Originally Posted by
dxzh
In this case, the centre tank reduced to a little under 750kg indicated at its lowest at the time of the engines switch to main tank supply, before apparently increasing for some reason by 125kg or so to 800 kg indicated a few minutes later.
The signal is slightly noisy - there are other blips - and that could account for this. Also, consider that when the OJ pumps switch off, does fuel already in the pipes / pump but upstream of the check valve fall back into the tank ? That might be enough to account for this signal.
How localised could that depth of water be given the geometry of the tank, position of ribs and the tank's shape during flight? Can anyone produce a diagram of exactly where everything is at the bottom of the tank near the OJ pump inlets?
I think there have been drawings posted way back in the various threads on this incident - but you'll have to trawl through a lot of posts to find them.
- I note from the rate of scavenge in Figure 1 the rate increased as the level of fuel indicated declined to 0kg. Could that suggest that as the level lowered: the rate of scavenge appeared to increase as less fuel/free water was accessible than expected as it was trapped where the scavenge lines were not working locally; and/or an expected increase given the vagaries and the geometry of tank?
Could be, but I suspect it is more likely a result of the way it's controlled from float valve in main tank - I think the final scavenge rate works out higher than the fuel burn rate when scavenge started.
Yet as a Mad Hatter is drawn to a tea party, I am curious about the possibility given the suggestion in the interim report that air at least might possibly pass through the OJ/jettison check valve (and presumably therefore through the OJ pump when selected OFF too) ...
I think that was only if
all the main tank inlets were blocked and suction feed woudl pull in either nothing or air from the CT. Nothing vs. air is not a great choice of fuels
. Also, if this happens they've found different effects to those recorded on this flight, so it didn't happen in this case.
Overall, I would have thought that the important thing to do with the fuel system FDR data (ie. fig 1 in the report) would be to compare with a similar but incident-free flight. We don't have the data to do that but the AAIB will - I would be very very suprised if this hadn't been done already and anthing unusual in the fuel system data would have been looked into.