What is known about the air crash between a passenger jet and a US Army helicopter

Congress will hold a hearing Thursday morning about what we know so far about the deadly midair collision above the nation's capital that killed 67 people in January and what might have caused it. The head of the National Transportation Safety Board that is investigating the crash will appear along with the leaders of the Federal Aviation Administration and the Army's aviation division to answer questions in the Senate. There will almost certainly be questions about the 85 dangerous close calls between planes and helicopters near Washington's Ronald Reagan National Airport that the NTSB identified in the three years before…