co-pilot andreas lubitz may have practiced a control descent on his outbound flight that day, potentially rehearsing for the crash that would take place hours later. tom costello with details. this is extraordinary. >> reporter: it is. it's a preliminary report that comes out from the french investigative agency. it describes the flight from dusseldorf into barcelona. the flight immediately prior to the fatal crash back to dusseldorf. it crashed into a mountain, of course. we have a graph that shows the information from the flight data recorder which has the co-pilot programming erratic altitudes into the flight computers while the captain was out of the cockpit, presumably at the lavatory. moves the plane from 100 feet in altitude -- down to 100, i should say, to 49,000 feet. the red sideline the actual altitude. what we believe is happening is the co-pilot was literally dialing in different altitudes. 100 feet up to 49,000 feet. down to 100 again.