WASHINGTON (AP) _ The percentage of delayed passenger flights has not improved since 1987 when airlines first were required to disclose their performance, a congressional study said Friday.

The study by the General Accounting Office criticizes the government reporting system for failing to show how late flights are and said some flights are not accurately reported by the airlines.

Since September 1987, on-time performance logged by the Transportation Department's monthly reports has ranged from a low of 66 percent in December 1987 to a high of 86 percent in September 1988. The data is based on reports airlines are required to file with the government.

In 1988, 20 percent of all flights were late; last year, late flights increased to 24 percent, the GAO said.

Airline industry and government officials agree that most flight delays are due to poor weather and that airport congestion is a growing factor. Delays generally are greater in winter months.

Transportation Secretary Samuel Skinner on Thursday said that if something is not done about airport capacity, the number of airports experiencing 20,000 annual hours of delay will increase from 21 now to 39 by 1997.

A flight is considered on time if it departs or arrives within 15 minutes of its scheduled time.

Delays due to mechanical problems are not counted, and the GAO said the department does little to verify information about the 23,000 such flights each month. That's about 5 percent of scheduled flights.

The GAO study said that the department's inspector general found last year that a tracing of sample flights of 12 major airlines showed that of 3,903 flights excluded from delay statistics because of mechanical problems, 564 should have been reported.

The study also said about 4,700 flights a month are cancelled for non- mechnical reasons but are reported simply as late flights and put in the same category as those running more than 15 minutes behind.

Some of the non-mechanical cancellations, the GAO said, are caused when a flight is cancelled for mechanical problems and other flights schedules are adjusted to accommodate the most passengers possible.

The GAO recommended changes in the reporting system to include the reasons for non-mechanical delays.