Safety areas to catch airplanes skidding off the ends of National Airport's main runway could be tripled in length without harm to the environment, a Federal Aviation Administration study has concluded. FAA officials expect to make a final decision on the long-discussed project by the end of May.

The study recommends a $10 million plan to lengthen the existing 230-foot overrun at the runway's northern edge to 750 feet by filling in 7.4 acres of Roaches Run, an inlet of the Potomac River. A marshy area of the same size would be dug out elsewhere on airport property to replace the area lost as a habitat for aquatic life.

Work crews would smooth and upgrade existing land at the runway's southern end to extend the 335-foot overrun there to 1,000 feet and would replace rigid posts that support landing lights with ones designed to give way if struck by a plane.

The FAA is requesting funds for the project in fiscal 1983. The study is part of a process that began in 1977 to bring the runway's overruns closer to current federal requirements of 1,000 feet of safety area at new airports.

Opened in 1941, National is exempt from those rules.

The Jan. 13 crash of Air Florida Flight 90, which killed 78 people, has refocused attention on National's runways, due to speculation that the Boeing 737's pilots might have aborted their takeoff if the runway had had a longer safety area.

Pilots also have expressed concern over the main runway's relatively short length (6,870 feet, compared with 11,000 feet at newer airports such as Dulles) and its curved approach path. Neither runway length nor approach path is addressed in the environmental impact study.

National FAA studies have shown that airliners overrun or undershoot their runways about once in 550,000 landings, creating the statistical likelihood of such an incident about once in five years at National. Overruns of 750 feet are sufficient to accommodate 82 percent of such incidents.

National's first recorded overrun occurred in 1976 when an Eastern Airlines 727 landing from the south went off the runway, coming to rest about 15 feet from the Potomac shoreline, according to the FAA. The next year an Eastern 727 overshot after aborting a takeoff. No one was injured in either incident.

Extension of the north overrun to 1,000 feet is not feasible, the study said, because it would then span Roaches Run to the public park on Gravelly Point. That would create security problems, close off water access to the 53-acre Roaches Run Waterfowl Sanctuary and necessitate removal of a boat ramp, according to the study.

The National Park Service has expressed concern about the effect of longer overruns on the parkland it controls north of the airport. But the environmental impact study said the recommended alternative would create "no significant impact" on aesthetic and noise conditions there.

The FAA also considered and rejected as impractical proposals to fill in the Potomac at the other end of the runway in order to avoid filling in Roaches Run; to fill in nothing new but to equalize the overrun area at the two ends; and to shorten the runway length to allow extra overrun area.