Domestic business and commuter aircraft makers told the U.S. International Trade Commission that their survival was being threatened by unfair competition from foreign manufacturers. Edward Stimpson, president of the General Aviation Manufacturers Assn., said foreign firms now have 43% of the U.S. business jet market. He also testified that 336 U.S. commuter and business planes were exported last year, against 4,200 in 1974. Stimpson charged that foreign builders were subsidized. Subsidization was denied by witnesses of the French firm Avions Marcel Dassault-Breguet and the British firm Short Bros. PLC.