The chairman of a key Senate subcommittee is expected next week to propose that the Senate support a reduction in the number of passengers allowed to use National Airport.

The proposal by Sen. Mark Andrews (R-N.D.), chairman of the Transportation Appropriations subcommittee, calls for improved access to the area's two suburban airports through new bus service and possible helicopter service from downtown Washington.

The proposal is intended to counter House opposition to suggested cutbacks in the passenger ceiling at National. Andrews' position is detailed in a draft report that he will submit to the subcommittee on Tuesday when it reviews the Senate version of the 1984 transportation appropriations bill, which includes the National Airport budget. A copy of the draft was obtained by The Washington Post.

Last month, the House voted 249 to 170 to block a proposal by Transportation Secretary Elizabeth Dole to reduce the current 16 million passenger ceiling at National to as low as 14.8 million. Dole's measure is intended to spur future growth at Dulles and Baltimore-Washington International airports. The current passenger traffic at National is about 13.5 million persons a year.

Many members of Congress have traditionally been wary of moves perceived to cut service at National, which is a few minutes' drive from Capitol Hill and is used frequently by many members. Airlines, favoring it over Dulles and BWI, have also fought restrictions at National.

Dole argued that the 16 million cap was based on expectations National would reach that level around 1985, after which new growth would have to go to the other airports. With growth proceeding more slowly than expected, a lower cap is needed to meet the scheduled shift in traffic.

Andrews' draft backs Dole's reasoning, saying the House "missed the mark" in its vote, and proposes removing the language the House inserted in the appropriations bill to block any reduction in the passenger cap.

In an apparent effort to answer concerns that the policy might cut flights to home districts, Andrews' proposal calls on Dole to make efforts to assure retention of "an appropriate number of medium-range flights out of National Airport."

It also suggests that Dole and the airlines meet and work out an agreement assuring that there would be no further changes in the airport policy for a specified period of time.

The report also backs another Dole proposal to allow two more takeoffs and landings by commuter airplanes at National per hour and supports the Federal Aviation Administration's (FAA) recent decision to allow the new Boeing 757 jetliners to use National.

The report argues that a major issue is "how quickly, conveniently and reliably" travelers can get from downtown to an airport. It notes that Amtrak service at BWI and the upcoming opening of a link between Interstate 66 and the Dulles Access Road will help.

But it says that further steps are needed. The report asks Dole to conduct a detailed study of helicopter service that the FAA has proposed between Washington and the two outlying airports.

Andrews would also provide $3.5 million for the purchase of 14 new buses to improve ground access to Dulles.