BA038 (B777) Thread
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they work all the time when the engine is running ( i of course assume u are talking about the engine mounted mechanical driven pumps) and not at maximum rate - maximum rate 100% of the time is not possible because the speed of the pump is determined by the speed of the gearbox and therefore the engine- at any given speed the fuel pumps supply more fuel than is need - this excess fuel is then bled back toward the engine pumps via actuated valves and not by preset pressure relief valves
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Did the centre tank LP pumps put air into the system?
My experience is on Airbus so I'd be very happy for someone with Boeing fuel system knowledge to correct me on this.
1) Centre tank pumps usually pump at a higher pressure than wing tank pumps so that the centre tank fuel is used up first.
2) When the centre tank is empty sensors in the tank tell the centre tank pumps to stop. By the latter stages of a flight one would expect the centre tank fuel to be used up.
3) It is important that LP tank pumps are immersed in fuel so that they don't introduce air into the system, also the fuel provides a cooling function for the pumps.
4) In some early configurations of the A320 the centre tank pump sensors had a bit of an issue and not switch off the pumps when the tank was empty. This would trigger an ECAM (FUEL CTR TK PUMP LO PR) and we'd switch off the pumps manually. To avoid damaging the pumps there was an SOP not to switch the centre tank pumps on in the first place unless there was a certain amount of fuel in the tank. Its fixed now - well on our fleet anyway.
5) So, you're probably ahead of me, what if the sensors in the 777 center tank failed or were fooled and the pumps operated whilst the tank was dry? Would EICAS alert on something or could they keep going long enough to put enough air into the system to balls things up?
Centre tank obviously feeds both engines albeit via different LP feeds but it is not impossible for two pumps to be running.
Happy to be shot down - I've only given it a few minutes thought.
Cheers,
Silent Badger
1) Centre tank pumps usually pump at a higher pressure than wing tank pumps so that the centre tank fuel is used up first.
2) When the centre tank is empty sensors in the tank tell the centre tank pumps to stop. By the latter stages of a flight one would expect the centre tank fuel to be used up.
3) It is important that LP tank pumps are immersed in fuel so that they don't introduce air into the system, also the fuel provides a cooling function for the pumps.
4) In some early configurations of the A320 the centre tank pump sensors had a bit of an issue and not switch off the pumps when the tank was empty. This would trigger an ECAM (FUEL CTR TK PUMP LO PR) and we'd switch off the pumps manually. To avoid damaging the pumps there was an SOP not to switch the centre tank pumps on in the first place unless there was a certain amount of fuel in the tank. Its fixed now - well on our fleet anyway.
5) So, you're probably ahead of me, what if the sensors in the 777 center tank failed or were fooled and the pumps operated whilst the tank was dry? Would EICAS alert on something or could they keep going long enough to put enough air into the system to balls things up?
Centre tank obviously feeds both engines albeit via different LP feeds but it is not impossible for two pumps to be running.
Happy to be shot down - I've only given it a few minutes thought.
Cheers,
Silent Badger
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Can fuel become stratified?
Hello,
First post; I'm not flight crew, nor engineer, just scientist.
Apologies for trespassing on your professional forum to ask the following but my curiosity has gotten the better of me.
Can fuel on a long, cool flight become stratified?
(Fuel is probably the wrong term considering it is a mass, held within wing tanks, consisting of fuel sensu stricto, other hydrocarbon derivatives, H20, emulsions and various physical particles all of which will behave differently given the same physical environment.)
If the answer is yes then follow-on questions are:
1) Can fuel stratify by density and/or thermal anomaly?
2) Does the ground testing of the fuel take account of possible in-flight stratification?
3) Given that the fuel in both wing tanks was of almost equal weight (bar 30Kg), is it possible for the tank outlets in both wings to encounter a 'bad' layer/stratification at almost the same time?
Regards, Tanimbar
First post; I'm not flight crew, nor engineer, just scientist.
Apologies for trespassing on your professional forum to ask the following but my curiosity has gotten the better of me.
Can fuel on a long, cool flight become stratified?
(Fuel is probably the wrong term considering it is a mass, held within wing tanks, consisting of fuel sensu stricto, other hydrocarbon derivatives, H20, emulsions and various physical particles all of which will behave differently given the same physical environment.)
