BA038 (B777) Thread
clarification
Just in case it isn't clear, I have no idea what caused this accident, other than ideas stimulated by this discussion. I am completely unqualified to have an opinion, still less a "theory"!
But I'd be interested in hearing someone explain cavitation in this context.
BTW I just spoke to a former colleague who was present at the same event I described and his recollection is that we had repeated reduction in pressure just before we lost it entirely each time this cavitation occurred.
I take the point that there has been a statement that the fuel flow was not impacted in this case. Can someone suitably qualified explain if a cavitating pump can sucessfully deliver at the correct rate. This doesn't imply that I think it can't!
But I'd be interested in hearing someone explain cavitation in this context.
BTW I just spoke to a former colleague who was present at the same event I described and his recollection is that we had repeated reduction in pressure just before we lost it entirely each time this cavitation occurred.
I take the point that there has been a statement that the fuel flow was not impacted in this case. Can someone suitably qualified explain if a cavitating pump can sucessfully deliver at the correct rate. This doesn't imply that I think it can't!
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Cavitation, again.
Originally Posted by skridlov
I have seen many references to cavitation in this thread; it seems to be a generally accepted factor in the events leading up to the accident.
In my ignorance I keep thinking that if there was cavitation, where did the air/gas come from if not out of solution? And if there was cavitation in a pump, it's hardly surprising that fuel ceased to be delivered (if indeed it did cease...)
Could someone please suggest why the cavitation isn't a sufficient explanation in itself?
Bernd
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I am sure the AAIB checked other 777 pumps to see if they had signs of cavitation, especially as the manufacturer stated that the pump (even with the cavitation damage/wear) was capable of meeting its design specification.
That information would establish if any deterioration in the pumps performance due cavitation was caused on the flight in question or could have been caused on a previous flight.
That information would establish if any deterioration in the pumps performance due cavitation was caused on the flight in question or could have been caused on a previous flight.
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Biesieker
It may not be clear to some that cavitation was caused by events just prior to the incident, but that is exactly what Boeing Captain Carbaugh suggested and AAIB is looking for a "restriction in the Fuel system upstream as a cause of the cavitation." See post #662 and the flightglobal.com article (3-14-08) quoting Dave Carbaugh.
It may be a linguistics issue, saying cavitation couldn't be the "cause of Fuel Starvation", but it certainly would show a "restriction upstream" as evidenced by damage to pump lobes. Said restriction might be the "proximate cause of the lack of Fuel". Carbaugh said the cavitation might have been caused BY restriction, making the pump damage a RESULT of restriction. (He also mentioned ice in fuel and made reference to "temperature").
It may be a linguistics issue, saying cavitation couldn't be the "cause of Fuel Starvation", but it certainly would show a "restriction upstream" as evidenced by damage to pump lobes. Said restriction might be the "proximate cause of the lack of Fuel". Carbaugh said the cavitation might have been caused BY restriction, making the pump damage a RESULT of restriction. (He also mentioned ice in fuel and made reference to "temperature").
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Originally Posted by airfoilmod
It may not be clear to some that cavitation was caused by events just prior to the incident, but that is exactly what Boeing Captain Carbaugh suggested and AAIB is looking for a "restriction in the Fuel system upstream as a cause of the cavitation." See post #662 and the flightglobal.com article (3-14-08) quoting Dave Carbaugh.
Note that even the flightglobal article talks about a "possible" fuel flow restriction, and that the cavitation signs "might indicate" such a restriction. The assessment that the cavitation occurred "not long before the impact" has not yet been publicly supported by facts. It is so far just an assumption, useful for creating test scenarios, but it should not be confused with an established fact.
It may be a linguistics issue, saying cavitation couldn't be the "cause of Fuel Starvation", but it certainly would show a "restriction upstream" as evidenced by damage to pump lobes. Said restriction might be the "proximate cause of the lack of Fuel". Carbaugh said the cavitation might have been caused BY restriction, making the pump damage a RESULT of restriction. (He also mentioned ice in fuel and made reference to "temperature").
