Washington area members of Congress yesterday said they are making headway in efforts to ensure that the proposed new policy governing future noise and air traffic levels at National Airport is not scuttled on Capitol Hill.

Twenty-nine senators and 137 members of the House of Representatives, it was announced, have signed letters supporting the plan, which would be implemented in October following years of debate over guidelines for National.

The area delegation's lobbying comes as Rep. Charles Wilson (D-Tex.) is preparing to submit an amendment that would delay implementation of the plan while further studies are conducted. Last year, congressional opposition killed attempts to establish a policy.

Yesterday, testifying at hearings at the Federal Aviation Administration, which owns and operates National, area members of Congress did express some criticism of the current plan, but endorsed it as a step in the right direction.

Civic leaders, meanwhile, were deeply critical of the proposal's failure to impose an outright ban on flights between 11 p.m. and 7 a.m., and said the plan would allow growth in traffic in the years ahead rather than the reductions they had sought.

Unveiled July 8 by Transportation Secretary Drew Lewis, the plan would hold "slots" for take-offs or landings to the current 60 per hour. Unlimited shuttle flights would be allowed, officials say, but loopholes that New York Air has used to expand service beyond its slot allotment would be closed.

The plan would cap passenger levels, now at 14.5 million per year, at 16 million.

Aircraft using National would have to meet specific noise standards, which would tighten between 10 p.m. and 7 a.m. and stop most jet traffic. Noise standards would tighten further in 1986, forcing airlines to switch to newer, quieter aircraft and engines now under development, according to federal officials.

Sen. John W. Warner (R-Va), who circulated the Senate letter of support, said "the plan admittedly is a compromise. But it is skillful one and a workable one." The letter was delivered to Secretary Lewis yesterday, he said. Among its signatories were Majority Leader Howard Baker (R-Tenn) and Robert Packwood (R-Ore.), chairman of the Committee on Commerce, Science and Transportation, which oversees the FAA.

Both Rep. Frank Wolf (R-Va.) and Rep. Michael Barnes (D-Md.) backed the plan but suggested major revisions. Wolf called for a ban on night flights, while Barnes proposed an immediate cutback on the number of flights in and out of National, which he said the plan probably will not do.

Wolf expressed optimism that Congress would not block the plan. "I think basically this policy has been well accepted by members of Congress," Wolf said.

However, an aide to Rep. Wilson predicted that his amendment to delay the proposal would attract wide support among many members who long have opposed efforts to curtail traffic at close-in National, which is so convenient to the Hill. The measure will be submitted when the transportation appropriations bill comes to the floor, according to the aide, who asked not to be named.

The aide said the Transportation Department had not followed its own rules requiring impact studies for new regulations like those proposed for National. Wilson's amendment would delay implementation of the plan until such a study had been completed.

Meanwhile, Rep. Barnes, together with fellow Democratic members of Maryland's delegation, Rep. Steny Hoyer and Sen. Paul Sarbanes, called for increased attention to Baltimore-Washington International Airport in solving the area's air traffic problems.

Sarbanes proposed new work on access roads to BWI, and consultation between the U.S. Department of Transportation and Amtrak concerning upgrading service to the airport. He also called for subsidized bus transport from downtown Washington to BWI, as the plan under consideration has suggested for Dulles International Airport.

Civic leaders, meanwhile, generally were far more critical of the plan than the members of Congress, but Eric Bernthal, president of the Coalition on Airport Problems, an alliance of civic groups, continued to call the policy "a first step in the right direction."

He said, however, that "we are being asked to accept a policy, which, in truth does not reduce air carrier traffic at National, not now and not ever . . . We cannot accept the idea that 300 jets overhead each day is a rational and acceptable volume of air traffic from this antiquated facility."

Bernthal called for a ban on the unlimited extra flights allowed for shuttles. He also asked for an absolute curfew, saying that noise levels as laid out would allow 24-hour-a-day traffic for certain types of airplanes.