The nation's airlines turned in an improved on-time performance during October, although nearly a fifth of all flights were delayed 15 minutes or more, the Department of Transportation said yesterday.

DOT released its second detailed report on airline performance, measuring how well airlines did in on-time performance, baggage handling, bumped passengers and overall passenger complaints.

American Airlines turned in the best on-time performance for the second month in a row. Pacific Southwest Airlines, which has been acquired by USAir Group Inc. but operates independently, replaced USAir as the airline with the worst on-time performance. And Northwest Airlines led the industry in reports of mishandled baggage for the second month, with 10.62 complaints per 1,000 passengers, although United Airlines was a close second with 10.26 complaints per 1,000 travelers.

The DOT said 86.1 percent of American's flights arrived on time in October, up from 84.5 percent in September. In contrast, 60.3 percent of PSA's flights were on time, down from 70.5 percent in September. Pan American World Airways turned in the best performance in baggage handling, with 2.89 complaints per 1,000 passengers. DOT said that each carrier uses a different system to track complaints about baggage.

Pan Am had the worst performance in another category, however, leading the list of airlines denying boarding to passengers on overbooked flights with 17.09 per 1,000 passengers.

Northwest Airlines received the largest number of passenger complaints of the major airlines, with 19.06 complaints per 100,000 passengers. Continental Airlines and Eastern Airlines, both subsidiaries of Texas Air Corp., were next, together accounting for about a third of all complaints registered with the DOT in October. Delta Airlines had the fewest complaints per 100,000 passengers of any major domestic airline.

DOT's statistics on delays do not include those caused by mechanical problems. The DOT said last month that it would not count mechanical delays on the theory that airlines might be tempted to improve on-time performance by spending less time than they should to make mechanical repairs.

Even excluding mechanical delays, 19.7 percent of all flights were late. In September, 23 percent of all flights were delayed.

Pacific Southwest had the largest number of flights arriving chronically late -- late 70 percent of the time or more -- followed by Pan American. Continental had the fewest chronically late flights, according to that DOT measure.

United Airlines had the highest percentage of flights arriving on time at Washington National Airport, with 90.7 percent of its flights arriving within 15 minutes of their scheduled times. Continental, which had the best record in September, had the second best statistics, with 89.8 percent of its flights arriving promptly