BA038 (B777) Thread
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Green-dot: okay, thanks. Not sure what the point of a water-detection system that only triggers at such a high volume is, though.
Airfoilmod: your physics is wrong. You seem to be muddling gas and liquid behaviour. Water freezes at 0°C. The pop-bottle phenomenon which you cite is to do with super-cooling. The release of pressure causes the appearance of finely dispersed CO2 bubbles which act as nucleation sites for crystallisation.
Airfoilmod: your physics is wrong. You seem to be muddling gas and liquid behaviour. Water freezes at 0°C. The pop-bottle phenomenon which you cite is to do with super-cooling. The release of pressure causes the appearance of finely dispersed CO2 bubbles which act as nucleation sites for crystallisation.
The Reverend
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Airfoldmod, I don't think you paid enough attention during physics lectures. Does altitude alter the freezing point of water?
By itself? If you put the water in a sealed, insulated container
and lofted it in a balloon? No. Altitude by itself is a measure of
how far away from the Earth's center you are, which in turn means how
strong gravity is. Gravity has very little influence on the freezing
point, because water is essentially incompressible.
However, you probably mean freezing in an open container, exposed to
the surrounding atmospheric pressure. In that case the answer is:
yes, for several reasons. First of all, the freezing point of water
increases with a decrease in the pressure applied to it. Hence the
freezing point of water will be less at high altitude and low
pressure. But this effect is small. The freezing point of water
rises a mere hundredth of a degree per atmosphere of decrease in
pressure.
It is also true that water collected at high altitude, for example
water droplets in high-altitude clouds, is exceptionally pure. This
water will not freeze easily, because ordinarily water needs some
``seed'' to grow a crystal of ice around. That seed is commonly a
speck of dirt or the walls of the container, but there isn't either at
high altitude, so liquid water droplets exist up there down to
ridiculous temperatures, -40 or so I believe.
and lofted it in a balloon? No. Altitude by itself is a measure of
how far away from the Earth's center you are, which in turn means how
strong gravity is. Gravity has very little influence on the freezing
point, because water is essentially incompressible.
However, you probably mean freezing in an open container, exposed to
the surrounding atmospheric pressure. In that case the answer is:
yes, for several reasons. First of all, the freezing point of water
increases with a decrease in the pressure applied to it. Hence the
freezing point of water will be less at high altitude and low
pressure. But this effect is small. The freezing point of water
rises a mere hundredth of a degree per atmosphere of decrease in
pressure.
It is also true that water collected at high altitude, for example
water droplets in high-altitude clouds, is exceptionally pure. This
water will not freeze easily, because ordinarily water needs some
``seed'' to grow a crystal of ice around. That seed is commonly a
speck of dirt or the walls of the container, but there isn't either at
high altitude, so liquid water droplets exist up there down to
ridiculous temperatures, -40 or so I believe.
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another scenario
This poses some questions that I cannot answer, so comments from the qualified would be welcome.
Lets assume that the liquid in the wing tanks is a mixture of fuel and ice crystals. This fuel will be warmer than the top surface of the tank during most of the flight. Presumably, the ice crystals, unless they never reach the surface, will sublimate into some water vapor, which will freeze on the colder surfaces of the tank in the form of frost crystals.
Take this tank into warmer air, and the mat of ice crystals will drop off the now-warmer tank top, putting a bunch of snow into the fuel in a short time, possibly enough to overwhelm the jet pumps and clog the boost pump inlet screens at the wrong moment. This could happen to both port and starboard tanks at roughly the same time.
If this abnormal ice crystal load got past the inlet screens, it might migrate
through the boost pumps to the engine plumbing, making the same trouble.
For a greater stretch, substitute fuel vapor for water vapor. I see the problem that waxed fuel would rapidly melt when it fell into the warmer fuel. The water ice wouldn't melt until fuel temp went above 0 C.
These theories could be tested in a lab, or in a real acft with a camera in the tank. The trick would be to find OATs to match the flight in question.
/bill_s
Lets assume that the liquid in the wing tanks is a mixture of fuel and ice crystals. This fuel will be warmer than the top surface of the tank during most of the flight. Presumably, the ice crystals, unless they never reach the surface, will sublimate into some water vapor, which will freeze on the colder surfaces of the tank in the form of frost crystals.
Take this tank into warmer air, and the mat of ice crystals will drop off the now-warmer tank top, putting a bunch of snow into the fuel in a short time, possibly enough to overwhelm the jet pumps and clog the boost pump inlet screens at the wrong moment. This could happen to both port and starboard tanks at roughly the same time.
If this abnormal ice crystal load got past the inlet screens, it might migrate
through the boost pumps to the engine plumbing, making the same trouble.
For a greater stretch, substitute fuel vapor for water vapor. I see the problem that waxed fuel would rapidly melt when it fell into the warmer fuel. The water ice wouldn't melt until fuel temp went above 0 C.
These theories could be tested in a lab, or in a real acft with a camera in the tank. The trick would be to find OATs to match the flight in question.
