NSEU
Steve....
I think I'm looking at the same blurry picture from the AMM, but enlarging it, I don't see the CT water scavenge system hooked up to wing tank pumps. So, with the CT O/J pumps OFF, I don't see the CT water scavenge system operating.
Yes I agree with you now. The centre tank water scavenge pump motive flow comes from the centre tank pump, so with the pump off it will be disabled.
From Boeing AFM:
Quote:
The design provides indication of low fuel quantity in the center tank based on input from the Fuel Quantity Processor Unit (FQPU) for a crew controlled wet-shutoff. If the crew fails to respond, or there is an error in the fuel gauging system, the ELMS system will de-power each center tank pump after 15 seconds of continuous low pressure based on input from the pump pressure switch.
So it looks like the pumps will shut themselves off and not run 'dry', based on pump outlet pressure. The FUEL LOW CENTER EICAS appears once the centre tank is <=900Kg.
I have been studying the manuals for the last two hours. I have found a statement in the Schematics manual that shows the Fuel Low Centre message coming from the FQPU, so I agree that it is quantity derived.
I cannot see any reference to a 15 sec T/D or ELMS auto shut off. There is an ELMS driven Centre pump inhibit that shuts down the centre tanks, but this is mainly to do with lack of power on the aircraft. It is a load shed device.
In the ELMS there is a 30sec T/D. With centre pump selected ON, and low output pressure, it will set the message FUEL PUMP CENTRE L/R Advisory/status.
I do not doubt your manual, just cannot prove it from the maint manuals that we have.
Shows how difficult the B777 is for engineers. All the signals are shown entering an ARINC 629 bus, and on the next page coming off into AIMS. Impossible to say if they go ellsewhere!
Perhaps we poor engineers need an AFM to find out how it works.