If the answer is yes then follow-on questions are:
1) Can fuel stratify by density and/or thermal anomaly?
2) Does the ground testing of the fuel take account of possible in-flight stratification?
3) Given that the fuel in both wing tanks was of almost equal weight (bar 30Kg), is it possible for the tank outlets in both wings to encounter a 'bad' layer/stratification at almost the same time?
Regards, Tanimbar
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I would think stratification would be unlikely given the aircrafts frequent manouvring and encounters with turbulence. The fuel would have been well and truly mixed up for most of the flight and certainly for the last 30 minutes.
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It is my understanding of the water scavenge jet pumps that they would, while trying to keep the water emulsified in the fuel, would also mix the fuel, preventing stratification.
A minor slip: the imbalance was 300kg, not 30kg, which makes it still less likely that both engines would be fed the same undigestible type of stratum at roughly the same.
Bernd
A minor slip: the imbalance was 300kg, not 30kg, which makes it still less likely that both engines would be fed the same undigestible type of stratum at roughly the same.
Bernd
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you asked about the speed of the HP fuel pumps and how they are controlled - it should be noted that the pumps are not electrical driven but are connected to the the gearbox and as such the gearbox is driven via a shaft which is in turn connected to the N2/N3 compressor spool so the pump speed is not and can not be directly controlled.
What are the sealing arrangements of the HP pumps? Do they have a double mechanical seal or are they magnetically coupled?
If they are magnetically coupled (mag drive) then they can suffer from magnetic slip and lose coupling all together. This would not necessarily however show signs of cavitatation. Magnetic slip can occur quite readily in large pumps when the demand on them is too sudden (on electrically driven ones, the star-delta needs to be set up to apply demand more gradually).
FOAMING
Does anybody know anything about the foaming tendencies of Jet A1, especially at low pressures and temperatures?
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Probably not a huge surprise, but BA has confirmed that 'YMMM is a total loss.
http://www.flightglobal.com/articles...ashed-777.html
http://www.flightglobal.com/articles...ashed-777.html
causes of aeration
Hi PPRUNE. Non-pilot,non-anorak, first post. I've been reading the Rumours board intermittently for years so I realise that taking the risk of posting on any technical issue is foolhardy, but here goes. I offer it only for what it is.
Many years ago I was involved in an operation that required pumping lots of water from a river, using a diesel engine (250hp) driving a 4 inch Ajax centrifugal pump. One day I replaced the cage which was fitted to the (non-return) foot-valve - which was intended to prevent large stones being drawn into the suction line. It was badly damaged, so I placed a stainless steel mesh directly across the valve opening (ie "2d" replacement for a "3d" predecessor).
To cut a long story short, we repeatedly encountered delivery failure after about an hour of operation. When checked, we discovered that the pump was partially full of air and consequently was cavitating. I assumed that this was due to a leak on the suction side and repeatedly refitted the suction hose (an 8", heavy, beast) with added sealant and many clamps.
Then it dawned on me to take into account the recent changes to the foot-valve protection. Clearly the restriction to the flow was causing air to gradually come out of solution, hence the eventual cavitation. After replacing the grid with a fabricated box of the same material, the problem vanished.
I have read a lot of the preceding thread. I understand a little of it - in very broad terms anyway. I don't recall if anyone has implied the cause of the "aeration" which has been referred to, except in relation to the possibility of leaky seals on the lp side. But I kept being reminded of my own experience described above. It would only be relevant if there is indeed any dissolved gas/air in jet fuel etc etc, about which I have absolutely no idea.
Many years ago I was involved in an operation that required pumping lots of water from a river, using a diesel engine (250hp) driving a 4 inch Ajax centrifugal pump. One day I replaced the cage which was fitted to the (non-return) foot-valve - which was intended to prevent large stones being drawn into the suction line. It was badly damaged, so I placed a stainless steel mesh directly across the valve opening (ie "2d" replacement for a "3d" predecessor).
To cut a long story short, we repeatedly encountered delivery failure after about an hour of operation. When checked, we discovered that the pump was partially full of air and consequently was cavitating. I assumed that this was due to a leak on the suction side and repeatedly refitted the suction hose (an 8", heavy, beast) with added sealant and many clamps.