My previous comments were in response to skridlov asking (linguistically unambiguous) why cavitation could not be a sufficient cause in itself.
Bernd
Last edited by bsieker; 7th May 2008 at 08:52. Reason: Typo.
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From experience observing complex fluids in experimental systems, I believe cold fuel in the lines leading to the pumps might have a tendency to 'shear' and locally develop some very abnormal physical properties if the supply flow is disrupted upstream and the downstream pumps are still doing their best to push max fuel toward the engines.
Any gasses dissolved in the fuel would increase in volume and come out of solution. Some volatile fractions of the fuel itself might gassify. Whipped around by the under-loaded and possibly over-heating pump impeller blades, the gas plus liquid would flow forward as a frothy foam, still burn-capable but much lower in density and fuel energy than normal.
Any gasses dissolved in the fuel would increase in volume and come out of solution. Some volatile fractions of the fuel itself might gassify. Whipped around by the under-loaded and possibly over-heating pump impeller blades, the gas plus liquid would flow forward as a frothy foam, still burn-capable but much lower in density and fuel energy than normal.
Cavitation damage is hard evidence
The cavitation damage may or may not be relevant to this incident, but it is incontrovertible evidence that at some stage in its life that pump has had to suck harder than it should. Or put another way, at some stage in its life it has found the fuel supply system under performing.
An analysis of the pump population will indicate how rare or common it is for the fuel supply to under perform.
We know that for it to critically under perform is a rare event. To estimate the probability of this rare critical under performance, it would be useful to know how much non-critical under performance occurs, and how it correlates with aircraft history.
Then we might be able to determine how and why it happens, and how often the under performance becomes 'nearly critical'.
An analysis of the pump population will indicate how rare or common it is for the fuel supply to under perform.
We know that for it to critically under perform is a rare event. To estimate the probability of this rare critical under performance, it would be useful to know how much non-critical under performance occurs, and how it correlates with aircraft history.
Then we might be able to determine how and why it happens, and how often the under performance becomes 'nearly critical'.
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arcniz makes an excellent point re cavitation:
Such conditions may well exist at the HP pump inlet, but the pump discharge must still be at a much higher pressure to overcome the burner (air) pressure. Otherwise we would see backflow, starving the burner and an immediate flameout. I'd don't think that's evident on the DFDR.
Any gasses dissolved in the fuel would increase in volume and come out of solution. Some volatile fractions of the fuel itself might gassify. Whipped around by the under-loaded and possibly over-heating pump impeller blades, the gas plus liquid would flow forward as a frothy foam, still burn-capable but much lower in density and fuel energy than normal.
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the pump discharge must still be at a much higher pressure to overcome the burner (air) pressure
Even if the foamy froth I described were re-compressed to a higher pressure, the new fluid likely would still have a notably lower density and some different physical properties due to the disruption of the original fuel fluid structure and consequent rejiggering of the intramolecular van der Waal's forces in the resulting froth. The aereated fuel would almost certainly have a lower density than the original JPxx for any given flow pressure.
The phenomenon is rather like those products one purchases at the store which once removed will never again fit into the original box.
Last edited by arcniz; 8th May 2008 at 02:15.
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Has anyone heard of RR fuel pumps being pulled for strip downs and comparison to ascertain whether this cavitation damage is a one-off or common occurrence?
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Everybody is still trying to outguess the AAIB. Amongst the more bizarre theories is frothing fuel! More like frothing at the mouth, give us a break. Put the lock on this thread.
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Everybody is still trying to outguess the AAIB.
Was it pilot error?
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Originally Posted by HotDog
...Amongst the more bizarre theories is frothing fuel! More like frothing at the mouth, give us a break...
There's a lot you can tell from a laboratory bench that you couldn't possibly guess at from any seat in the cigar tube.
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Engine Refresher
barit1 raises an interesting point concerning the pressure to be overcome by the HP engine fuel pumps at the burner nozzles at low thrust. This could be expected to be the addition of the engine core compressor output and the back pressure from the turbine/s which would not be inconsiderable. My guess would be about 100 to 200 psi.
