Thanks a lot, that is actually very helpful.
- Yes, when I try to enable HDG, it does blink briefly and then does not stay engaged.
- Also, you are correct: I incorrectly said APU, but I actually meant GPU / external power.
- My test case is with the 4-bladed GNS / Garmin variant.
- What I observed is this:
1.- With ther aircraft with on battery power only, HDG does not latch.
2.- With GPU connected (without batteries on) HDG can latch on the ground. And the aircraft powered up, after engine start, HDG can still remain engaged but when I remove GPU/external power, HDG drops out immediately after a couple os seconds.
So the behavior really seems tied to the electrical source / bus transfer.
I can keep testing and provide a more exact switch-by-switch sequence if that helps, but from my side it definitely looks like X-Plane is dropping HDG right after the command is sent.
Thanks again for looking into it.