BA038 (B777) Thread
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Originally Posted by Green-dot
If the temporary reverse logic is corrected in less than 15 seconds, the spar valve moves [open->close->open] and will be in transit for almost but less than 30 seconds without ever reaching the closed position. Result, obvious limitations to the fuel flow. No alerts are presented and no visual indications if fuel synoptic and/or fuel management maintenance page are not displayed at moment of occurrance
According to AAIB, both EPRs reduced and maintained steady values, 1.03 and 1.02, therefore I don't see as plausible scenario that spar valves were constantly in transit, which would have produced, I think, anything but steady values.
I'd rather go for your previous hypothesis:
Focussing on the T7 dual engine roll back again, just suppose both spar valves had temporarily closed for, say, 80 percent (only 20 percent capacity remaining) with boost pumps on. Could that have resulted in engine roll backs to a stabilized thrust level above flight idle and cause cavitation at the engine pumps as the engines initially responded and then rolled back because reduced fuel flow did not meet engine demand?
I assume similar simulations such as mentioned above would have to be performed to find answers or are there other means (computer simulations?) to observe engine behaviour under such conditions?
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Originally Posted by CONF iture
I would easily conceive a fuel flow restriction by mechanical means, like a partial spar valve closure, but I don't see how the fuel itself would produce its own restriction in a way that such a restriction stabilizes for the remaining of the flight and don't deteriorate any further ?
a) That there is a location for the "slush" particles to accumulate. (a screen, a partially open valve, a matrix of tubes (FOHE?))
b) That the particles are of sufficient size and properties to accumulate.
c) That the particles can move from their formative location, or that they form in a location with the right characteristics to restrict fuel flow.
d) That there is sufficient time for the particles to accumulate. (that there is enough time at the end of the flight and during the descent that the engines require no more than low/moderate fuel flow.)
The "slush" would allow fuel to flow through at a steady state as long as the fuel flow demand was low/moderate. At high fuel flow levels, the fuel would be restricted to near the low "steady state" value. (sucking fuel through ice using a straw so to speak)
---------------------------------
There are some hurdles to the theory:
a) That the AAIB report identifies that there were "no significant quantities of water." (Definition of significant?) This can be explained if the "slush" was mostly pulled through the system at the time of the crash. Even at ppm of water, 79,000 kg of fuel would result in a few kg of water/ice, which would be enough to cause mischief. Not to mention the legacy water in the system.
b) That the FOHE were clean of blockage and that the fuel feed lines did not reveal any defects or restrictions. This could be explained also with the "slush" being pulled through the system. There may also have been some loss of the fuel downstream of the HP fuel pump from the crash impact, therefore losing the evidence.
If the "slush" was more waxy components than ice, then they could have redisolved into solution, again leaving minimal evidence.
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In my own realm of fluid dynamics I have witnessed some few examples over many years of systems entering states of very unintended and undesired resonance created by the combination of a free moving valve component and some elasticity in pipework and/or structures.
Acknowledging many of the very informative and interesting previous posts on the subject of BA038, I would like to suggest that just such a resonating system as described could very well have propagated involving one of the non return valves in the booster pump and the associated pipework on the right hand side of the aircraft.
The liquid flow through such a system in resonance will be restricted in proportion to variations of the frequency and amplitude these both being influenced by changes to any physical property within the system and most likely very much exacerbated by any increase of differential pressure across the oscillating valve component.
Before any problem becomes apparent, all individual components function perfectly and none are damaged, nothing is dangerously cold, the fuel is within standards, there is no ice present or any EMI, but at some time in the last minutes of the flight a very rare and of course quite unintended combination of factors starts a small harmonic interference in the fuel flow. These might include all or any of a change in fuel demand, the fuel temperature and viscosity, the airframe temperature, the level of fuel in the tanks, the aircraft attitude and normal vibrations within the airframe etc.
Regardless of the exact moment that this resonance started, as the demand for fuel increases the differential pressure across the now oscillating poppet valve hugely exacerbates the resonance. The HP pump firstly manages to draw the bulk of the liquid fuel from the delivery pipe and the engine accelerates as required but the supply of fuel quickly becomes limited by the non return valve now jumping on and off its seat. During the last seconds of the flight this system is in stasis with the HP pump developing its apparent cavitation damage but delivering adequate fuel to the engine to maintain the recorded 1.03 EPR output.
