WASHINGTON -- Airlines that operate some Boeing Co. 787s and 747s will be required to steer clear of some very large thunderstorms under a new rule from the Federal Aviation Administration.The airworthiness directive to be published Wednesday is meant to prevent ice build-up inside General Electric engines. The storms include those with clouds more than 60 miles across.The FAA says it knows of nine instances where ice being sucked into an engine caused it to lose power. Two of those incidents caused engine damage on 747-8s.Japan Airlines has changed some routes already to avoid those storms. In the U.S., the rule covers seven 787s flown by United Continental Holdings Inc., as well as some cargo-hauling 747-8s. A spokeswoman says United has not yet changed any routes because of icing.