprisingly, passengers can get confused. The fact that complaints actually number as few as they do suggests that the airlines -- against big odds -- really do make a conscientious effort to have wheelchairs and escorts available.

Essentially each airline is responsible for providing wheelchair service for the passengers it is carrying at all of the airports it serves. If a problem develops, travelers who need help should always contact an employee of the airline on which they are traveling, not an airport employee. The airport itself does not provide the service. "Airports don't handle people and bags; we're just the landlord," says Art Kosatka, spokesman for the Washington-based Airports Association Council International.

Among the variables:

Who actually pushes the wheelchairs? At large airports, some carriers contract with private firms for wheelchair duty. United, for example, uses a firm called Andy Frain Services to provide the service at its big hubs in Chicago and Denver, says Hopkins. At Washington Dulles, employees of Page Avjet, another airline servicing firm, handle United's wheelchair aid at the airport's midfield terminal. At Washington National, three separate contract services each provide the service for different airlines.

On the other hand, Delta Air Lines generally makes use of its own specially trained employees as wheelchair escorts system-wide, even at its principal hub in Atlanta, according to spokesman Neil Monroe.

At smaller airports, almost invariably it is an airline's ground crew that is detailed to help anyone needing assistance. This is the case, for example, at Gallatin Field in Bozeman, Mont., which is served by five airlines but gets only 16 arriving flights a day, says airport manager Ted Mathis.

What other forms of transport are used? At some airports, an airline may provide assistance in the form of an electric cart that can carry up to three or four passengers. With United as the major exception, most airlines serving Denver dispatch electric carts to assist passengers. At Washington National, passengers needing assistance in transferring between the Main Terminal and the Interim Terminal are carried in a van.

How extensive is the service provided? Again, this depends on the airline and the airport.

At the optimum, some airlines offer door-to-door service to their passengers at some airports; that is, a wheelchair and escort are provided from the entranceway of the airport terminal through security to the airplane and off again through baggage claim to a waiting car or taxi at the destination. American Airlines limits its responsibility, however, to providing assistance from the check-in gate, says spokeswoman Lise Olson, and assistance at baggage claim is not part of the regular service -- although an escort may volunteer it.

At Washington Dulles, Ogden Aviation Services provides wheelchair service on a contract basis for most airlines serving the airport. The firm stations an employee with a two-way radio at the terminal entrance, and when a departing passenger shows up needing a wheelchair, help will be summoned by radio, says Hilda Small, Ogden's operations manager. At Washington National, wheelchair escorts are on duty at curbside or may be summoned by a skycap at curbside check-in.

If necessary, an Ogden escort at Dulles may stay with passengers for an hour or more, says Small, waiting with them if necessary until the plane is ready for boarding. Such service is free, but some travelers volunteer tips, sometimes as much as $10 or $20.

On United, travelers who have checked a wheelchair as luggage may retrieve it between flights if there is a long wait between flights, says spokesman Hopkins, and the service may be requested of other airlines.

To have a wheelchair waiting at the end of a flight, travelers should make the request when booking their seat. But any traveler who feels weak or tired or unable to negotiate a forbidding length of corridor can ask for a wheelchair on the spot. There may be a short wait, however, until one is free.

For any wheelchair service, travelers usually have several options. They can request a wheelchair without escort, and an accompanying passenger can assist; they can request both a wheelchair (or electric cart) and an escort; or, if traveling with a personal wheelchair, they can request an escort only.

What can go wrong? As the Kossars discovered, the expected wheelchair may not be waiting, which can be a big problem for travelers with a tight connecting flight.

This happens when an airline, for whatever reason, has failed to notify its wheelchair escort service that one is needed -- which, says Small, is a recurring problem. Bad weather also can create havoc with flight schedules, which in turn may upset Small's careful planning. Or there may be an unanticipated need for wheelchairs from travelers who ask for one on the spur of the moment.

On a busy day, Delta handles as many as 50,000 passengers in Atlanta, says spokesman Monroe, "and we may not always have a wheelchair available."

If you need wheelchair assistance for a flight, these precautions may assure a smoother trip:

Make your request for a wheelchair when you reserve your flight. It gives the airline an idea of how many wheelchairs it will need at a specific time, which is to your benefit.

Query the reservation clerk on the type of service the airline provides at the airports you will be using. Do you get door-to-door assistance, or are you or an accompanying passenger responsible for retrieving luggage from the baggage claim area? Will the escorts assist you onto and off the plane or leave you at the boarding gate with a gate attendant?

Arrive at the airport early. Maneuvering a wheelchair through a busy airport takes extra time. On arrival, you may have to wait until a wheelchair can be summoned, and you should not expect an escort to race you down a corridor to catch a flight.

Renew your request for a wheelchair when checking in or at the boarding gate. The request should be forwarded to your connecting and destination airports. While aloft, ask a flight attendant to have your request reaffirmed once more by radio. The Kossars said they attempted this tactic -- suggested by the Society for the Advancement of Travel for the Handicapped -- but the flight attendants refused.

Be wary of tight connections. A 35-minute connection may not be sufficient if you must be helped off a plane and wheeled to a distant gate. This is especially true if no escort has been assigned or the escort is delayed. Ask for a routing that does not require hectic -- and stressful -- scrambling.

If a chair is not waiting at your destination, get help from an employee of the airline you are flying. The most obvious sources of help are flight attendants. The attendants should assist or at least see that some member of the ground crew does, but remember they may be pressed to catch another flight themselves. If you can exit on your own, go to the nearest customer service desk for help.

Airport Access A free 48-page pamphlet, "Access Travel: Airports," details facilities, services and accessible design features at 553 airport terminals worldwide. The publication is aimed at older travelers and those with disabilities.

The guide notes the availability of such aids as rental cars with hand controls; wheelchair-accessible hotels on site; telecommunication devices for the deaf; handicapped parking spaces, ramps and elevators; and some 60 other items. It is published by the Airports Association Council International. For a copy, write the Consumer Information Center, Pueblo, Colo. 81009.