BA038 (B777) Thread
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Flight Safety
Not a bad suggestion. I'd guess it would be comparatively easy to do a test to find out where any vacuum induced temporary buckling/collapsing of the fuel piping might occur. Those locations could then be investigated on BA038.
But getting back to the main problem that remains so elusive: Could it be worthwhile to look at the facts from a 180 degrees opposite angle? I.e. perhaps the problem was actually too good fuel? Factually, large numbers of B777s operate sucessfully every day on fuel that freezes already at -47 degrees. Could the fact that this fuel was good all the way to at least -57 degrees cause some indirect effects that eventually led to the fuel starvation problem? What about the water content in the fuel, will it behave differently at such low temperatures?
Just my layman's $0.02.
But getting back to the main problem that remains so elusive: Could it be worthwhile to look at the facts from a 180 degrees opposite angle? I.e. perhaps the problem was actually too good fuel? Factually, large numbers of B777s operate sucessfully every day on fuel that freezes already at -47 degrees. Could the fact that this fuel was good all the way to at least -57 degrees cause some indirect effects that eventually led to the fuel starvation problem? What about the water content in the fuel, will it behave differently at such low temperatures?
Just my layman's $0.02.
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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.
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.
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
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Not really one for posting or speculating, but have been more or less following this thread and have been involved in fluid control engineering for several years in my previous occupation.
I would say that given the latest report available that maybe we are all looking at the wrong end of the fuel starvation chain. I am sure that the AAIB are also checking this but it would fit with the known facts that it could possibly be a problem with the venting system to the tanks, if it were the case that ice or other foreign matter were to clog the vents to the tanks then this would give a similar situation to the engines being able to produce thrust at low EPR and when commanded to increase thrust, then the vacuum effect would not allow sufficient fuel flow to the engines and associated pumps, resulting in an initial spool up followed by a reduction in thrust and thus causing the pumps to cavitate, just my 2p´s worth but it figures in my head.
I would say that given the latest report available that maybe we are all looking at the wrong end of the fuel starvation chain. I am sure that the AAIB are also checking this but it would fit with the known facts that it could possibly be a problem with the venting system to the tanks, if it were the case that ice or other foreign matter were to clog the vents to the tanks then this would give a similar situation to the engines being able to produce thrust at low EPR and when commanded to increase thrust, then the vacuum effect would not allow sufficient fuel flow to the engines and associated pumps, resulting in an initial spool up followed by a reduction in thrust and thus causing the pumps to cavitate, just my 2p´s worth but it figures in my head.
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My own pet theroy
Way back on March 3 I posted a theory for this accident. It looks better and better as the investigation advances.
Post #589 Page 30
http://www.pprune.org/forums/showthr...325095&page=30
1. There was no malfunction of any of the systems, mechanical or electrical.
2. The freezing point of the fuel had nothing to do with the cause of the accident.
3. There was no water ice or physical blockage in the fuel pathway to the HP pumps.
The short version ...
1. A cold spot developed in the fuel significantly below the temperature of the bulk of the fuel.
2. The viscosity of this cold fuel was higher than what the pumps can handle.
3. The cold spot in the fuel did not reach the pumps until the AC was on short finals.
4. The high viscosity made the pumps cavitate and the commanded thrust could not be maintained.
For those who did not understand my March post, the long version ...
Given a suitable input pressure, every pump subjected to increased RPMs will reach a point where cavitation occurs, regardless of liquid type or viscosity. A higher viscosity results in cavitation at a lower RPM. The operation of all pumps can be graphed and though the values may vary the form will be simular.
A positive displacement pump as is on the 777 has a predictable and certain output flow for any given pump speed. Displacement per rpm multiplied by the rpm gives output independent of pressure up to the point of cavitation.
Since fuel required tracts power produced, a positive displacement pump can be directly connected to the engine and geared to produce a varying output slightly above that needed at all engine speeds.
A smaller and lighter in weight pump can be used so long as the higher pump speed needed to produce the output does not push the pump into cavitation. In aviation, lighter is better, but in my opinion , the Boeing boys cut things a bit tight with the 777.
