WASHINGTON (Reuters) - The United States and the European Union reached a tentative deal on Friday to dramatically expand aviation service and boost competition on both sides of the Atlantic. Negotiators made substantial progress in the State Department-led talks this week on the breakthrough agreement, which must be reviewed by EU transport ministers, who will meet next month. Europe is conditioning approval on a crucial side issue -- the U.S. proposal to ease some limits on foreign investment in American carriers. European negotiators first want to ensure the Transportation Department plan is finalized and that it would facilitate greater investment...