WASHINGTON — Senator Amy Klobuchar of Minnesota demanded that the Pentagon explain an “unbelievably dangerous” incident at Reagan National Airport across the Potomac River from the Capitol. “Why are your planes flying 500 feet below passenger jets full of Minnesotans headed from DCA to my state,” she asked. A collision between the Delta airliner and an Air Force T-38 was barely avoided. On the Delta flight were two pilots, three flight attendants and 131 passengers. The Delta flight, Number 2983, is flown regularly by members of the Minnesota congressional delegation for visits to their home districts.