At the point of cavitation the percentage of design flow begins to drop and continues to worsen with additional pump speed.
The only standard for the viscosity of jet fuel I can find is based at minus 20C, where the maximum viscosity is stated to be 8 centistokes. The viscosity of most liquids increases as temperature drops. Many single component liquids, like pure water, have a linear function of viscosity to temperature up to the point of freezing. Liquids mixtures of hydrocarbons tend not to be linear. Ever hear of multi-viscosity motor oil?
Different formulations of jet fuel can have different viscosity profiles. All that is needed to meet standard is that it be below 8 centistokes at minus 20C. All that is known for certain is that below minus 20C the viscosity will be higher and the greater the difference of temperature from the standard the more uncertain actual viscosity of a particular fuel becomes as illustrated in the graphic below. Note that these viscosity changes apply across the full temperature range and have nothing to do with the point of freezing.
If my theory for BA038 is correct, a cold spot developed in the fuel, though the bulk of the fuel did not go below minus 34. (see my March post for particulars) This cold fuel was not mobile in the tank until lower flight levels. Once mobile, the cold fuel, maybe around minus 40C, began drifting toward the boost pump inlet. It arrived at HP pumps on short finals.
In the graphic below I have combined the concepts of the graphics above. One line represents the engine/pump rpm/thrust and the associated maximum viscosity for those values. Added are some possible temperatures that might be associated with a particular fuel viscosity.
From flight idle the engine responded to command with an increase in speed/thrust marked with green dots. Pumping minus 34C fuel there wasn't a problem. When the super cold fuel hit the pumps the viscosity increased above the maximum value and cavitation started. Starved of fuel the engines rolled back. The rollback to lower rpm improved the cavitation problem and when the slightly cavitating pump matched the needs for the thrust level, the situation stabilized at thrust levels of 1.03 & 1.02 EPR.
Post #589 Page 30
http://www.pprune.org/forums/showthr...325095&page=30
1. There was no malfunction of any of the systems, mechanical or electrical.
2. The freezing point of the fuel had nothing to do with the cause of the accident.
3. There was no water ice or physical blockage in the fuel pathway to the HP pumps.
The short version ...
1. A cold spot developed in the fuel significantly below the temperature of the bulk of the fuel.
2. The viscosity of this cold fuel was higher than what the pumps can handle.
3. The cold spot in the fuel did not reach the pumps until the AC was on short finals.
4. The high viscosity made the pumps cavitate and the commanded thrust could not be maintained.
For those who did not understand my March post, the long version ...
Given a suitable input pressure, every pump subjected to increased RPMs will reach a point where cavitation occurs, regardless of liquid type or viscosity. A higher viscosity results in cavitation at a lower RPM. The operation of all pumps can be graphed and though the values may vary the form will be simular.
A positive displacement pump as is on the 777 has a predictable and certain output flow for any given pump speed. Displacement per rpm multiplied by the rpm gives output independent of pressure up to the point of cavitation.
Since fuel required tracts power produced, a positive displacement pump can be directly connected to the engine and geared to produce a varying output slightly above that needed at all engine speeds.
A smaller and lighter in weight pump can be used so long as the higher pump speed needed to produce the output does not push the pump into cavitation. In aviation, lighter is better, but in my opinion , the Boeing boys cut things a bit tight with the 777.
At the point of cavitation the percentage of design flow begins to drop and continues to worsen with additional pump speed.
The only standard for the viscosity of jet fuel I can find is based at minus 20C, where the maximum viscosity is stated to be 8 centistokes. The viscosity of most liquids increases as temperature drops. Many single component liquids, like pure water, have a linear function of viscosity to temperature up to the point of freezing. Liquids mixtures of hydrocarbons tend not to be linear. Ever hear of multi-viscosity motor oil?
Different formulations of jet fuel can have different viscosity profiles. All that is needed to meet standard is that it be below 8 centistokes at minus 20C. All that is known for certain is that below minus 20C the viscosity will be higher and the greater the difference of temperature from the standard the more uncertain actual viscosity of a particular fuel becomes as illustrated in the graphic below. Note that these viscosity changes apply across the full temperature range and have nothing to do with the point of freezing.