Each side of the fuel system has matching components – and non return valves - and is constructed as a virtual mirror image of pipework and connections. Particularly given the ever increasing perfections and reduced tolerances of mass production is it unreasonable to therefore suggest that the two similar systems will have almost identical resonant qualities? Especially when we know that the left side has almost exactly the same amount of the same fuel at the same temperature etc, etc?
The range of vulnerability to harmonics created by all the variable conditions mentioned could well be adequate for the left side system to independently propagate the harmonic effect exactly as the right side.
Alternately, could it be that the resonance is transferred through the crossfeed pipework which creates a robust physical connection between the two boost pumps on the right hand side to the two boost pumps on the left hand side of the aircraft. Even though the crossfeed valves are properly closed, the resonance can be very effectively transmitted either through the metal of this pipe or the mass of liquid that fills it.
Whichever, within the period of 7 seconds the left hand system imitates the right but the fuel flow on this side reaches stasis with the engine at the slightly lower output of 1.02 EPR.
This scenario seems to me to very adequately explain all the phenomena that are apparent in this incident including the lack of any physical evidence beyond the damage found on the HP pumps.
I can think of two very pertinent questions that need to be asked.
Is it mechanically possible or actually quite impossible that one or other of the non return valves in the booster pumps could be induced to go into a state of oscillation?
Would any of the data recording systems be sensitive enough to identify and record the anomaly of pulsations in the fuel rather than just interpreting a reduced but steady flow?
Further considerations:
Given the very short period of the malfunction, it seems perfectly possible that no evidence of abnormal vibration within the fuel system would be apparent; in normal operational use all pipes and fittings will show signs or wear relative to normal and expected movement. The most likely appearance of unusual wear would be on the poppet valve assemblies but especially if the limits of movement are constrained by resilient materials this could be almost impossible to detect as being any different to normal wear and tear..
While it is very hard to accept the idea that such an event would happen on two such independent systems, very clearly something has happened. The similarity of the components and the mechanical connection of the crossfeed assemblies however do lead towards an illustration that perhaps the two sides are not quite as independent as they appear to be!
The frequency of the harmonics in the fuel system is most likely going to have a relationship to other natural frequencies within the airframe and hence it seems very reasonable that neither anyone on board nor any of the data recording equipment would detect any vibrations or noises created in the fuel systems as being either out of the ordinary or in excess of those to be expected of an aircraft in the landing configuration.
If the fuel delivery, rather than being a normal consistent flow - even though much reduced - is actually arriving at the burners in high frequency spurts, could this not perhaps explain the unusual engine noise suggested by onlookers, this likely to be far more apparent to those outside rather than those on board the aircraft?
Comments would be very welcome….
JG
Acknowledging many of the very informative and interesting previous posts on the subject of BA038, I would like to suggest that just such a resonating system as described could very well have propagated involving one of the non return valves in the booster pump and the associated pipework on the right hand side of the aircraft.
The liquid flow through such a system in resonance will be restricted in proportion to variations of the frequency and amplitude these both being influenced by changes to any physical property within the system and most likely very much exacerbated by any increase of differential pressure across the oscillating valve component.
Before any problem becomes apparent, all individual components function perfectly and none are damaged, nothing is dangerously cold, the fuel is within standards, there is no ice present or any EMI, but at some time in the last minutes of the flight a very rare and of course quite unintended combination of factors starts a small harmonic interference in the fuel flow. These might include all or any of a change in fuel demand, the fuel temperature and viscosity, the airframe temperature, the level of fuel in the tanks, the aircraft attitude and normal vibrations within the airframe etc.
Regardless of the exact moment that this resonance started, as the demand for fuel increases the differential pressure across the now oscillating poppet valve hugely exacerbates the resonance. The HP pump firstly manages to draw the bulk of the liquid fuel from the delivery pipe and the engine accelerates as required but the supply of fuel quickly becomes limited by the non return valve now jumping on and off its seat. During the last seconds of the flight this system is in stasis with the HP pump developing its apparent cavitation damage but delivering adequate fuel to the engine to maintain the recorded 1.03 EPR output.
