$FG_AIRCRAFT is refered to as the aircraft directory or directories of FlightGear. It is available since FlightGear 2.4.0.
It can be set on the first page of FGRun (previous from aircraft selection). $FG_ROOT/Aircraft should never be added to $FG_AIRCRAFT, else aircraft will be listed twice in FGRun.
See this forum post for a description of the algorithm used by FlightGear (development version of January 2016) to choose between the various directories listed in $FG_AIRCRAFT and $FG_ROOT/Aircraft, as well as in the arguments of --fg-aircraft options (for the fgfs command line).