BA038 (B777) Thread

[chris weston]

It's fuel

SuperT

I'm not sure we can imply that a problem with cavitation is only a function of low inlet pressure on a pump - with or without nucleation within the liquid phase, although I agree, nucleation will help considerably.

Presumably you're imply waxing or freezing of the fuel thereby restricting flow rates to the donkeys, lowering fuel pump inlet pressures and causing the cavitation. My own non specialist view is that the waving/freezing hypothesis died the death aeons ago on this most worthy of threads in the light of the published AAIB data showing fuel fpt to be < - 50 C - a suspiciously low figure.

The following, somewhat familiar by now hypothesis, alleges no failure of the pumps.

I'm rusty on this but cavitation can easily produce a mixture of true gas phase mixture fuel components (which may or may not contain air as well) from the mixture alone by mechanical induction in a liquid phase mixture if component(s) in the fuel mixture have a low enough Mr or levels of intermolecular bonding and the kinetic energy input is high enough.

I'm guessing, but I reckon any gas phase component's t½ at say +80C will be long enough for it to be a nuisance.

And and ……if the fuel has stratified in the tanks as well, with the low Mrs with their lower densities at the top of the tanks, they'll be fed to the donkeys at higher concentrations just when the FADECs go with throttle up so to speak, at the end of the flight.

As an aside, the pumps are defined as serviceable by the AAIB interim report. Now I take that to mean that they're within spec and I presume spec refers to volumetric efficiency against a liquid phase. If the poor things are trying to pump lots of eminently compressible of bubbles we will have considerably reduced flow rates and outa-gas symptoms.

Just what were the additives? What was their Mr?

It's already been said and it's still true, AAIB have done their job entirely to spec and we have to wait for the full report, but hey this is PPRuNe after all!