Each side of the fuel system has matching components – and non return valves - and is constructed as a virtual mirror image of pipework and connections. Particularly given the ever increasing perfections and reduced tolerances of mass production is it unreasonable to therefore suggest that the two similar systems will have almost identical resonant qualities? Especially when we know that the left side has almost exactly the same amount of the same fuel at the same temperature etc, etc?
The range of vulnerability to harmonics created by all the variable conditions mentioned could well be adequate for the left side system to independently propagate the harmonic effect exactly as the right side.
Alternately, could it be that the resonance is transferred through the crossfeed pipework which creates a robust physical connection between the two boost pumps on the right hand side to the two boost pumps on the left hand side of the aircraft. Even though the crossfeed valves are properly closed, the resonance can be very effectively transmitted either through the metal of this pipe or the mass of liquid that fills it.
Whichever, within the period of 7 seconds the left hand system imitates the right but the fuel flow on this side reaches stasis with the engine at the slightly lower output of 1.02 EPR.
This scenario seems to me to very adequately explain all the phenomena that are apparent in this incident including the lack of any physical evidence beyond the damage found on the HP pumps.
I can think of two very pertinent questions that need to be asked.
Is it mechanically possible or actually quite impossible that one or other of the non return valves in the booster pumps could be induced to go into a state of oscillation?
Would any of the data recording systems be sensitive enough to identify and record the anomaly of pulsations in the fuel rather than just interpreting a reduced but steady flow?
Further considerations:
Given the very short period of the malfunction, it seems perfectly possible that no evidence of abnormal vibration within the fuel system would be apparent; in normal operational use all pipes and fittings will show signs or wear relative to normal and expected movement. The most likely appearance of unusual wear would be on the poppet valve assemblies but especially if the limits of movement are constrained by resilient materials this could be almost impossible to detect as being any different to normal wear and tear..
While it is very hard to accept the idea that such an event would happen on two such independent systems, very clearly something has happened. The similarity of the components and the mechanical connection of the crossfeed assemblies however do lead towards an illustration that perhaps the two sides are not quite as independent as they appear to be!
The frequency of the harmonics in the fuel system is most likely going to have a relationship to other natural frequencies within the airframe and hence it seems very reasonable that neither anyone on board nor any of the data recording equipment would detect any vibrations or noises created in the fuel systems as being either out of the ordinary or in excess of those to be expected of an aircraft in the landing configuration.
If the fuel delivery, rather than being a normal consistent flow - even though much reduced - is actually arriving at the burners in high frequency spurts, could this not perhaps explain the unusual engine noise suggested by onlookers, this likely to be far more apparent to those outside rather than those on board the aircraft?
Comments would be very welcome….
JG
Last edited by johngreen; 16th May 2008 at 18:51. Reason: grammar correction
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For clarity: 79,000 kg of fuel containing 1 ppm of water would equate to 79g of water (w/w)
1 ppm=1/1 000 000
79 000/1 000 000 gives alot less than 79kg!!
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Firelight :-
......... There are some hurdles to the theory: .........
But you have to explain how the engines each separately initially responded and then each managed (at slightly different times) to have an almost exactly similar "problem" which limited the thrusts to 1.02 and 1.03 EPR.
.
......... There are some hurdles to the theory: .........
But you have to explain how the engines each separately initially responded and then each managed (at slightly different times) to have an almost exactly similar "problem" which limited the thrusts to 1.02 and 1.03 EPR.
.
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To abandon ice in Fuel. I have not been 100% comfortable with what water levels in Fuel infer re: AAIB "less than significant" findings. Debating the Authority from a distance with perhaps 1% the information they possess has been frustrating, to say the least.
What the discussion has suggested to me is a different direction; without the data necessary to support it, I'm envisioning a theory that is at least compelling to me.
"contamination" can involve other than chemical or foreign substance intrusion in the Fuel supplied.
Fuel can also be contaminated by SOUND.
Fuel can be contaminated by TEMPERATURE.
