After months of uncertainty, the Reagan administration's proposed policy to regulate noise and congestion at National Airport appeared yesterday to be gaining crucial support on Capitol Hill and in the airline industry.

Marking up an appropriations bill, a Senate subcommittee yesterday removed language inserted by the House to block the plan's limits on traffic levels. And Rep. Charles Wilson (D-Tex.), who led the House fight against the plan, said he understood that airlines previously opposing the plan had reached an accommodation with Transportation Secretary Drew Lewis.

New York Air was among the most vigorous opponents of the policy, on the grounds that it would effectively shut the airline out of National. Last night, its president, Neal Meehan, said "we've been reassured by the Secretary of Transportation that New York Air will be left with a strong competitive position" when the plan takes its final form.

He said that would be done through ways that "slots" for landings and takeoffs are allocated, but he declined to elaborate further. Because of these assurances, Meehan said, "we have elected not to lobby against the policy any further."

"If the airlines are all satisfied that they're being treated fairly, why should anybody object?" Wilson said yesterday. He said he had no objection to limiting traffic at National, as long as all airlines were cut back equally.

Yesterday, Lewis said through a spokesman that certain "misunderstandings" concerning the policy had been cleared up with airlines. "We have fine-tuned our policy," he said, "and are in the process of clearing it through the normal OMB process. But the basic structure of the policy remains the same."

He declined to disclose what changes had been made to the policy.

Lewis's statements came shortly after the transportation subcommittee of the Senate Appropriations Committee deleted an amendment inserted by the House when it considered the bill last month.

That amendment would block funding for any plan that would reduce traffic below levels of July 31, the busy Friday just before the air controllers went on strike. On that day 808 commercial take-offs and landings were recorded at National.

The measure was removed by unanimous consent, at the recommendation of subcommittee chairman Mark Andrews (R-N.D.), a staff source said. The bill could go to full committee next week and onto the Senate floor by the end of the month.

To completely clear obstacles to the plan, conferees from both House and Senate would then have to agree to keep the language out of the conference bill, which would have to pass both houses of Congress.

Unveiled by Lewis in July, the airport plan was billed as the final chapter in a decade of debate over controlling growth at National. The plan originally was to take effect late this month but the Transportation Department now plans to seek a one-month extension while public comment on the policy is evaluated.

The policy would establish new quotas on "slots" for take-offs and landings at National, impose noise regulations and put a maximum figure on the passenger load at the airport. Transportation to the under-used Dulles International Airport would be improved in an attempt to draw more airline business there.

As originally worded, the plan would close loopholes by which some airlines had flown into National in excess of their slot allocations. New York Air, for instance, regularly recorded 48 takeoffs and landings daily, even though it officially had slots for 18. The provision to close the loophole was the basis of the airline's objection to the policy.

New York Air spokesmen were unavailable for comment late yesterday on their discussions with the Transportation Department.

Congressional opponents of the airport plan argue publicly that it is anticompetitive, would inconvenience constituents and conflict with other federal regulations. With National only minutes from Capitol Hill, some opponents of the plan also have stressed the inconvenience to legislators if flights to their home districts were curtailed.

The exact amount of air traffic the plan would allow cannnot be predicted, as it would depend on airline demand, federal enforcement and a complex slot formula. However, it is generally agreed that the reduction would be nowhere near the 25 percent cut caused by the controllers' strike.

After the plan was made public, supporters gathered the endorsement of all of the Washington area's senators and representatives and major leaders of local governments. But in September, the House inserted the amendment by a vote of 204-188 and the fight's focus shifted to the Senate.

Sen. John Warner (R-Va.) coordinated the fight there for the administration, with Transportation Department officials lobbying senators known to object to the plan. A spokesman for Warner said that many senators were now satisfied that service to their home states would not be seriously disrupted.

In another development, the bill reported out yesterday by the Senate panel would appropriate $290 million in capital funds for Washington's transit authority for the current fiscal year. Metro's construction plans are based on getting $315 million, the sum specified in the House version of the bill.