The Federal Aviation Administration is investigating why multiple commercial flights about to land at Ronald Reagan National Airport, near Washington DC, repeatedly received midair collision alerts over the weekend when there were no other aircraft nearby.The alerts occurred only miles from the site of the deadly January 29 midair collision of American Eagle 5342 and a US Army Blackhawk helicopter.Several flight crews reported the mystery alerts on their Traffic Collision Avoidance System, known as TCAS, on Saturday “indicating another aircraft was nearby when no other aircraft were in the area,” the FAA said in a statement on Monday.“We were about...