on which operating system? windows or linux or mac? it makes a difference in the instructions to fix the problem... but then, there's this from the wiki
http://wiki.flightgear.org/Navdata_cachehttp://wiki.flightgear.org/Navdata_cache#Known_issuesgenerally speaking, IF i am understanding your error message correctly, the fix has been to simply delete the navdata cache file from the users' local cache directory... that directory is generally either
C:\Users\<yourusername>\AppData\Roaming\flightgear.org\
or
C:\Documents and Settings\<yourusername>\AppData\Roaming\flightgear.org\
the AppData directory is a hidden folder so you must enable seeing hidden folders... then just rename or delete the navadata cache file...
on linux, the directory is generally ~/.fgfs which is also a hidden dot directory... again, just delete the navdata cache file...
i don't know what the steps are for mac systems but all of these have been talked about numerous times in these forums... if you search (top right of the forum logo header) for your error message, you should easily find the information you need...
"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."