Fuel stratification - follow-up comments
Follow-up to my earlier posting (#195, page 10) on the possibility of fuel stratification.
Warning: I'm not crew nor engineer, just scientist.
Hand Solo, you wrote:
"I would think stratification would be unlikely given the aircrafts frequent manouvring and encounters with turbulence. The fuel would have been well and truly mixed up for most of the flight and certainly for the last 30 minutes."
I think you might be mistaken given that the g forces applied to the aircraft during manoeuvers are very low and the resulting force is typically normal to the wing surface (apologies, being very simplistic). The g forces would not be sufficient to disturb a stratification to cause it to break-up, i.e. the fuel becomes fully mixed.
Turbulence could cause a break-up of stratification if violent and with the imposed g forces being applied largely parallel to the wing surface. Moderate turbulence, i.e. the aircraft falls and rises rapidly, and applying g forces normal to the wing surface, might cause disturbance of stratified layers, even cause disruption, but I don't think it would take long for the full stratification to return in calm air. Plus, there are no reports of such events on this flight.
Bsieker, you wrote:
"It is my understanding of the water scavenge jet pumps that they would, while trying to keep the water emulsified in the fuel, would also mix the fuel, preventing stratification."
Thanks for that. Can anyone comment on how these pumps operate and to what 'depth' in the fuel their effect is propogated? Are they designed to fully mix fuel within the total volume of the tanks and so prevent stratification?
Bsieker also wrote:
"A minor slip: the imbalance was 300kg, not 30kg, which makes it still less likely that both engines would be fed the same undigestible type of stratum at roughly the same."
Sorry, a typo. When considering the imbalance, and its possible implications, I had in mind the 300Kg figure. A 5.8% weight imbalance equates to a few centimeters in depth of fuel (for example, 2.9 cm for a fuel depth of 0.5m). Of course, I don't know the dimensions of the fuel tank and so cannot calculate the exact difference in fuel depth between the two wing tanks but, I'd be surprised if the actual difference is greater than 2-3cm. This would possibly suggest that stratification, if it exists, in
both wing tanks would be essentially identical and might be fed into the tank outlet ports at the nearly the same time. Fascinating!
Further, I note now, having missed the text before, that the AAIB Special Bulletin 1/2008 concludes with:
"In addition, comprehensive examination and analysis is to be conducted on the entire aircraft and engine fuel system; including the modelling of fuel flows taking account of the environmental and aerodynamic effects."
Also, someone elsewhere on this thread mentioned that it had been previously thought that the mixing of fuels, provenanced from different sources, was not likely to cause any problems but was now being investigated.
And finally to politics, the elephant in the room: the fuel was sourced from Peking in an Olympic
year.
Regards, Tanimbar