The Federal Aviation Administration said yesterday it has found "no fundamental shortcomings in the design" of the much-maligned DC10 engine support pylon and proposed a relaxed inspection schedule for the plane involved in the nation's worst aviation disaster.

The finding is one that DC10 manufacturer McDonnell Douglas has insisted was inevitable. It adds to the growing body of official opinon that blames an American Airlines maintenance procedure for a crack in a load-bearing pylon bulkhead in the DC10 that crashed May 25 in Chicago, killing 273 people.

If the DC10 pylon "is not damaged by maintenance or other causes," the FAA report said, the airplane should fly safely for about 25 years, longer than it was designed to last.

Also yesterday, McDonnell Douglas agreed to pay a $300,000 fine to the FAA for violations of quality control requirements during the manufacture of some DC10 pylons. The "quality control problem" was found on some DC10 pylons inspected after the Chicago accident and was not related to the cause of the Chicago accident. Although it paid the fine. McDonnell Douglas said that the "discrepancies . . . are not violations of" the appropriate regulations.

McDonnell Douglas and FAA lawyers reached the settlement yesterday afternoon, after the pylon study results were released. Deputy FAA Counsel Jonathan Howe said "there is no connection" between the events.

The FAA's finding was based on a study performed by McDonnell Douglas itself under the Supervision of the FAA. The results of the study were reviewed and concurred in by both the Air Force and a team of independent experts. McDonnell Douglas was chosen to do the $1.5 million study because "they are clearly the people who know the airplane best," an FAA official said. "The FAA doesn't have enough people who could have been devoted to this task to get it done" in a reasonable period of time, he said.

The DC10 was grounded for more than 30 days after the Chicago accident because of concerns that the pylon bulkhead might be too fragile. When it was returned to the air last July, FAA Administrator Langhorne Bond imposed a rigorous inspection schedule on the airlines using the plane, and promised a review of the pylon leading to a possible required change in design.

That schedule, which has required about a once-a-week look at the pylon and its support, can be relaxed dramatically because of the study's findings, the FAA said. Under the proposed new inspection schedule, the pylon and the points where it is attached to the wings would be examined only once a year or once every 5 1/2 years, depending on the pylon part involved.

Some changes in the pylon itself would be made if, for some reason, the pylon had to be removed from the airplane. However, an FAA official confirmed late yesterday, it is possible that the DC10s could fly to the end of their natural design life without any of the changes ever being made because it might never be necessary to remove the pylon.

"It seems kind of silly," he said, "to remove a pylon to fix something that happens only when you remove the pylon."

If the pylon should have to be removed, two bolts on the aft bulkhead flange would be replaced with countersunk bolts whose heads are flush with the flange.The heads on those two bolts are believed to have provided the leverage necessary to have caused the cracks in the flange that led to the crash. Furthermore, special tooling or blocking would be required during removal and reinstallation of the pylon to prevent that kind of cracking.

The thrust link, the centermost of the three attach points between the pylon and the wing, would have to be inspected every year, and replaced before the airplane reached 48,000 flight hours, about 13 years. Those links have been replaced on some planes.

The FAA's proposed rule is open for comment for 60 days. A final, binding rule, called an "airworthiness directive" will follow and should be in place by this summer officials said.

The FAA had earlier required that the DC10's stall warning systems be modified to provide completely redundant warning systems to both pilot and copilot. A key finding in the Chicago crash was that the crew might have saved the plane if their single warning system had not been knocked out when the pylon fell. That modification must be made by August.

The FAA also said yesterday it saw no reason to require many other design changes in the pylon or in the plane's controls that have been suggested by various and sundry critics, including members of Congress, the National Transportation Safety Board and American Airlines.

American Airlines declined to comment on all questions until it has had time to study the 2 1/2-inch-thick report.

Jack Cooke, a spokesman for McDonnell Douglas, said, "None of the proposed changes give us any heartburn." Nobody would estimate potential cost to the airlines or McDonnell Douglas of incorporating the changes.

FAA Administrator Bond said he was convinced that "You can pin any one of 15 different occasions in the process where if one person had intervened more strongly," the Chicago accident might not have happened. He cited the lack of aggressiveness of his own inspectors and the attention of maintenance crews, among other factors.