By Alwyn Scott
SEATTLE, Nov 27 (Reuters) - The U.S. aviation regulator on Wednesday required U.S. airlines to inspect any General Electric Co engines on Boeing Co 787 Dreamliner and 747-8 aircraft that experience icing problems which could lead to loss of thrust and forced landings.
The Federal Aviation Administration's decision to issue an airworthiness directive had been expected. The agency also required airlines to prohibit pilots from flying into high-altitude, icy weather conditions.
The FAA said that in certain kinds of high-altitude thunderstorms, ice crystals entered and damaged the engines, causing temporary loss of thrust, or in some cases increased vibration with no loss of thrust. The problems affected nine flights, and in two cases involving 747-8 planes it affected two of four engines.
The FAA said its concern was that "unrecoverable thrust loss" on multiple engines "could lead to a forced landing."
General Electric noted that there had been no unrecoverable thrust loss in any of the nine incidents.
The directive is effective immediately and requires airlines to inspect General Electric GEnx type engines if the "engine thrust" indicator appears in flight.
The FAA said its order only applies to 14 aircraft operated by U.S. airlines. Foreign aviation regulators typically follow the FAA's lead.
Last Friday, Boeing and GE said they had alerted airlines about the risk of engine icing and the loss of thrust that could result, and were working with them to resolve it.
Boeing and GE had said there were six incidents in which icing occurred, but the FAA's higher tally included three incidents in which the engines "experienced a vibration during the ice-crystal storms, but didn't lose any thrust," GE Aviation spokesman Rick Kennedy said in an email.
The issues occurred with GEnx-2B engines on Boeing 747-8 passenger and 747-8F freighter planes. It also occurred on GEnx-1B engines on 787-8 passenger planes.
Japan Airlines Co Ltd has said it is replacing 787 Dreamliners on two routes because of the problem.
United Airlines, the operating unit of United Continental Holdings Inc and the only U.S. carrier currently flying the 787, said it was not making changes because of the issue.
Atlas Air Worldwide Holdings, which flies 747-8F freighter jets in the U.S., said it was alerted to the problem by Boeing on Nov. 14, and had already taken steps to avoid the weather condition. It expected only minimal impact on Atlas and its customers, spokeswoman Bonnie Rodney said.
GE said it has a software fix and will put it into effect early next year. The regulator has not called the engines defective, and that the situation occurs only when "encountering weather conditions at 40,000 feet for which the engines were not tested at certification," Kennedy said.
"We had encountered a similar situation with our CF6 engine in the 1990s and early 2000s, and we successfully resolved the issue with a modification to the control system," he added.
In those earlier incidents, the FAA said, temporary loss of thrust occurred when the engine throttle level was changed, or when a flight was descending.
The latest incidents happened at cruising altitude and in icing weather conditions that are invisible to radar and unknown to the pilots. The events "caused permanent damage" to part of the engine, the FAA said.
The FAA said five of the nine incidents involved 747s, and the rest were with 787 jetliners.
Three of the 747 incidents did not involve loss of thrust, but "at least one engine showed elevated vibrations" while the airplane was flying in bad weather.
After the plane left the storm, the engines ran normally for the rest of the flight, the FAA said.