Yeah, hard-coded instruments are initialized by the so called "instrument manager" which reads in requested instruments/systems from the aircraft-set.xml file/property tree, a few default configurations are provided and stored in $FG_ROOT/Aircraft/
For details, see:
https://gitorious.org/fg/flightgear/sou ... r.cxx#L110You'll see a few default instruments loaded via preferences.xml:
https://gitorious.org/fg/fgdata/source/ ... s.xml#L454Which in turn refers to :https://gitorious.org/fg/fgdata/source/Aircraft/Generic/generic-instrumentation.xml
Obviously, aircraft can specify their own defaults and override such settings - so the details depend on the your particular aircraft.
All this is done while booting, so wasn't designed to be supported at run-time - also, scripted Nasal instruments are a completely different mattter.