Flightgear has changed the way heading indicators are electrically powered. The 707
used to use /systems/electrical/outputs/DG to power the two fluxgate heading indicators.
Use /systems/electrical/outputs/heading-indicator-fg instead. Before this change,
the directional gyros would always indicate the aircraft pointing due north, and be
unusable.
Directional gyros, as opposed to flux gates, would require additional work in
instrumentation.xml, to use heading-indicator-dg with appropriate parameters instead of
heading-indicator-fg.
See:
https://wiki.flightgear.org/Instrumentation#Heading_Indicator_(Directional_Gyro)
https://wiki.flightgear.org/Instrumentation#Heading_Indicator_(Electric_Flux-Gate_Compass)
