Some of the nation's airlines have discovered a simple way to improve their on-time performance: They've started telling passengers the truth about flight times.

American Airlines, for example, has added 39 minutes to its scheduled flying time for flight 75 from Dulles to Los Angeles, providing passengers with a more realistic estimate of when they actually might arrive. As a result, what once was a 4 hour 55 minute coast-to-coast flight is now scheduled as a 5 hour 34 minute flight.

That move is among a number of changes made recently by air carriers in an effort to improve performance after months of growing public indignation over the sorry state of air travel.

Last week the Department of Transportation released its first shoppers' guide to airline performance, comparing major airlines in terms of on-time performance and baggage handling and noting which airlines received the most passenger complaints. The DOT's decision in September to require the airlines to report performance data was clearly designed in part to head off tougher action by Congress.

Critics of DOT last week found fault with the data, but the prospect of having to account publicly for performance appears to have had a tonic effect on the airlines. In the past two months, passenger complaints have dropped by 50 percent, and in October flight delays -- calculated by the Federal Aviation Administration on a different basis from the new DOT figures -- dropped 45 percent from the same month a year ago.

The DOT report card didn't satisfy some critics, who said the report should have been more comprehensive -- for instance, by including delays for mechanical reasons. DOT officials argue, however, that including mechanical delays in the performance ratings might tempt airlines to take short cuts on needed repairs.

The airlines are quick to point out that problems beyond their control will continue to contribute to delays. One major culprit, according to airline officials, is the air traffic control system, which employs 2,500 fewer full-performance air traffic controllers than it did in 1981, despite a tremendous increase in air traffic.

And weather still accounts for about 70 percent of all flight delays. When Wednesday's snowstorm shut down Washington National Airport, "we all flunked the report card," said Mike Gunn, American's senior vice president for marketing.

But one thing is clear: Once the airlines felt pressure from the public, the Congress and DOT, they found a variety of ways to improve performance.

"Their mind was off the store for awhile," said Matthew V. Scocozza, DOT's assistant secretary for policy and international affairs. "I've seen that this rule has had an impact on on-time performance since it was first conceived." Even so, nearly a quarter of all flights operated by the 14 largest airlines were late by 15 minutes or more.

One key to improving on-time performance was in the schedule changes the airlines made. American estimates that it adjusted times for about 1,500 of its 2,000 scheduled flights to reflect actual flight times more closely.

Airlines had an incentive to squeeze flight times reported in airline schedules because computerized reservation systems had listed the shortest flights first until July, when DOT changed the rules to say that the shortest flight time no longer would give an airline priority in the listings. To the extent that schedules were unrealistic, they added to the perception of a badly backed-up system.

American and other airlines also added gate personnel for flights that routinely are full or almost full to improve on-time performance.

"One of the key reasons for delay is late passenger processing," Gunn said. "We provided additional manning on flights that are booked at heavy levels so we could take care of that high level of activity in the last 10 minutes or so, so we could get people on and close the doors on time."

Once on board, passengers have found themselves being urged by flight attendants and even pilots to take their seats so that flights could take off. The FAA requires that all passengers be seated, with seat belts fastened, before an airplane can begin to move.

Airlines also have shifted scheduled departures to avoid the congestion at peak travel hours.

DOT has pointed out throughout the year that airlines have scheduled more departures at certain popular departure times than airports can handle. Scocozza said last week that in one intensely competitive market, 22 flights were scheduled to depart in a single minute.

In August, American, Delta, United, USAir, Continental and Eastern agreed -- under pressure from DOT -- to modify schedules at four major airports (Atlanta's Hartsfield, Dallas-Fort Worth, Chicago's O'Hare and Boston's Logan) to spread out traffic more realistically. Airlines have done so in other markets as well.

For instance, Continental shifted the departure of a widebody jet flying from Newark to San Francisco from 9 a.m. to 8:45 a.m. in order to improve on-time performance.

The Continental jet had departed at the same hour as several other widebody flights that left John F. Kennedy International Airport. Noted Bruce E. Hicks, vice president for corporate communications at Continental, "Air traffic control took all the 9 a.m. requests and threw them into a queue" -- which meant that the Continental flight was sometimes delayed.

Changing the departure time was "kind of like getting ahead of the rush hour," he said. "If you can leave the office 10 minutes before the peak, you can get there 30 minutes faster."

The Continental flight was not competing directly with other nonstops to San Francisco, so giving up the attractive 9 a.m. departure time was easier than it might have been in more competitive situations, he said.

Continental had two of six flights that were late every time they flew during September, according to the DOT statistics. Hicks said that a review of those flights found that in one case, a flight from Denver to Newark, an air traffic control problem along the route was to blame.

The flight consistently ran into delays because it had to fly through clogged air space. When Continental realized that the flight was running into a problem its other flights from Denver to Newark did not encounter, it built extra time into the schedule for that one flight to reflect the reality, Hicks said.

Continental, which held the top of the consumer complaint charts for months earlier this year, also began keeping "hot spares" available at its hubs. "Hot spares" are extra airplanes with crews ready to go in case another aircraft doesn't arrive or develops problems once it has landed.

United Air Lines added several hundred public-contact employes, in part to keep up with growth but also to improve performance, said Steve Steers, senior vice president for airport operations.

"We have better curbside coverage, better coverage in the lobby, better coverage at the gates and Red Coats at the hubs who give customers connecting information," he said. "All those things help."

United also opened a long-planned new terminal at its principal hub in Chicago, which the airline said allows it to handle planes, passengers and baggage more efficiently.

Some of the changes made to improve on-time performance were simpler.

American found that computerized seat assignments had some drawbacks when it came to efficient boarding. Passengers who received advanced seat assignments might not show up, leaving empty seats to be assigned to waiting customers. Airline personnel at the gate would not know those seats were vacant until a flight attendant on board reported back, Gunn said.

The airline resumed using an old tool: an airline seating chart with seat-number stickers on it. Now passengers pick up a sticker when they board, and the gate attendants know by looking at the chart which seats are still available for standby passengers.

Several airlines claim their performance in October will be better than in September. Airline officials note, however, that the first two months of reporting have been months in which it is easier to look good than it may be later.

"Air traffic control has operated pretty well in September and October, and they're also good-weather months around the nation," Steers said.

"There's still a great deal of fixing to be done to the ATC system," Gunn said. "I think the airlines are coming to the party with their part of the fix. Now, to make substantial further improvement, it's up to the government with the air traffic control system and up to God when it comes to the weather."

Airline officials say they think performance would have improved without the new reporting requirements.

DOT's Scocozza said one factor that may have prevented airlines from doing what they could to improve service may have been the difficulties airlines encountered in absorbing other airlines. About 70 percent of the airline industry has been touched by mergers in recent years, he said.

"I think it's part of the evolution of the airlines since deregulation," said United's Steers.

"For the first eight years of deregulation, the only thing that mattered a great deal was low prices, and it became less than fashionable to talk about airline service," Gunn said. "Now it's come full circle."

Con Hitchcock, legal director for the Aviation Consumer Action Project, had a different explanation.

"I think one of the reasons we didn't see any reforms before is there wasn't enough incentive and certainly weren't enough penalties in terms of what consumers could do about it, or in terms of telling people who was doing a good job and who was doing a bad job," he said. "DOT has been fast asleep on consumer protection for the past five years, and it was only when Congress decided to pass consumer protection legislation that DOT woke up and decided to do something.

"The question is, is this something that DOT is going to give priority to, or will it go back to sleep in another couple of months when the headlines disappear?" Hitchcock said.