Hi fellow Aermacchi-MB-339 fans.
The compass card in the Fgaddon version doesn't turn.
Fixed mine as follows.
With Aermacchi-MB-339 installed from addon hangar, I copied it from Documents>FlightGear>Aircraft>org.flightgear.fgaddon>Aircraft to my personal hangar; after which I did an uninstall from the regular hangar.
I modified the copied version in Documents >my hangar> as follows:
In Aermacchi-mb-339\Models\Interior\Panel\Instruments\hsi\hsi.xml I modified line 171 from <property>instrumentation/heading-indicator/indicated-heading-deg</property>
To <property>orientation/heading-degree</property>
Compass card now rotates and indicates correct magnetic heading beneath rhumb line; and all other functions-orange VOR pointer and white Tacan pointer- work correctly, as before.
Halleluja.
Best Regards