I realize the following question has little practical use .. but I am actually quite interested. I also understand it's dangerous to draw conclusions from incomplete electric diagrams such as the FCOM… but;
About the forward airstair, exterior control - standby mode. My FCOM says that holding the switch in STANDBY provides DC power from the
battery bus for airstair operation. The BAT switch on the flight deck does
not need to be ON when operating the airstair on standby from the exterior control panel.
I was under the impression that the BAT switch and the STANBY POWER SWITCH (in BAT) were the two switches that could power the Battery Bus while on the ground, cold & dark, before TR3 is available.
Is there any easy explanation to this?
Thanks in advance