Here is a brief summary of what the pilot warning system does, what we know about what went wrong and background about the safety notices provided to pilots, known as NOTAM. The FAA system that is meant to distribute notices to pilots on hazards failed at about 2 a.m. Eastern Time, officials said. The FAA ordered airlines to put a halt on all domestic departures until 9 a.m. Eastern time while it tested whether crews had managed to restore the system and bring it back online.