Fuel can be contaminated by VIBRATION.
It seems to me that introducing ENERGY into a liquid system can create all manner of mischief. (Witness Cavitation, Foaming, Vaporisation)
Out of my field (by light years), I will suggest that I have seen gasoline "foam" at 1 atm. pressure simply by being subjected to vibration. The corollary, and of the most interest here, would be, given the VERY low temps involved (The lowest flying that day, arguably), what would it take to solidify very cold Fuel? What combination of factors can be entertained that would explain "Bubbling", "Congealing", "Waxing", etc. of tested Fuel?
1. Stasis at descent is demonstrated.
2. Sudden introduction of Massive energy into a balanced system
3. New (Commanded) Thrust level presents dozens of instant challenges to very cold Fuel, not the least of which is acceleration in the lines of the liquid fuel, reducing pressure.
4. Although snubbed against vibration and sound, Fuel passages create several local and isolated environments, each of which must react in its own way to intense vibration (mechanical) and harmonics (Acoustic) while containing extremely frigid Fuel, which has differing resistance of its own to the new "impacts".
5. Flutter. The ancient enemy of powered flight. Any articulating mass in a fluid medium can perform badly at certain levels of tuned energy. This would include, but not be limited to: Fans, Turbines, Pumps, Valves, Filters, Solenoids, and Fuel.
Holding, Airfoil
What the discussion has suggested to me is a different direction; without the data necessary to support it, I'm envisioning a theory that is at least compelling to me.
"contamination" can involve other than chemical or foreign substance intrusion in the Fuel supplied.
Fuel can also be contaminated by SOUND.
Fuel can be contaminated by TEMPERATURE.
Fuel can be contaminated by VIBRATION.
It seems to me that introducing ENERGY into a liquid system can create all manner of mischief. (Witness Cavitation, Foaming, Vaporisation)
Out of my field (by light years), I will suggest that I have seen gasoline "foam" at 1 atm. pressure simply by being subjected to vibration. The corollary, and of the most interest here, would be, given the VERY low temps involved (The lowest flying that day, arguably), what would it take to solidify very cold Fuel? What combination of factors can be entertained that would explain "Bubbling", "Congealing", "Waxing", etc. of tested Fuel?
1. Stasis at descent is demonstrated.
2. Sudden introduction of Massive energy into a balanced system
3. New (Commanded) Thrust level presents dozens of instant challenges to very cold Fuel, not the least of which is acceleration in the lines of the liquid fuel, reducing pressure.
4. Although snubbed against vibration and sound, Fuel passages create several local and isolated environments, each of which must react in its own way to intense vibration (mechanical) and harmonics (Acoustic) while containing extremely frigid Fuel, which has differing resistance of its own to the new "impacts".
5. Flutter. The ancient enemy of powered flight. Any articulating mass in a fluid medium can perform badly at certain levels of tuned energy. This would include, but not be limited to: Fans, Turbines, Pumps, Valves, Filters, Solenoids, and Fuel.
Holding, Airfoil
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Not sure here of units used but!!
1 ppm=1/1 000 000
79 000/1 000 000 gives alot less than 79kg!!
1 ppm=1/1 000 000
79 000/1 000 000 gives alot less than 79kg!!
So once again, for clarity, 1 ppm of water in 79,000 kilograms (kg) of fuel = 79 grams (g) of water (w/w)
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Cold pockets
Please note i'm not a pilot, i've just been following the discussion.
Does anyone know what the recommended minimum OAT is for a Boeing 777 and where the fuel temp sensors are located? There seems to be some suggestions that there might have been a localised region of cold fuel. Presumably for that to happen there would have had to be a preliminary failure of the systems in place to mix the fuel up. If the min rec. OAT for a 777 is colder than -76 (which i'm guessing it is), then there should not have been any concern about even localised regions of cold developing unless there was then a secondary failure of another safety system for warming the fuel.
Which seems a little tentative, none of the system failures have been reported yet, but if it is the case it might explain why it's such a rarity. Presumably, then there would need to be a reason why there was a delay of several hours in the effects of the cold....
