EASTERN AIR LINES didn't waste much time in putting on display in Washington the new European-built airplane it plans to buy - or in launching an advertising campaign to defend its decision. That's understandable. Eastern needs to get permission to operate the A-300 Airbus at National Airport to get the best possible return on its money. And it needs to convince Congress, among others, that its decision to buy European was not contrary to the best economic interests of this nation.

Eastern should have little trouble with the first of those problems if a full set of tests supports the initial reaction to the Airbus's operations at National. The people who heard it fly in and out said it was much quieter than the smaller jets operating there now, and the instruments near the runway supported their observations. If that holds true in later tests, the trade of greater congestion on the ground for less noise in the air will be worth while. But we add a note of caution: It is not the noise levels at the airport, where the initial measurement were made, that are critical. It is the noise levels up and down the river - over Arlington, Alexandria and the District of Columbia - that count.

Eastern's problems with Congress may be tougher.Questions about the deal have already been raised there and an investigation has already been promised. At issue will be the effect of this deal on the balance-of-payments situation and complaints by American aircraft manufacturers that they are at an unfair-disadvantage because of the financial assistance foreign governments give their competitors. The dominance of these American companies in the international airplane market has been an important factor in the trade balance for the last two decades. But they now see that dominance threatened just as the airlines embark on a new round of purchases that may total as much as $75 billion in the next dozen years.

Standing alone, Eastern's decision to spend its $778 million on 23 planes made by a French-West German-Spanish consortium does not have major significance. No other airline has bought an airplane abroad in the last 15 years.And a third of the components of the plane Eastern has chosen are made in the United States - a point it stressed in three-page advertisements in the newspapers this week. But that is not likely to satisfy the aerospace industry and its friends on Capitol Hill. They have the view that all commercial aircraft ought to be built in this country and that Congress ought to do whatever is necessary in terms of financing or subsidy to make that possible. International trade doesn't work that way, and the American companies aren't in a strong position to abroad for its purchase when the U.S. government is busy offering similar financial help to foreign companies who buy American.