If my theory for BA038 is correct, a cold spot developed in the fuel, though the bulk of the fuel did not go below minus 34. (see my March post for particulars) This cold fuel was not mobile in the tank until lower flight levels. Once mobile, the cold fuel, maybe around minus 40C, began drifting toward the boost pump inlet. It arrived at HP pumps on short finals.
In the graphic below I have combined the concepts of the graphics above. One line represents the engine/pump rpm/thrust and the associated maximum viscosity for those values. Added are some possible temperatures that might be associated with a particular fuel viscosity.
From flight idle the engine responded to command with an increase in speed/thrust marked with green dots. Pumping minus 34C fuel there wasn't a problem. When the super cold fuel hit the pumps the viscosity increased above the maximum value and cavitation started. Starved of fuel the engines rolled back. The rollback to lower rpm improved the cavitation problem and when the slightly cavitating pump matched the needs for the thrust level, the situation stabilized at thrust levels of 1.03 & 1.02 EPR.
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Tex37
This clogged fuel vent theory seems to be a recurring favourite here on Pprune.
It's certainly feasible in theory. At our airfield we had a case of a totally "imploded" fuel tank on a light airplane where the vent was iced up. Fortunately the forced landing was successful.
However, airliner tank vents are designed to cope with exactly that hazard. And I would also think it highly improbable that if vent clogging were to occur, it would happen in both tanks within seven seconds.
Further, I'd suspect that a vacuum strong enough to restrict fuel flow would also cause visible buckling on the wing skin, noticeable to any passenger who looks outside.
Just my $0.02
It's certainly feasible in theory. At our airfield we had a case of a totally "imploded" fuel tank on a light airplane where the vent was iced up. Fortunately the forced landing was successful.
However, airliner tank vents are designed to cope with exactly that hazard. And I would also think it highly improbable that if vent clogging were to occur, it would happen in both tanks within seven seconds.
Further, I'd suspect that a vacuum strong enough to restrict fuel flow would also cause visible buckling on the wing skin, noticeable to any passenger who looks outside.
Just my $0.02
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Post 1120
Cavitation, to metallic fuel pipes, did happen on RR Spey engined F4's, but the evidence was obvious. Also on same (USA) AC I remember reading some US articles about flexible TX fuel pipes inside the tanks going porous and causing mayhem........
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Sufficient to collapse tanks and Fuel Lines would "Boil" the lights out of the Fuel, not to mention have to be serendipitously synchronized to each "side": they responded, initially (read: simultaneously). The lobes on the pump were Deformed, Pitted (Cavitation). My theory on the "damage" reported by Boeing and AAIB #2. Are we overlooking some rather obvious available energy that can boil some liquids at a fair distance? The vibration and acoustics from 180,000 pounds of thrust made by two 10 foot fans at full go. A closed system, Fuel with no gas present, can be made to vaporize without any mechanical prodding, just vibration and or sound energy. I know, why hasn't it happened before?.
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snowfalcon2
I hear what you say and must admit I may have missed a few pages of this thread. Probably been covered before, but so far as I am aware, the fuel pumps on commercial aircraft (not x feed) are gravity-fed pumps as the aircraft is not expected to fly inverted or in any aerobatic attitude, also as a result, the outflow from the wing tanks is at the lower point.
Not knowing the full specs of the pumps on the 777 would they be able to draw such a vacuum as to result in physical distortion to the wing tanks, given the stresses they are designed to cope with in normal flight?
Give the fact that thrust was commanded on both engines at the same time with a slight imbalance of fuel in each wing tank, if vents were iced then the problem would replicate in each engine at more or less the same time.
Will have to wait for the final report by those who know but if fuel is ok and no apparent systems problems then the fault is either narrowed down to almost nothing or exponentially/impossibly widened to the point of never being identified
I hear what you say and must admit I may have missed a few pages of this thread. Probably been covered before, but so far as I am aware, the fuel pumps on commercial aircraft (not x feed) are gravity-fed pumps as the aircraft is not expected to fly inverted or in any aerobatic attitude, also as a result, the outflow from the wing tanks is at the lower point.