Does anyone know what the recommended minimum OAT is for a Boeing 777 and where the fuel temp sensors are located? There seems to be some suggestions that there might have been a localised region of cold fuel. Presumably for that to happen there would have had to be a preliminary failure of the systems in place to mix the fuel up. If the min rec. OAT for a 777 is colder than -76 (which i'm guessing it is), then there should not have been any concern about even localised regions of cold developing unless there was then a secondary failure of another safety system for warming the fuel.
Which seems a little tentative, none of the system failures have been reported yet, but if it is the case it might explain why it's such a rarity. Presumably, then there would need to be a reason why there was a delay of several hours in the effects of the cold....
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Good analysis, johngreen.
A spring loaded diaphragm, such as found in a non-return valve, would make a very interesting fluid motor.
A drawing of the valve would be very helpful, as would valve specifications.
Even if resonance damping was applied to the spring or the diaphragm, a resonance might occur in such a valve due to gradual wear of the damping material or increased stiffness of the spring due to low temperature or some perfect combination of both.
Given the rarity of the event, it may be that some perfect combination of conditions and shock waves from other fuel supply components would need to occur at the valve to trigger this resonance.
Recording a low frequency resonance in a fuel flow probably depends on how often the fuel flow is sampled.
If the sample rate is less that four times the resonance, a fuel flow resonance will probably not be detected.
A spring loaded diaphragm, such as found in a non-return valve, would make a very interesting fluid motor.
A drawing of the valve would be very helpful, as would valve specifications.
Even if resonance damping was applied to the spring or the diaphragm, a resonance might occur in such a valve due to gradual wear of the damping material or increased stiffness of the spring due to low temperature or some perfect combination of both.
Given the rarity of the event, it may be that some perfect combination of conditions and shock waves from other fuel supply components would need to occur at the valve to trigger this resonance.
Recording a low frequency resonance in a fuel flow probably depends on how often the fuel flow is sampled.
If the sample rate is less that four times the resonance, a fuel flow resonance will probably not be detected.
Last edited by shawk; 16th May 2008 at 20:29. Reason: Ineptitude.
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Sampling at two times the rate of the fundamental will suffice1, but your point is still well taken -- if the fundamental frequency of the system at resonance is, say 30Hz, and if the sampling rate of the sensor is, say, every second since a profile is not intended to be continuously constructed, but merely the point value sampled, then cyclic perturbations could be missed altogether, or aliased as noise/variance in the discrete samples, but the resonance signal would not be reconstructed.
I think the idea of resonance is interesting, but improbable. I think we will find a rare, but relatively mundane cause of dual fuel supply transitory obstruction.
1 "Nyquist (1928) pointed out that, if the function is substantially limited to the time interval T, 2BT values are sufficient to specify the function, basing his conclusions on a Fourier series representation of the function over the time interval T."
I think the idea of resonance is interesting, but improbable. I think we will find a rare, but relatively mundane cause of dual fuel supply transitory obstruction.
1 "Nyquist (1928) pointed out that, if the function is substantially limited to the time interval T, 2BT values are sufficient to specify the function, basing his conclusions on a Fourier series representation of the function over the time interval T."
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Location of temp sensor
How is it decided (apart from practical issues) where to install the sensor? Was the a/c once instrumented with tens of sensors in various places, then the worst case was chosen?
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Thanks soem dood.
I oversimplified my description of the performance of a switched channel A/D by just combining the Nyquest minimum sample rate and the switching rate to sample two separate fuel system samples.
I plead laziness.
To atone, I looked up the performance of the 777 DFDR.
The DFDR in the 777 appears to record 128 samples / word per second. The parameters of interest to fuel flow and engine performance are sampled once a second.
This gives a maximum sampled frequency of 0.5Hz.
No fluidic resonance recording for you.
http://www.tc.faa.gov/its/worldpac/techrpt/ar06-11.pdf
I oversimplified my description of the performance of a switched channel A/D by just combining the Nyquest minimum sample rate and the switching rate to sample two separate fuel system samples.
I plead laziness.
To atone, I looked up the performance of the 777 DFDR.
The DFDR in the 777 appears to record 128 samples / word per second. The parameters of interest to fuel flow and engine performance are sampled once a second.
