The airline reservation business has burst into a frenzy of litigation, and after Sabre brought suit against British Airways and Covia’s Apollo lodged a complaint against Sabre, System One Direct Access Inc, the up-and-coming third player in the US market and affiliate of Texas Air yesterday announced that it was suing United Airlines and its Covia Corp alleging that they have excluded System One from the market through exclusive dealing contracts with travel agents, punitive damages clauses and other tactics in violation of antitrust law.