Avrflr
Can the experts also explain how the > 138 gallons of water then chokes the engines, causes the plane to crash and leaves not a trace of water in the wing tanks?
We’re talking about the centre tank, not the wing tanks.
NSEU
but even if all 138 gallons of ice thawed during the descent and was scavenged, would mixing it with over 3000 gallons of remaining wing tank fuel degrade the engine performance so much?
No, it would be fine.
Thanks for the pics. Got a system schematic?
Tanimbar
SyEng, you say the paragraph is general, not 777-specific, so is it possible to confirm the destruction of any stratification in the wing tanks by the systems you mention for the 777?
No, but I can’t imagine the 777 tank environment is so different to the types I’ve worked on. Secondly, in many years experience, I’ve never come across a single reference to fuel stratification as an issue for aircraft fuel systems.
First, several folks here seem to be assuming that all the aircraft systems were working as designed and that, e.g. “the CT would have emptied early on” or that the water would have been scavenged into the wing tanks. This approach doesn’t make too much sense. If it did, the corollary is that this aircraft systems were operating as the designers intended, and 777s would be dropping out of the sky every other day. There must have been failures (not necessarily equipment failures).
My inferred design philosophy for the CT transfer system is:
-CT boost pumps operate until the inlets uncover. Inlets are positioned forward in the tank to avoid picking up large quantities of water in climb/cruise attitude.
-Wing tank boost pumps now take over supply to engine feed.
-CT (fuel) scavenge jet pumps with motive flow from wing tank boost pumps empty the CT at a leisurely pace. Any water picked up is safely mixed with wing tank fuel before being fed to the engines.
-CT water scavenge jet pumps get motive flow from the CT boost pumps, so only operated when CT boost pumps do.
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.