WASHINGTON — About 51 million Americans are expected to fly on U.S. airlines in the three weeks around Christmas and New Year’s Day, a 3.5 percent increase over last year. That’s the forecast Wednesday from Airlines for America, a trade group for most of the leading U.S. airlines. The group credits an improving economy and airfares that have lagged below the rate of inflation. Related ArticlesDecember 7, 2017 DOT drops proposal to force airlines to disclose bag fees December 7, 2017 Frontier pilots demonstrate in Denver amid pay fight December 5, 2017 Centennial’s Boom Supersonic partners with Japan Airlines on orders, $10M investment December 2, 2017 Meet the woman who’s spent 60 years making the skies a little friendlier November 30, 2017 Even procrastinators can find deals on holiday travel The forecast counts domestic and international flights between Dec.