I have done another test and got a pleasant surprise. I still have the electric_always_available=0 setting in the aircraft.cfg. I will remove it completely from the aircraft.cfg to try a couple of things that will become clear through reading this post.

I exited and then restarted FSX and from Free Flight selected Load then 727-100 Cold-n-Dark (KSEA) and clicked Fly Now.

I left the plane sitting there for a while with the GPS displayed and ON with the Battery Switch OFF. Again after about 11 minutes the GPS went OFF. I turned the Master Battery Switch ON and the GPS came on too. I then decided to do a flight from KSEA to KIAD, so I started the APU and then the engines. After starting the engines, I moved both the AC Meters Selector from APU to GEN 2, the Essential Power Selector from APU to GEN 2 and the DC Meters Selector from BAT to ESS TR and after more than one (1) hour I have not lost any power to any avionics instruments. Have I found a solution other than having the electric_always_available=1?

The other thing that surprised me was that ALL the sounds (cockpit announcements and "dings" for other switches) worked without having to reload the 727. After my current flight, which still has 3 hours to go, I will do a whole computer restart and see if this works again or if I just got lucky this one time. Sorry, but I hate not completing a flight I have started.

Mark

EDIT. It is now some 2 hours since I posted this. I never finished my flight as my FSX went to a completely black screen where I couldn't see anything anymore, so there was no hope of landing. It does this every now and then for some strange reason.

I exited FSX and rebooted my computer. I did all the same things to start my flight in the 727. The sounds were NOT working, so no fix there!

As for what I did to start the 727. I followed the same as I did before. I did remove the electric_always_available=1. It is not there anymore, not even electric_always_available=0 is there. I'm doing a flight from KSEA to KSFO. I am flying at FL300 315 KIAS with 388nm to go and I still have avionics power after far more than 11 minutes since the start.  So my guess is that the power solution seems to be where I set those knobs that I mentioned.