two minutes after talking with air traffic control, 40 minutes into the flight, the plane's transponder goes dark. >> the plane's transponder is effectively the instrument that sends out a signal to air traffic control. it tells you what height it's at, which direction and what speed it's traveling. suddenly this giant 777 is blind to the world. >> and there's no easy explanation for why it happened. >> either it was intentional and someone tried to turn all those systems off at once or the pilot was unable to communicate, kept from communicating or there was a mechanical failure of some kind that took all those systems out at one time. >> then, minutes after the transponder stops, the 777 makes an unexpected turn heading west and way off course. >> that the plane turned immediately after the transponder went off is completely inexplicable and very worrisome. >> peter goelz is a former managing director of the ntsb. >> we don't know whether this was done voluntarily, whether it