Alaska's largest commuter airline has been temporarily shut down by the Federal Aviation Administration for correction of safety problems in the wake of a series of crashes. Ryan Air Service must make a series of operational and management changes, including the replacement of the firm's president and other top company officials, FAA officials in Anchorage said. Company officials said they hoped to have their planes back in the air within a month. The move follows an extensive FAA investigation that was prompted by a November crash in Homer that killed 18 people. Ryan has had 12 crashes since 1980 that killed a total of 30 people, including nine accidents in the last three years. The plane that crashed in Homer is suspected of having been overloaded. Ryan flies to 70 communities in western Alaska, many of them inaccessible by road.