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BA038 (B777) Thread

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BA038 (B777) Thread

Old 6th May 2008, 15:46
  #1021 (permalink)  
 
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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!
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Old 6th May 2008, 17:29
  #1022 (permalink)  
 
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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.
Not so. Although the AAIB report mentions cavitation signs on the HP pump outlet, it is by no means clear that this damage was caused by an event immediately preceding this 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...)
The cavities can be either air or fuel vapors. The fact that cavitation damage was apparent shows that at least some of the bubbles collapsed again on the outlet, causing abrasion. The manufacturer of the pump and experts will know whether or not it was able to sustain sufficient flow under cavitating conditions, all other things being equal. This information does not appear to be publicly available.

Could someone please suggest why the cavitation isn't a sufficient explanation in itself?
Cavitation does not happen under normal conditions in a well-designed and tried-and-tested fuel delivery system with (as the AAIB said) on-spec fuel. It has to be caused by something, which may be upstream flow restriction or excessive fuel aeration. Cavitation may then exacerbate that problem, but will not be a root cause.


Bernd
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Old 6th May 2008, 18:04
  #1023 (permalink)  
 
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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.
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Old 6th May 2008, 21:24
  #1024 (permalink)  
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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").
 
Old 7th May 2008, 08:50
  #1025 (permalink)  
 
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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.
I agree that it is a likely scenario, and it fits the rest of the scenario just fine, an upstream fuel flow restriction, causing both pump cavitation and restricted flow (the former possibly exacerbating the latter). All I was saying is that we (in the uninformed public) do not (yet) know at what point in time the cavitation occurred that caused the visible signs. This is a matter of level of confidence in the factors in our causal reasoning. If I were to make a Why-Because-Graph of this, pump cavitation would be marked as an "Assumption".

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").
Yes, I almost completely agree. Except that it's not a linguistic issue, but an issue of causality.

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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Old 7th May 2008, 19:01
  #1026 (permalink)  
 
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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.
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Old 7th May 2008, 21:29
  #1027 (permalink)  
 
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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'.
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Old 7th May 2008, 22:29
  #1028 (permalink)  
 
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arcniz makes an excellent point re cavitation:
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.
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.
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Old 8th May 2008, 01:42
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the pump discharge must still be at a much higher pressure to overcome the burner (air) pressure
Seems like a great spot for a backflow check valve, wot? Of course absence of fuel flow for any duration will tend to cause flameout also, but a pulsing flow mode might be the natural result in a fuel choking-foaming situation if some type of check valve is present inline at the outlet side of the pumps. Isn't it likely the igniters were full on at that late stage in the descent?

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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Old 8th May 2008, 06:48
  #1030 (permalink)  

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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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Old 8th May 2008, 11:03
  #1031 (permalink)  
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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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Old 8th May 2008, 11:17
  #1032 (permalink)  
 
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Everybody is still trying to outguess the AAIB.
Not really, as I trust the AAIB are not guessing much at this point... maybe weighing up, but not guessing as in pizzing in de vind!
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Old 8th May 2008, 13:43
  #1033 (permalink)  
 
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Was it pilot error?

Whats goin on, the aaib must know at this stage if it was a mechanical problem.
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Old 8th May 2008, 15:21
  #1034 (permalink)  
 
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Was it pilot error?
Always a possibility. However, I can't see a realistic way of duplicating what happened to BA38, even if I had to. How could you cause both engines to 'hang' above flight idle when you're sitting on the flight deck?
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Old 8th May 2008, 16:38
  #1035 (permalink)  
 
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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...
Actually HotDog, I think you will find that information was offered by someone who understands more about the properties of pumped fuels at various temparatures and pressures than most of us know about hot dinners or dogs

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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Old 8th May 2008, 23:53
  #1036 (permalink)  
 
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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?
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Old 9th May 2008, 22:26
  #1037 (permalink)  
 
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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 hasnít 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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Old 10th May 2008, 00:52
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Originally Posted by PETTIFOGGER
...I only spent another 45 minutes aloft. Very inconsiderate all the same.
You don't make it clear who or what was 'inconsiderate'. Did the BA038 fuel not take your needs into consideration, or was it the engines themselves?
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Old 10th May 2008, 01:12
  #1039 (permalink)  
 
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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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Old 10th May 2008, 08:02
  #1040 (permalink)  
 
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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?
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