American Airlines had the best on-time performance of any major U.S. airline and USAir had the worst, but Delta Air Lines had the largest number of chronically late flights during September, according to the Department of Transportation's first shopper's guide to airline performance.

Even excluding flights delayed for mechanical reasons, nearly a quarter of all nonstop flights were at least 15 minutes late during September, according to the agency.

The statistics indicate that airlines are improving their performance record, which was threatening to touch off passenger revolts earlier this year.

The detailed report on airline performance, which the agency released yesterday, measures on-time performance in several ways, including by airport and by hour, and reporting on baggage handling and over-booking.

The new reports -- required by a new disclosure rule issued in September -- will help consumers select the best flight for their circumstances.

Later this year, data on on-time performance for individual flights will be available to passengers through travel agents and at ticket counters.

That information will allow consumers to select airlines according to their performances, the agency said. For instance, a passenger confronted with a choice between two airlines with convenient schedules and similar prices could ask for on-time information. If an airline's performance is coded "8", indicating an on-time arrival record of between 80 and 89.9 percent for the previous month, the consumer might want to select that airline over one that is coded "5" for on-time performance of between 50 and 59.9 percent.

Overall, American had the best on-time performance record of the 14 major carriers surveyed, with 84.5 percent of its flights arriving on time (within 15 minutes of the scheduled arrival time). USAir had the worst, with only 67.4 percent of its flights on time.

"We're aware that our on-time performance for September was not acceptable and have been taking steps to make substantial improvements, particularly in scheduling at congested East Coast airports," said USAir spokesman David Shipley. He said the airline improved its on-time performance to 77 percent in October.

DOT broke down the data that it received from airlines in several other ways as well. For instance, DOT listed 150 flights that were late 80 percent of the time or more in September, including six flights that were late every time they flew during September. Delta accounted for 50 of the 150 chronically late flights.

Northwest Airlines, suffering from labor relations problems, led the industry in complaints about baggage handling, with 13.06 complaints per 1,000 passengers. Pan American World Airways, with 4.06 complaints per 1,000 passengers, had the best record. But DOT cautioned that baggage handling figures reflected different tracking systems used by different airlines.

DOT's statistics indicated which airlines had the best and worst performanced at selected airports. At Washington National Airport, Continental Airlines had the best on-time performance, with 83.5 percent of its flights arriving on time.. Delta had the worst on-time performance: only 52.5 percent of its flights arrived on time at National.

A passenger flying into National had the best chance of arriving on time between 7 a.m. and 8 a.m., which is usually before air traffic begins to stack up, according to the agency's statistics. The worst chance of arriving on time is after 10 p.m., even though relatively few flights are scheduled to arrive then. A passenger departing from National during September had the best chance of leaving on time on the first flights out and the greatest chance of being delayed between 4 p.m. and 6 p.m.

DOT also listed for each airline the percentage of flights arriving late 70 percent of the time or more. The airline that performed worst based on that measurement was Pan Am, while United Air Lines rated best. The airport with the largest percentage of flights arriving on time was in Charlotte, N.C., and the airport with the smallest percentage of flights arriving on time was the San Francisco-Oakland airport.

DOT did not include delays caused by mechanical problems in its reporting requirements "because we do not want any carrier to feel pressured to raise its on-time performance at the expense of safety," said Matthew Scocozza, assistant secretary for policy and international affairs. Excluding mechanical delays, only 77 percent of flights arrived on time.

DOT officials said that they watched closely to make sure that the airlines were not reporting delays as mechanical to improve their standings. There were no sudden increases in the numbers of mechanical delays, they said. DOT officials also said that mechanical delays accounted for only about 5 percent of delays and that incorporating those numbers in the overall statistics would not change the relative standings of the airlines.

DOT also issued its data on consumer complaints for October yesterday. Complaints declined 36 percent from September, the first month in which the torrent of consumer complaints began to subside. In September, complaints dropped 50 percent from August's 7,280. In addition, DOT reported on overbooking during the first six months of 1987. Presidential Airways, which operates as Continental Express, was the worst offender in the number of passengers denied boarding because of oversales, the agency said.

On Monday, the Federal Aviation Administration reported that flight delays, calculated on a different base from the figures released yesterday, dropped 45 percent in October.

DOT said that subsequent reports on airline performance would be released closer to the reporting period. The first report was delayed because of inherent problems in a new reporting system, the agency said.

Scocozza cited several factors that are helping airlines improve on-time performance. Airlines are scheduling more realistically -- posting later arrival times -- and many airlines are working their way through operational problems that resulted from mergers, he said.

DOT's data is likely to produce an advertising blitz by carriers, which will use the data to their advantage.