SEATTLE, OCT. 17 -- The Federal Aviation Administration has warned commercial airlines that an emergency system on the Boeing 757 jet might not operate properly if both engines fail in flight.

Dick Schleh, a spokesman for Boeing Commercial Airplane Co., said today the FAA notice follows a service bulletin Boeing issued last July to airlines that use 757s.

"The service bulletin followed efforts on our part to correct what had occurred in a few cases," he said. "We noticed that there was some contamination of a circuit that could possibly prevent the ram-air turbine from deploying."

The turbine is a device that "pops out" of the jet's underside when both engines fail in flight. Air spins the turbine, generating enough power to keep key parts of the plane's hydraulic system working. This allows the crew to operate such devices as wing flaps, to maintain control of the plane.

The service bulletin was issued "only because we conducted some routine ground tests and discovered in one particular instance it didn't operate when it should have," he said. Some airlines conducted their own tests and also found instances of failure, he said.

The system has never had a chance to fail in flight, because no 757 has ever lost power in both engines, Schleh said.

Modifications in the turbine's circuitry to avoid the problem were implemented on the 757 assembly line "some time ago," he said.

The notice gives airlines until Nov. 10 to respond. The FAA then can issue an "airworthiness directive" to have the problem corrected.