Mine were all gone so I re-synced them and all came back. I do sync after every flight. I still get flights with wild and crazy gps stuff (I am pretty sure I didn't fly to Africa again yesterday). I wonder if that has to do with putting it in airplane mode? The GPS was still on and 17 satellites.
I also use AirData, but you don't need it for regular flight sync. What I like about AirData is the battery info.