Originally Posted by Flight Safety
What about this possibility.
Pumping high volumes [...] of very cold viscous fuel [...] where some part of the fuel system plumbing [...] starts to deform and narrow due to the suction presssure of said cold viscous fuel, thus creating a temporary restriction.
In other words, some part of the pipe work partially [...] collapses due to the suction pressure and the viscosity of the fuel, causing a restriction. This theory is not unheard of, but does require the pipe work to rebound after the suction pressure is removed (thus leaving no evidence of a restriction). This might also explain how it could happen on both sides of the airplane.
(my emphasis)
Interesting theory. However, where I live, it is unheard of.
A few remarks.
1/ With the boost pumps working, the fuel manifold is under positive pressure. There is no suction in normal operation, except in the very short pipe from the suction point to the boost pump. This ensures that no underpressure and thus no cavitation occurs.
2/ The AAIB specifically mentioned that the boost pumps were working correctly.
3/ Even if we did assume suction-feeding, the highest suction under-pressure possible happens when the engines run at high thrust with no boost-pump operating. This is within design specifications, expected, and will not cause a significant deformation of the pipes, sufficient to restrict flow to such a degree as happened in this case. The Flight Manual warns that dissolved air will be released during climb due to decreasing ambient pressure and may restrict fuel flow, and this restriction may cause thrust deterioration or flameout
at high altitude. Also see point /2.
4/ The AAIB said that the piping had been examined:
Originally Posted by AAIB Special Bulletin S3/2008
Detailed examination of the fuel system and pipe work has found no unusual deterioration or physical blockages.
Bernd