SEATTLE — The Federal Aviation Administration has issued a warning regarding potential weaknesses in parts of the tail of Otter seaplanes, the same type of aircraft that crashed off Whidbey Island last month. The emergency airworthiness directive, issued Tuesday, warns of potential cracks and corrosion in the movable surface of the horizontal tail that controls the plane’s pitch. The directive is not a result of the investigation into the fatal crash in Mutiny Bay, which killed 10 people, but does suggest one possible cause. The directive warns that, if not addressed, the cracks and corrosion …