The Federal Aviation Administration has issued a warning to commercial airlines that an emergency system on the Boeing 757 jetliner may not operate properly if both engines fail in flight. But Dick Schleh, a spokesman for Boeing Commercial Airplane Co., said in Seattle the FAA notice comes in the wake of a service bulletin Boeing issued last July to airlines that use 757s. "We noticed that there was some contamination of a circuit that could possibly prevent the ram-air turbine from deploying," he said. Without an operative turbine, power cannot be generated to keep key parts of the plane's hydraulic system working if both engines fail. However, Schleh said the system has never had a chance to fail in flight because no 757 has ever lost power in both engines. The FAA notice gives airlines until Nov. 10 to respond. The agency then can issue an "airworthiness directive" ordering airlines to correct the problem.