This gives a maximum sampled frequency of 0.5Hz.
No fluidic resonance recording for you.
http://www.tc.faa.gov/its/worldpac/techrpt/ar06-11.pdf
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The DFDR in the 777 appears to record 128 samples / word per second. The parameters of interest to fuel flow and engine performance are sampled once a second.
This gives a maximum sampled frequency of 0.5Hz.
No fluidic resonance recording for you.
The maximum sampled frequency is correct, but whether you can record something of interest is not so clear. If the sampling isn't low-pass filtered, you will still record aliases of the higher frequency. Basically you record a frequency on the difference between the signal frequency and the nearest integer multiple of the sampled frequency. So if there was a resonance at 21.25Hz, you'd record an alias frequency of 0.25Hz (21Hz being the nearest multiple of the 1Hz sampling). You couldn't tell from the record what the original frequency was, but you would get a recording of something unusual, so long as the sensors really are capable of doing point-sampling rather than one-second averages.
This gives a maximum sampled frequency of 0.5Hz.
No fluidic resonance recording for you.
The maximum sampled frequency is correct, but whether you can record something of interest is not so clear. If the sampling isn't low-pass filtered, you will still record aliases of the higher frequency. Basically you record a frequency on the difference between the signal frequency and the nearest integer multiple of the sampled frequency. So if there was a resonance at 21.25Hz, you'd record an alias frequency of 0.25Hz (21Hz being the nearest multiple of the 1Hz sampling). You couldn't tell from the record what the original frequency was, but you would get a recording of something unusual, so long as the sensors really are capable of doing point-sampling rather than one-second averages.
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Awesome, shawk -- I love your concept -- that we atone for our sins here by providing heretofore unpresented information that is potentially relevant to the investigation!
And Fzz is right on, as well!
(Heck, if everyone did that, we could have this thing wrapped up in a week!)
And Fzz is right on, as well!
(Heck, if everyone did that, we could have this thing wrapped up in a week!)
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Excuse the interruption in the jitters/flutters/harmonics/surges and PPMs, but from another thread comes 'confirmation' that a BA internal report says Flaps went 30 to 25. Hope the AAIB read this
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With a Johnson bar, I learned alot about flaps in ground effect and thereabouts. How to "pop over" an obstacle on very short final, how to make my brakes more effective for short ground roll, how to drop like a stone or float like a moth; this thread is about to get very interesting.
How's YOUR Johnson, Mav?
How's YOUR Johnson, Mav?
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The profoundly toxic Honeywell Aerospace site lists the Digital Flight Data Acquisition Unit used in the Boeing 777.
http://www.honeywell.com/sites/aero/...74EACA8348.htm
To read the specification sheet, you need to register for a Honeywell ID. This will take several days to get.
The Honeywell unit is a 12 bit A/D with a single 12 bit sample per second for fuel and engine parameters.
A sub-harmonic that is 7 octaves below the fundamental is quite unlikely to be resolved with a 12 bit A/D and a single sample.
A better bet might be to analyze the audio track of any amateur video shot of the landing.
If anyone has a pointer to these, post it here and I can take a look at the audio track.
http://www.honeywell.com/sites/aero/...74EACA8348.htm
To read the specification sheet, you need to register for a Honeywell ID. This will take several days to get.
The Honeywell unit is a 12 bit A/D with a single 12 bit sample per second for fuel and engine parameters.
A sub-harmonic that is 7 octaves below the fundamental is quite unlikely to be resolved with a 12 bit A/D and a single sample.
A better bet might be to analyze the audio track of any amateur video shot of the landing.
If anyone has a pointer to these, post it here and I can take a look at the audio track.
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Shawk,
> This gives a maximum sampled frequency of 0.5Hz.
> No fluidic resonance recording for you.
I agree it wouldn't accurately record higher frequencies but that doesn't mean the samples would all appear "normal" - unless the resonance was somehow synchronised to the sampling.
> This gives a maximum sampled frequency of 0.5Hz.
> No fluidic resonance recording for you.
I agree it wouldn't accurately record higher frequencies but that doesn't mean the samples would all appear "normal" - unless the resonance was somehow synchronised to the sampling.