Not knowing the full specs of the pumps on the 777 would they be able to draw such a vacuum as to result in physical distortion to the wing tanks, given the stresses they are designed to cope with in normal flight?
Give the fact that thrust was commanded on both engines at the same time with a slight imbalance of fuel in each wing tank, if vents were iced then the problem would replicate in each engine at more or less the same time.
Will have to wait for the final report by those who know but if fuel is ok and no apparent systems problems then the fault is either narrowed down to almost nothing or exponentially/impossibly widened to the point of never being identified
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Bsieker, a few counter-remarks.
Understood about the positive pressure from the boost pumps. However I'm not sure what parameters are recorded in the FDR for the boost pump. An underperforming boost pump (underperforming due to cold fuel viscosity perhaps) could create a vacuum at the outlet, if the HP pump was trying to move more volume. However if the boost pump RPM is monitored and recorded and shown to be within normal parameters, then I agree with you.
Just fine parsing of the text, a collapse/rebound event might not necessarily show up in a deterioration/physical blockage detailed examination. I really do think you'd have to specifically go looking for the kind of evidence this type of event would create. But to be honest, I don't even know if the materials necessary for a collapse/rebound event even exist in the pipe work of this fuel system.
You assume that the design of this fuel system is sufficient for all "normal" operation conditions. However it's possible this is not the case for the specific conditions of this flight, and I conclude this mainly on the evidence that the same event happened on both sides of the airplane to 2 independent fuel systems.
Remember also that both engines initially responded with an increase in power, then both rolled back at slightly different times. Could the flow rates between the boost and HP pumps have been different enough that a vacuum developed over a fairly short period of time between the pumps? Might be plausible in theory anyway but could also be a real stretch.
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.
2/ The AAIB specifically mentioned that the boost pumps were working correctly.
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.
Detailed examination of the fuel system and pipe work has found no unusual deterioration or physical blockages.
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.
Remember also that both engines initially responded with an increase in power, then both rolled back at slightly different times. Could the flow rates between the boost and HP pumps have been different enough that a vacuum developed over a fairly short period of time between the pumps? Might be plausible in theory anyway but could also be a real stretch.
Last edited by Flight Safety; 13th May 2008 at 22:00.
Some water in the fuel?
Can anybody work out what the effect of a few litres of water per minute mixed in with the fuel would be? Would it be enough to account for the observed loss of power?
I ask because water could be introduced into the fuel supply of each engine at something like that rate after a few seconds if the initial power increase caused a surge of melt water from the centre tank.
I ask because water could be introduced into the fuel supply of each engine at something like that rate after a few seconds if the initial power increase caused a surge of melt water from the centre tank.
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Ice in the mix?
The fuel has been tested extensively; it is of good quality, in
many respects exceeding the appropriate specification,
and shows no evidence of contamination or excessive
water.
many respects exceeding the appropriate specification,
and shows no evidence of contamination or excessive
water.
These tests are collectively aimed at understanding and, if possible,
replicating the fuel system performance experienced on
the day and the potential for formation of restrictions
replicating the fuel system performance experienced on
the day and the potential for formation of restrictions
region of particularly cold
air, with ambient temperatures as low as -76şC, in the
area between the Urals and Eastern Scandinavia.
air, with ambient temperatures as low as -76şC, in the
area between the Urals and Eastern Scandinavia.
These tests are collectively aimed at understanding and, if possible,
replicating the fuel system performance experienced on
the day and the potential for formation of restrictions.
replicating the fuel system performance experienced on
the day and the potential for formation of restrictions.
It does sound as though ice in the fuel... formed from whatever water there was present... might be what they are actively considering as a possible culprit...
Seems there is nowhere else they can go really...all other systems seem hunky dory...
But then again I appreciate there is still a fair way to go in this investigation...but if an extremely rare combination of environmental and system parameters came together in a certain manner...it might explain a good deal...
A pressurised fuel tank might complicate the phase change...but at temps below -20 ...not a lot methinks...you still get ice....