Does anyone have a typical pressure plot through the engine core at low thrust or the range of air pressures at the burner nozzles over the thrust range at low altitude?
barit1 raises an interesting point concerning the pressure to be overcome by the HP engine fuel pumps at the burner nozzles at low thrust. This could be expected to be the addition of the engine core compressor output and the back pressure from the turbine/s which would not be inconsiderable. My guess would be about 100 to 200 psi.
Does anyone have a typical pressure plot through the engine core at low thrust or the range of air pressures at the burner nozzles over the thrust range at low altitude?
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BA038
BA038
Milt, I do not know the answer to your question, but the following may be of help.
The final delivery path of fuel to a gas generator is designed to be more than capable of overcoming pressure variations in the burner. The only way (that I know of) that the final delivery can be compromised is by a defect or number of defects further up (or is it down) the line. According to the AAIB preliminary report everything between the tanks and the gas generators have been individually tested and pass muster, although cavitation damage on the impellors was noted. This might suggest a restriction in fuel flow, since pumps are carefully designed not to cavitate in the normal and extreme operating parameters that can reasonably be expected.
Hence the suggestion in my only other post (28 Feb #500) that it might be useful to run a computational fluid dynamics programme (CFD) to see where the problem might lie, which may be valuable in any subsequent physical simulation. A CFD programme may not however be able take account adequately of the effect of high frequency sound, and there is a lot of that about. High frequency sound (including structure-borne resonance) can cause disturbance in fluids, including cavitation, which may not be evident after the event, except when the entrained disturbed fluid meets a pump and leaves its mark.
Is this the area in which the AAIB should be looking? Maybe. Why hasn’t it occurred before in identical aircraft? Probably because of the infinite number of temperature/pressure/sound frequency/fuel velocity combinations.
As an aside, the incident happened about 15 minutes before I was due to land (as a passenger) but I only spent another 45 minutes aloft. Very inconsiderate all the same.
rgds
Milt, I do not know the answer to your question, but the following may be of help.
The final delivery path of fuel to a gas generator is designed to be more than capable of overcoming pressure variations in the burner. The only way (that I know of) that the final delivery can be compromised is by a defect or number of defects further up (or is it down) the line. According to the AAIB preliminary report everything between the tanks and the gas generators have been individually tested and pass muster, although cavitation damage on the impellors was noted. This might suggest a restriction in fuel flow, since pumps are carefully designed not to cavitate in the normal and extreme operating parameters that can reasonably be expected.
Hence the suggestion in my only other post (28 Feb #500) that it might be useful to run a computational fluid dynamics programme (CFD) to see where the problem might lie, which may be valuable in any subsequent physical simulation. A CFD programme may not however be able take account adequately of the effect of high frequency sound, and there is a lot of that about. High frequency sound (including structure-borne resonance) can cause disturbance in fluids, including cavitation, which may not be evident after the event, except when the entrained disturbed fluid meets a pump and leaves its mark.
Is this the area in which the AAIB should be looking? Maybe. Why hasn’t it occurred before in identical aircraft? Probably because of the infinite number of temperature/pressure/sound frequency/fuel velocity combinations.
As an aside, the incident happened about 15 minutes before I was due to land (as a passenger) but I only spent another 45 minutes aloft. Very inconsiderate all the same.
rgds
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Originally Posted by PETTIFOGGER
...I only spent another 45 minutes aloft. Very inconsiderate all the same.
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Ba 038
For clarity, I was not (not) on BA038, just following (as pax) from HKG. The 'inconsiderate' bit on which I should have given a fuller explanation was knowing of the incident just after it occurred (announced by pilot) but not knowing the fate of those involved until after we landed. I think we were either first or second to land after the incident/accident.
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I suspect everyone was rather busy working out what to do with the large number of planes delayed and where they were going to go. Watching BBC News during the event I was quite impressed how fast the airport handled such a major incident. I suspect there wasn't spare radio bandwith to give updates on the health of the passengers in those 45 mins. It's busy enough on a good day. Which airport did they divert you to?