Europeans Act To Clear Way For Air Pacts, Conditionally
European regulators propose preliminary conditions that appear to clear way for approval of alliance between American Airlines and British Airways PLC that will dominate air traffic across Atlantic; European Commission angers UAL Corp's United Airlines, Lufthansa and Scandinavian Air System by proposing retroactive restrictions on their two-year-old alliance; moves sharpen tensions between European Commission and Clinton Administration as well as between commission and its own member states concerning international aviation competition at a time when increasing number of world's airlines have entered into cross-border alliances that have received exemptions from US antitrust laws (M)