Before I get thrown to the proverbial...or even in the proverbial...I do not have slightest idea of what actually occurred...but something most certainly did...I offer the above from the prospective of just reading the latest report...tis not a conspiracy theory...just speculation...
But I accept I might be ...possible am... in total error...
And other are conclusions are possible....
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U. S. Government AD Notice Search Location
A good place to look for the latest in U.S. Government actions regarding almost anything, but most importantly for us who are following the BA-38 situation is is the following site. You might want to copy this site to your web browser and check it now and then.
http://regulations.justia.com/search...&search=Search
http://regulations.justia.com/search...&search=Search
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Green-dot,
From memory, on 757 there was an indication light for SPAR VALVE status. That one was amber as long as its physical condition was not in total accordance with commanded request.
I believe such indication light does not exist on 777 (?)
My questions:
How long does a spar valve take from fully closed to fully open ?
What kind of electrical input does it need to operate ?
- Is it a continuous input ?
- Is it just a simple impulse which initiate closure / opening ?
- Can it produce an only partial closure / opening ?
Before going in all kind of studies and theories regarding fuel composition and / or pipe structure, wouldn’t it be common sense to simply test the effect a partial and temporary spar valve closure could produce on HP fuel pump ?
Temporary partial closure of that valve, isn’t it the easiest way to restrict a fuel flow ?
Interesting how our mass media is willing to omit or even change wording … and therefore format mind of the masses:
BBC
A now-discounted theory was that radio signals from Gordon Brown's motorcade interfered with the Boeing 777… But the report from the Air Accidents Investigation Branch (AAIB) rules out electromagnetic interference.
When AAIB only says
There is no evidence of any anomalous behavior of any of the aircraft or engine systems that suggests electromagnetic interference
BBC
The plane was operating in what are described as unusually low temperatures, and one theory is that this affected the fuel, though recorded temperatures were within safe limits.
AAIB words
The Met Office described the temperature conditions during the flight as ‘unusually low compared to the average, but not exceptional’
BBC on may Th
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Originally Posted by Tex37
Probably been covered before, but so far as I am aware, the fuel pumps on commercial aircraft (not x feed) are gravity-fed pumps as the aircraft is not expected to fly inverted or in any aerobatic attitude, also as a result, the outflow from the wing tanks is at the lower point.
Please see posts #284, #461, #467 and #478 for details.
Not knowing the full specs of the pumps on the 777 would they be able to draw such a vacuum as to result in physical distortion to the wing tanks, given the stresses they are designed to cope with in normal flight?
Give the fact that thrust was commanded on both engines at the same time with a slight imbalance of fuel in each wing tank, if vents were iced then the problem would replicate in each engine at more or less the same time.
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The plane was operating in what are described as unusually low temperatures, and one theory is that this affected the fuel, though recorded temperatures were within safe limits.
AAIB words
The Met Office described the temperature conditions during the flight as ‘unusually low compared to the average, but not exceptional’
AAIB words
The Met Office described the temperature conditions during the flight as ‘unusually low compared to the average, but not exceptional’
=> Unusual implies "not average"
=> Unusual is not the same as "exceptional"
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Unusual sound - cavitation?
Warning: I'm non-professional; not crew, not engineer - just scientist guest and thanks.
Shawk in post 1110 wrote while discussing fluidic resonance in a tuned circuit:
If the reports are reliable then it seems more probable that the sound might be due to the high pressure fuel pumps cavitating.
Apologies if this has been mentioned before.
regards, Tanimbar
Shawk in post 1110 wrote while discussing fluidic resonance in a tuned circuit:
I believe that witnesses reported that the aircraft made an unusual sound as it approached the airport. While witnesses tend to be unreliable, some weight might be given to this observation as witnesses who work at airports tend to have experience with typical aircraft noise.
Apologies if this has been mentioned before.
regards, Tanimbar
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Well, if we're getting picky about correctness, you should note that your statement is only true for some (eg. normal) distributions. If the distribution is highly skewed, then the average value would be unusual, and the most commonly seen values (ie. "usual") would be "not average".