Then it dawned on me to take into account the recent changes to the foot-valve protection. Clearly the restriction to the flow was causing air to gradually come out of solution, hence the eventual cavitation. After replacing the grid with a fabricated box of the same material, the problem vanished.
I have read a lot of the preceding thread. I understand a little of it - in very broad terms anyway. I don't recall if anyone has implied the cause of the "aeration" which has been referred to, except in relation to the possibility of leaky seals on the lp side. But I kept being reminded of my own experience described above. It would only be relevant if there is indeed any dissolved gas/air in jet fuel etc etc, about which I have absolutely no idea.
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Simple question. How does fuel quality varies when we mix different kinds of fuel in the tanks?
From what I've learned here, normal Jet A1 has a different chemical composition from the fuel this crew used out of China. A chemical reaction between different additives could probably lead to quality degradation regarding its published freezing point.
From what I've learned here, normal Jet A1 has a different chemical composition from the fuel this crew used out of China. A chemical reaction between different additives could probably lead to quality degradation regarding its published freezing point.
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Sky9 asked if there has been a random (say 6) check of other BA 777s HP fuel pumps with a time history the same as the crashed a/c. I hope they have. All mechanical parts wear with usage. If they find much the same wear then this is not significant.
What I would consider is the flow path as to why? the engines slowed to just above flt idle, was it commanded and did the throttles close?, or did the pwr just drift away.
I realise that both the a/p & crew asked for more pwr once they became aware but by then it was too late.
I also note the a/c used 5% less fuel than predicted, on a 10hr flt this is 30mins saved. Unless the on route conditions (w/x) where significantly different this is an almost impossible thing to happen.
What I would consider is the flow path as to why? the engines slowed to just above flt idle, was it commanded and did the throttles close?, or did the pwr just drift away.
I realise that both the a/p & crew asked for more pwr once they became aware but by then it was too late.
I also note the a/c used 5% less fuel than predicted, on a 10hr flt this is 30mins saved. Unless the on route conditions (w/x) where significantly different this is an almost impossible thing to happen.
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Simple question. How does fuel quality varies when we mix different kinds of fuel in the tanks?
From what I've learned here, normal Jet A1 has a different chemical composition from the fuel this crew used out of China. A chemical reaction between different additives could probably lead to quality degradation regarding its published freezing point.
From what I've learned here, normal Jet A1 has a different chemical composition from the fuel this crew used out of China. A chemical reaction between different additives could probably lead to quality degradation regarding its published freezing point.
"The specified freezing point for Jet A-1 fuel is -47ºC; analysis of fuel samples taken after the accident showed the fuel onboard the aircraft had an actual freezing point of -57ºC."
There is also a major discrepancy with the planned fuel burn and the actual. Planned burn including taxi fuel would work out at 72,000 kgs actual burn to short finals was only 68,500 based on the AAIB figures. Was this based on FMC fuel indications or gauge indications?
Unless there was a reason for a 5% difference such as an improved route or altitudes it seems a little large to me.
Unless there was a reason for a 5% difference such as an improved route or altitudes it seems a little large to me.
Shorter routings
Better levels
Lower ZFW
Shorter taxi
Lower Cost Index
Over a 12-hour flight, lower actual aircraft mass can have a significant effect, especially if it allows a climb to a more efficient level earlier than planned.
The AAIB is explicit in saying that flap 30 was selected. No mention is made of decreasing the flap selection.
The usual jet airliner L/D of 18 is for the clean configuration and I suspect that flap 30 yields substantially less. A simulator run would present the raw F30 L/D, but in any case we did have some thrust and the achieved slope and L/D is derivable from the FDR.
There are a number of flapped gliders that use flaps to add drag. The big caution with flapped gliders is not to add too much flap as you can lose considerable altitude reducing flap. Many glider approach accidents have happened when flap was reduced with insufficient altitude.
I would not want to explore in the air how the 777 with thrust restricted to the accident setting reacts to a flap reduction attempt at 600', but it's possible the AAIB will decide to investigate that in a simulator.
In a high drag situation, increasing airspeed increases drag substantially. With flap 30, the best L/D speed would be lower than with lesser flap selections.
With gliders that have powerful spoilers and/or flaps, you can select full spoilers and/or flaps full on and if you are still not coming down steeply enough, add airspeed to steepen the slope.
