Stby Lighting on Ovhd

[farrokh747]

Hello...

Q: on switching gen 3 & 4 off, and isolating bus tie 3 & 4, we see 4 panels illuminated by the standby bus -

IRS
eng start/jett
anti ice
cabin alt control

Does anyone know the reasoning behind this, and on what basis did boeing decide that  these 4 should remain illuminated in case of failures... ? Why not the elec panel, or hyd ...
cheers!

fc

[Hardy Heinlin]

Hello,

I guess, because they are the most important controls on the overhead to get the aircraft safely from A to B.

Hydraulic controls are automatic. Electrical bus control is automatic, or irrelevant if you just have battery power anyway. Fuel control is automatic, or irrelevant if your engines are shutdown anyway.

You want to save battery power and need to find the best compromise re power consumption versus "user friendliness". E.g. you have the dome lights, the aisle and the captain's flood lights fixed at 12 V, just enough to see the controls on the panels, and just low enough to provide battery power for the duration of a complete idle descent from FL450 to sea level.

Part of this compromise is probably those few essential controls being  additionally backlit (also at fixed 12 V). Some of this stuff is also part of the towing control, when you enter a cold and dark ship just for towing: e.g. the brake pressure indication, the nav lights etc. are then essential.


Cheers,

|-|ardy

[farrokh747]

Thanks hardy!