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.
Failure 1) possible contributory factors:
1a) Water remains frozen during turnarounds precluding effective drains operation (See NSEU post 292).
1b) Water remains frozen during most of flight precluding effective water scavenge operation.
1c) FOD
Failure 2) possible contributory factors:
1a) The most nose-down attitude of the whole flight (including descent) occurs when landing flaps are selected. This I think is likely true of many civil types. Perhaps someone can confirm for 777. So any residual liquid in the CT moves forward at this point.
1b) CT boost pumps remain running throughout flight (I imagine this should generate a warning).
1b) CT boost pumps switch back on uncommanded when pick-ups become covered (is there any way (including failures) that this can happen in the 777 system?).
1c CT boost pumps switch back on by crew action.
Please remember that the CT was breached and contaminated by firefighter’s foam and hydraulic fluid after the landing. It is not clear from the AAIB report that they even tested for water in the CT. It sounds like it may have been a pointless exercise.
It is conceivable that in the final seconds of the approach, with the increase in pitch, the CT boost pump inlets uncovered again allowing engine feed to resume from the wings, helpfully flushing evidence from the feed lines but not in time for the engines to spool up enough to make a great difference to the outcome.
Like I said before, I’ve not seen anything here or from the AAIB that rules out this theory. I’m open to offers, though.
I see some problems with it (maybe not fatal):
1. CWT boost pumps stop at 900kg - which is a fair bit of "something" to be in the tank.
2a. Given that CWT empty is the expected state (late in flight), CWT reading 900kg+ would be a serious anomaly that they would have picked up on by now.
or
2b. If theres 900kg of something (ice) in CWT
but it is reading empty, then you've got a ton
less fuel than expected in your wing tanks after fuel scavenge - which doesn't seem to fit the data either.
3a. If CWT boost pump operation is recorded, then having them switch on unexpectedly would be a serious anomaly that they would have picked up on by now.
or
3b. Even if CWT boost pump operation is
not recorded directly, I would expect the CWT low pressure advisory (when they turn off) to be. By your theory, they turn
off again close to landing (when it goes nose-up, as we know it did), which would generate this warning, which would have been spotted.
Overall, I can't see CWT feeding the engines direct at the end of flight without it showing one way or another in the recorded data, and I think this would have been flagged by now if it was the case. Contamination of some sort (at least water based) from CWT via fuel scavenge would go via wing tanks, where they would have found evidence of it.
I don't have an answer, but I'm struggling to see, with the info we have now, how the center tank could be involved.
