Posted by airfoilmod:
The Spar valves shut at the same time could initiate cavitation but would make both environments (fuel lines) dependent on the "same" "set" of conditions. The volume of the lines, though similar, would have been susceptible to varying results from suction due to connections, tubing strength, indeed even the "position" of the valves themselves.
There may have been a time lag between the two spar valves closing, either at initiation or in travel time. One valve may have been slightly faster than the other. Depending on what hypothetically would have caused such an anomaly, one valve may have closed further than the other before restoring to normal condition.
When I think of cavitation, I visualize a smallish tightly sealed space subject to great energy at one end and collapse close by (shear). I'm looking for a block close by the HP's, one that reacted to parameters present in spite of all engineered precautions and anticipated challenges.
Not necessarily so. Remember the engines were initially accelerating, demanding a considerable increase in fuel flow. Sudden deprivation of demanded fuel feed, depending on timing and rate of the reduction reaching the HP pumps could lead to the same results.
On the other hand, the delay of 7 seconds could represent the "differences" in systems' reactions to the fault . . . .
Fuel lines (LH and RH) between the spar valves and the engines are similar and almost a mirror image of each other. From the spar valve the fuel line initially runs parallel to the rear spar and then makes a 90 deg. turn forward following a rib then bends slightly inboard before making another turn outboard then forward and inboard again (going around the drybay) before finally going relatively straight forward, entering the engine pylon and finally going downward connecting to the engine fuel line. Viewed from the top the fuel line path roughly represents a Z shape followed by a U shape (around the dry bay) and then straightening out into the pylon area.
However, (depending on many "ifs" yet to be answered) if the spar valves were affected in the way i described at the very beginning of this thread, it should not be ruled out that the APU fuel shutoff valve (and perhaps the APU remote shutdown) electrical circuitry was also affected. If the valve was electrically opened in the same action the spar valves were closing, all of the fuel lines between (partially closed) spar valves (closed Xfeed valves) and boost pumps in the fuel feed manifold of the LH fuel system could have been acting as an accumulator providing just that much more buffer in feeding the LH engine before rolling back 7 seconds later than the RH engine did? The APU fuel supply line is a very long line going to the tail before ending at the APU fuel control unit. With a system out of wack, that APU fuel control would probably have continued to shut off fuel supply to the APU. Just as a reminder, there is still no official answer as to why the APU door was in the open or partially open position at "touch down", of which there is photographic evidence.
FS: I think the empty center tank would have performed more as an insulator than a chiller. Think Thermos.
An empty center tank filled with air and fumes will adjust faster to ambient temperature than a main wingtank (partially) filled with cold soaked fuel. Furthermore, the center tank temperature is also affected by the heat radiated from the airconditioning systems directly below the center tank.
Green-dot