The congested airports, missed connections and flight cancellations bedeviling air travelers today are changing the travel habits of some of the airlines' most valued customers -- business fliers.

Poor service is driving business travelers away from certain airlines and airports, and in some cases it is driving them off of commercial carriers altogether. It is leading them to book flights at off-peak hours and to schedule trips so they arrive the night before a morning meeting.

Those strategies are adding significantly to corporate travel budgets, but most business travelers believe they have no choice.

It may mean spending a little more to ensure a business executive's timely arrival for a key meeting, but "it depends on what you define as costly. Missing a business meeting is very costly," said Mitchell York, the editor of Business Travel News, a weekly publication for business travel planners.

"If you have an 11 a.m. meeting and you leave at 8 a.m. and miss the meeting because you don't arrive until early afternoon, it might cost $1 million, compared with $125 for a {hotel} room. It's opportunity costs, not just costs" that are critical, he said.

Some companies are using corporate aircraft more. "From a pure dollar standpoint, there's no way you can justify a corporate airplane. You can fly people first class forever on the interest you pay on a corporate aircraft," said David Curtiss, manager of corporate travel for Upjohn Co. But "if that multimillion investment in that airplane means you can be someplace to make a decision that wouldn't have been made or couldn't have been made properly before, you can recoup that money in increased business opportunities and profits."

Edward T. Chu, director of aviation and travel for Warner-Lambert Co., said his corporation's helicopter and two jets "are being scheduled now pretty close to optimum usage." The costs are sometimes justified on overseas or transcontinental flights, particularly if the company's airplanes are full, he said. "On short hauls, it's not the most economical way to travel, but the speed and convenience does offset the economics there," he said.

There are no comprehensive figures on the added costs that businesses are incurring, but even a small percentage increase can add up to tens of thousands of dollars for an individual company and millions to business as a whole.

Upjohn, for example, spent more than $7 million last year in corporate travel from its headquarters in Kalamazoo, Mich., an expenditure that Curtiss said puts it in the middle range for large corporations.

The critical nature of most business trips and the large sums spent on travel make corporations understandably touchy when they don't believe they are getting their money's worth.

When business travelers book flights on commercial air carriers, "they're buying a schedule," said Travis Tanner, president of retail travel for Carlson Travel Group, which owns Ask Mr. Foster, one of the largest travel agencies in the United States. "If they buy a schedule, and it never flies on time, is that really worth the price?"

Despite what a traveler may think after sitting on a runway for two hours, airlines are conscious of the problems, and they understand how much they have to lose if business travelers desert them.

Business customers are a large and profitable part of the market for airline services. Last year, 46 percent of all travel on airlines was for business purposes, according to Airline Economics Inc., a research and consulting company.

But even that percentage understates the importance of business travel to the airlines.

Because business travelers are more likely to pay full fare rather than travel on discount tickets, they represent big profits for airlines.

Business travelers account for 49 percent of United Airlines' passengers, but the airline wants to increase that percentage, said Matt Gonring, a spokesman for United. "One percent of our passengers contribute 11 percent of our revenue," he said.

Put still another way, 11 percent of the airline's travelers -- the frequent fliers who shuttle back and forth for business -- generate almost 50 percent of the trips on the airline, said Gonring. "That gives you an idea of how important our business travelers are," he said.

As a result, United is designing a program to "discriminate in favor of business travelers," Gonring said.

United is designing a computer profile of its most frequent fliers, those who fly a minimum of 25,000 miles a year. In 1986, United had about 150,000 of those passengers. The profile will list each of these passengers' seating and meal preferences and allow United personnel to address them by name. The passengers will get first chance at standby seating and first-class upgrades and will be able to board along with first-class passengers, said Gonring.

Airlines have a variety of ways to attract business travelers, including "frequent flier" mileage bonus programs designed to build consumer loyalty.

United recently stopped handing out seating assignments to passengers buying its deep discount fares "because our business travelers were telling us that because of the advance purchase requirements for the discount tickets, those passengers were getting all the good seats," Gonring said.

Airlines have used discounting to attract a larger number of "discretionary" or pleasure travelers to their flights. But times are changing. The industry has consolidated through mergers and acquisitions, and fares have been going up to allow airlines to recoup increased costs.

"Now that there are just a few survivors left, there's going to be very little price discrimination left in the industry, so over the next few years the key is going to be service," said Paul P. Karos, an airline industry analyst with L.F. Rothschild.

Complaints about airline service have hit record highs, and the airlines are emphasizing their attention to service.

At the top of the list of airlines provoking consumer complaints to the Department of Transportation have been Continental Airlines and Eastern Air Lines, both subsidiaries of Texas Air Corp.

Several corporate travel planners said that they often get requests to avoid booking flights on those airlines.

Not everyone is suffering from the difficulties that business travelers encounter.

Travel agents say they expect to reap benefits from corporations and individuals who will turn to their services more in hopes of getting assistance in avoiding the worst of the delays and inconvenience.

Airport hotels also reap benefits when flight delays leave crowds of passengers stranded.

Peter Kaiser, general manager of the Clarion Hotel near Denver's Stapleton Airport, said that he recently had 200 guests whose rooms and a meal were paid for by Continental. The hotel tries to help ease frustrations by offering the guests a glass of champagne and speedy checkout the next morning, he said.

"I try to put myself in the position of sitting in an airport for five hours and all of a sudden discovering that I'm going to have to spend the night in the city. If I had a meeting scheduled the next morning and had to stay in Denver overnight, I would not be the most happy customer," he said.

Despite the devices adopted to help ease the pain of flying -- including avoiding the peak travel hours of 4 p.m. to 7 p.m., carrying baggage on board, using airline clubs in airports as work places, and trying to fly early in the morning before delays start backing up the system -- most travelers still encounter problems.

"There's really not very much the traveler can do except to plan as much as possible to minimize exposure," said Phil Davidoff, president and general manager of Belair Travel Consultants in Bowie, Md., and national vice president and secretary of the American Society of Travel Agents.

Much of the difficulty that travelers encounter arises from airlines' use of the so-called hub-and-spoke system of routing.

In this system, carriers arrange their routes so that many flights converge at "hub" airports. The idea is to maximize traffic. An airline might not be able to justify more than one flight a day from city A to city B. It might, however, be able to justify eight flights a day from City A to its hub, where passengers could connect with flights to 30 different cities, including City B.

But hub-and-spoke routes have proved prone to disruption. The hubs are often congested, and airports at the tips of the "spokes" are short on backup equipment and repair capabilities.

Some corporate travelers suggest that there is a market out there waiting for the airline that offers point-to-point service, eliminating the stopovers in hubs.

"The carrier who does step ahead and who does a good analysis of the market and puts in nonstop travel at a convenient hour can get the bulk of travel for city pairs {direct flights from one city to another}," said Chu.

For the time being, however, business travelers are stuck with the hub-and-spoke system. Most business travelers have little choice about traveling. "There could be a cutback in discretionary travel, but most business travel has to take place," York said.

"Although people complain about it, they generally adapt," said Bernard Barovian, manager of corporate travel for The Standard Oil Co. "If you get hit in the head often enough, you make adjustments."