In this accident, we see a trade of airspeed for glideslope, mostly at the behest of the autopilot. Given the touchdown point and the 108 kt. cited by the AAIB, they were amazingly lucky.
The usual jet airliner L/D of 18 is for the clean configuration and I suspect that flap 30 yields substantially less. A simulator run would present the raw F30 L/D, but in any case we did have some thrust and the achieved slope and L/D is derivable from the FDR.
There are a number of flapped gliders that use flaps to add drag. The big caution with flapped gliders is not to add too much flap as you can lose considerable altitude reducing flap. Many glider approach accidents have happened when flap was reduced with insufficient altitude.
I would not want to explore in the air how the 777 with thrust restricted to the accident setting reacts to a flap reduction attempt at 600', but it's possible the AAIB will decide to investigate that in a simulator.
In a high drag situation, increasing airspeed increases drag substantially. With flap 30, the best L/D speed would be lower than with lesser flap selections.
With gliders that have powerful spoilers and/or flaps, you can select full spoilers and/or flaps full on and if you are still not coming down steeply enough, add airspeed to steepen the slope.
In this accident, we see a trade of airspeed for glideslope, mostly at the behest of the autopilot. Given the touchdown point and the 108 kt. cited by the AAIB, they were amazingly lucky.
"The specified freezing point for Jet A-1 fuel is -47ºC; analysis of fuel samples taken after the accident showed the fuel onboard the aircraft had an actual freezing point of -57ºC."
Kero is not known for incompatibility problems so mixing differing compositions not currently* thought to be an issue in the industry
*Take it from me, this is currently being investigated
Last edited by Mariner9; 20th Feb 2008 at 14:57.
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The AAIB is explicit in saying that flap 30 was selected. No mention is made of decreasing the flap selection.
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Quote (re. fuel pumps) (from Swiss Swiss) :-
they work all the time when the engine is running ( i of course assume u are talking about the engine mounted mechanical driven pumps) and not at maximum rate - maximum rate 100% of the time is not possible because the speed of the pump is determined by the speed of the gearbox and therefore the engine- at any given speed the fuel pumps supply more fuel than is need - this excess fuel is then bled back toward the engine pumps via actuated valves and not by preset pressure relief valves
unquote
Thanks again - very helpful,
So, if the spill valves are actuated to actually relieve the pressure between the fuel pump and the control valves (which were confirmed as being open) are the actuation messages recorded ? If so would their operation have been covered by any of the "satisfactories" in the AAIB report ? The Special Bulletin does say that the EEC worked correctly, but the listing of items tested does not include the spill valves.
they work all the time when the engine is running ( i of course assume u are talking about the engine mounted mechanical driven pumps) and not at maximum rate - maximum rate 100% of the time is not possible because the speed of the pump is determined by the speed of the gearbox and therefore the engine- at any given speed the fuel pumps supply more fuel than is need - this excess fuel is then bled back toward the engine pumps via actuated valves and not by preset pressure relief valves
unquote
Thanks again - very helpful,
So, if the spill valves are actuated to actually relieve the pressure between the fuel pump and the control valves (which were confirmed as being open) are the actuation messages recorded ? If so would their operation have been covered by any of the "satisfactories" in the AAIB report ? The Special Bulletin does say that the EEC worked correctly, but the listing of items tested does not include the spill valves.
Scanx3
I am concerned about the quoted fuel figures, they planned to use 72100kgs of fuel and according to the recorded quantity they had used 68500kgs, saving 3600kgs. If this amount was saved by flying higher than planned why not fly at that height everytime. I note that the FQPU was tested but I assume that this unit gathers information from the quantity sensors in the tanks or whatever system the 777 has for fuel quantity, testing of those sensors should be straightforward and I would have thought been mentioned in the report.
I am concerned about the quoted fuel figures, they planned to use 72100kgs of fuel and according to the recorded quantity they had used 68500kgs, saving 3600kgs. If this amount was saved by flying higher than planned why not fly at that height everytime. I note that the FQPU was tested but I assume that this unit gathers information from the quantity sensors in the tanks or whatever system the 777 has for fuel quantity, testing of those sensors should be straightforward and I would have thought been mentioned in the report.
Last edited by Oldlae; 20th Feb 2008 at 21:46. Reason: scanx3 amended his post.
