Originally Posted by SyEng
Now, here are the 2 functional failures necessary to support my theory (post 216):
1) Failure to scavenge effectively CT water.
2) Engine feed source switches from wings to CT during approach.
Re 2).... This scenario seems very unlikely. Remember that with the crossfeed valves closed, each (L/R) CT pump feeds its respective engine. the control of the CT pumps would have to fail almost simultaneously to make both engines receive the contents of the CT.
Each CT pump is controlled by an
independent ELCU (electronic load control unit). I haven't yet seen an internal diagram of the ELCU, but the control relay in the ELCU probably needs to see "inhibit circuit" deactivated AND the pump switched ON (by the pilots) before it will switch on: The pilot has to have the ability to turn OFF the pump in the air by deslecting the switch, irrespective of automatics.
Note that the ELCU control relay needs to be energised to turn on the pump. If the coil of the relay failed, relay would relax, turning OFF the pump. If relay contacts had fused ON (earlier), the CT pump would run out of fuel and the crew would have several indications of this.
The inhibit signal comes from ELMS. The inhibit is only for loadshedding under certain circumstances. As far as I can see, the pump doesn't switch itself off if pump pressure is low. Normally, the pilot responds to an EICAS message to turn off the pump during normal ops.
Note that low pump pressure turns on the overhead PRESS light for the pump, but unfortunately, the Boeing Maintenance Manual D&O section doesn't offer any insight into what turns on the EICAS message (low pump pressure and/or fuel quantity). We know that 900Kg is the trigger point, but whether this is how high the fuel pump pickup is in the CT or whether this is FQUIS generated, I don't yet know. Swedish Steve?
Note that fuel jettison system cannot turn on the CT pumps automatically if they have been manually turned off.
Rgds.
NSEU