Use the property browser to navigate to /ai (2nd entry from the top in the screenshot below):
http://wiki.flightgear.org/Property_browserThis will tell you the location/name and type of each entry.
Next, you can use arbitrary I/O means to access those properties and send them to another process - e.g. via the mongoose/httpd bindings (web sockets), the generic protocol, telnet/props etc.
http://wiki.flightgear.org/Telnet_usageYou can also use Nasal scripting (built-in Flightgear scripting language) to do all of this easily:
http://wiki.flightgear.org/Howto:Workin ... properties(It's really just 5 lines of code to do this)
Background info at:
http://wiki.flightgear.org/Howto:Work_w ... y_Tree_APIhttp://wiki.flightgear.org/Nasal_scripting_language