A projected 150.7 million people will fly in March and April this year on U.S. airlines, according to Airlines for America. The industry trade group, also known as A4A, represents the major airlines. The estimate represents a more than 4 percent increase over last spring, when 145 million people took to the skies. "An expanding economy, employment gains and surging household net worth are also contributing to the growth in demand for air travel," said John Heimlich, A4A vice president and chief…