Design of B777 overhead buttons
 

Hi,

I'm just curious.

Why is Pri Hyd buttons labeled as "ON/FAULT" and not just ON/OFF?

Why is L ENG bleed air switch "ON/OFF" and not "AUTO/OFF" like the APU Bleed?

Anyone know the meaning behind this?

That should be fairly clear from the sim..the "fault" bit on the switch is a light not a label. Take a look when you are in the box..turn the primary pumps on and the fault lights will go out when appropriate, but it/they will remain on if there is a system fault...so it is not just an ON/OFF switch..., it's an ON/off switch with an attached FAULT light.

As for the Bleed switches, it's a bit more vague..ARMing the the APU valve puts it into the hands of system logic, whereas turning the engine Bleed valves ON will open the bleed valve as long as bleed air is available and there are no faults..but TBH I don't think there's much practical difference between the two.

I was thinking that the AUTO/OFF on the APU was correct, as it is on until it senses engine bleed above a certain pressure (idle) and turns off to prevent backpressure damage to the APU. Thus AUTO prevents the driver from turning engine feeds on, going above idle, and overcharging the APU.

Thanks for the enlightenment!