Quoting infrequentflyer789:
Interesting theory...
Problem: I would expect spar valve position would be recorded (although I don't know details on 777 FDR or QAR), and that a difference between recorded and commanded position would raise a warning (which would also be recorded). AAIB reported no recorded anomalies.
Counter: I think you've previously theorised that spar valve could be detected / recorded as only open/shut and therefore "partly shut" could go unrecorded ?
That is indeed the question, spar valves in transit from open- but not reaching the closed position may perhaps not be recorded. Warning to the crew in case of unscheduled closing of the spar valves (with the fuel switches still in "Run" position) is unlikely. I refer to the AMM and/or WDM for further information as to why this may be the case. Also, if the fuel synoptics page was not displayed (or not observed) at the time, the crew would have no visual indication of actual spar valve position.
Problem: Spar valves were found to be open.
Counter: [Again, I think you've theorised this] could be a transient effect that reset/cleared before impact.
Correct, also see my comments below.
Problem: The issue didn't clear before impact, because we know the engines stayed at below commanded thrust until impact (AAIB).
Thoughts: impact cleared the problem - bounced the valves open again, or the electrical disruption ceased at impact. Hmm.
- I would not know if the disruption (if present) did or did not clear before impact. If, as i have theorized, spar valve control was disturbed by reverse logic: the valves take 15 seconds to fully close and another 15 seconds to fully open again. Theoretically the valves could have been in transit for over 25 seconds (from open - to not fully closed - and back to open) limiting fuel flow to the engines for this period of time and perhaps causing cavitation to the pumps when the engines demanded more fuel but did not receive it.
- The AAIB Special Bulletin 1/2008 from page 5 onward explains why the spar valves were found open. Aircraft wiring was in pre SB 777-28-0025 configuration. The SB was introduced to ensure the spar valve closes when the fire handle is pulled regardless of sequencing. I do, however, still have a question regarding the AAIB findings and subsequent explanation related to the spar valve control relays found in open position. Were they found in the open position because of incorrect sequencing as reported or perhaps because of possible reverse logic still in effect at the time of- or just before impact? With reverse logic this would imply:
- Fuel switch in "Run": control relay in Cutoff position;
- Fuel switch to "Cutoff": control relay in Run position.
In other words, could selecting fuel switches to "Cutoff" at the time of the fire drill have inadvertently re-opened the spar valves (already closing or closed) due to a reverse logic condition still in effect before the fire switches were activated, thus
assuming in this case that the sequence was correctly performed, contrary to the report? In the
pre SB configuration the second wire path goes directly to the spar valve actuator and de-energizes the control relay circuit, latching the relays in last selected position if i am correct, i could be wrong. Depending on when exactly the fire switches were activated, the impact and damage to the landing gear may have severed the wiring to the valve actuators, preventing the close signals from reaching the valves?
Or, there is a return flow of fuel around the HP pump, and that feedback makes me wonder if this system could possibly get kicked into an abnormal flow state which then self-sustains. ie. your transient spar valve incident causes reduced flow and cavitation at the HP pumps, affects the return flow, which then continues to cause reduced flow even after the spar valve is open again.
My gut feeling is it can't happen like that, on the other hand, it's a long time since I did any fluid dynamics, so maybe...
To my knowledge there is a return flow path of excessive fuel returning from the fuel control unit to the HP pump. As fuel flow reduces due to the suspected flow restriction, return flow to the HP pump reduces as more fuel is demanded by the now "fuel hungry" FCU.
See also my post #590 on page 30 of this thread which describes engine behaviour when fuel flow to the engine is deliberately restricted. The test with a 95 percent closed fuel shutoff valve was done because the aircraft type involved had encountered unscheduled closing of the valve due to reverse logic on several occasions, resulting in incidents and accidents.
Green-dot