The Federal Aviation Administration is planning tighter rules on when airlines may postpone maintenance amid findings that work at Eastern Airlines--and possibly other carriers--often has been delayed to avoid disrupting schedules, FAA officials said. They said the agency has been looking at 10 airlines to determine if federal maintenance requirements are being abused. Except for Eastern, none of the other airlines were named. An FAA report made public this week said that Eastern had interpreted FAA maintenance requirements so broadly that it amounted to a "misuse" of the latitude given to air carriers on repairing items that do not in themselves pose a danger to flight. Eastern executives said that, at the request of the FAA, the airline changed its maintenance policy last August.