/bill_s
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I think we are all amazed by the 138 gallons quoted by Green-dot, thanks for a very informative post.
Could you please explain the working of the centre jet pumps. As operating crew on the aircraft in question the only control of the fuel leaving the centre tank is the two centre tank pumps (via their switches on the overhead fuel panel). As has been quoted before these (due to the attitude during cruise) stop feeding (uncovered) with roughly 900 kgs still remaining in the tank.
This 900kgs remains there until the wing tanks drop below a certain figure where the jet pumps remove it from the centre tank and pump it into the wing tanks. No annunciations on EICAS of the commencement or completion of this fuel transfer.
I think we can assume that the investigators are looking at how often the "water in tank maintenance message" has occurred on other aircraft.
Let us assume for a moment (and there is no reason tho think it was not working properly) that the message was correct having being flagged up on the last two sectors. My thought is, with this 138 gallons settled at the back of the centre tank (cruise attitude) the water would have remained there when the centre tank stopped feeding with 900kgs still in the tank.
In "normal" operation (without any water present) the centre tank (as covered above) is emptied automatically later in the cruise. Perhaps this 138 gallons of water which, sitting at the back of the tank had frozen. Normally it would have thawed during the descent and the jet pumps picked it up (I never in six years saw any "fuel" left in the centre tank on shutdown so the automatic system obviously works well). But, and it is a big but the report suggests the temperatures in Beijing were around minus 7 centigrade (on the ground) so there would have been no melting of any ice prior to arriving at Beijing. Hence it may have still been in the tank in the form of a sheet of ice.
After refuelling the maintenance message reoccurred regarding this 138 gallons of water. Even if it had thawed with the introduction of "warmer" fuel during refuelling (no mention made in the report of the actual fuel temperature available at Beijing but the OAT was minus 7C) this large quantity of water could have either remained frozen, or re-froze during the return sector.
It would have remained frozen until descending into the warm airmass around the UK, but with the attitude during the descent and initial approach being nose down (even during the "one hold" at LAM the report states that the aircraft was descending) the first time the jet pumps may have been able to pick up this water, all 138 gallons, was when following flap extension and speed reduction on the ILS glidepath (the perfect continuous descent) when the attitude increased above zero.
Experts in the layout of the pickup points of the centre jet pumps, and I appeal to the better knowledge of Green-dot, might enlighten us as to whether a thawed amount of water in the centre tank could have been transferred to, and then picked up, by the wing booster pumps then feeding it onto its associated engine once the aircraft attitude increased above zero during the final approach.
Could you please explain the working of the centre jet pumps. As operating crew on the aircraft in question the only control of the fuel leaving the centre tank is the two centre tank pumps (via their switches on the overhead fuel panel). As has been quoted before these (due to the attitude during cruise) stop feeding (uncovered) with roughly 900 kgs still remaining in the tank.
This 900kgs remains there until the wing tanks drop below a certain figure where the jet pumps remove it from the centre tank and pump it into the wing tanks. No annunciations on EICAS of the commencement or completion of this fuel transfer.
I think we can assume that the investigators are looking at how often the "water in tank maintenance message" has occurred on other aircraft.
Let us assume for a moment (and there is no reason tho think it was not working properly) that the message was correct having being flagged up on the last two sectors. My thought is, with this 138 gallons settled at the back of the centre tank (cruise attitude) the water would have remained there when the centre tank stopped feeding with 900kgs still in the tank.
In "normal" operation (without any water present) the centre tank (as covered above) is emptied automatically later in the cruise. Perhaps this 138 gallons of water which, sitting at the back of the tank had frozen. Normally it would have thawed during the descent and the jet pumps picked it up (I never in six years saw any "fuel" left in the centre tank on shutdown so the automatic system obviously works well). But, and it is a big but the report suggests the temperatures in Beijing were around minus 7 centigrade (on the ground) so there would have been no melting of any ice prior to arriving at Beijing. Hence it may have still been in the tank in the form of a sheet of ice.
After refuelling the maintenance message reoccurred regarding this 138 gallons of water. Even if it had thawed with the introduction of "warmer" fuel during refuelling (no mention made in the report of the actual fuel temperature available at Beijing but the OAT was minus 7C) this large quantity of water could have either remained frozen, or re-froze during the return sector.
It would have remained frozen until descending into the warm airmass around the UK, but with the attitude during the descent and initial approach being nose down (even during the "one hold" at LAM the report states that the aircraft was descending) the first time the jet pumps may have been able to pick up this water, all 138 gallons, was when following flap extension and speed reduction on the ILS glidepath (the perfect continuous descent) when the attitude increased above zero.
Experts in the layout of the pickup points of the centre jet pumps, and I appeal to the better knowledge of Green-dot, might enlighten us as to whether a thawed amount of water in the centre tank could have been transferred to, and then picked up, by the wing booster pumps then feeding it onto its associated engine once the aircraft attitude increased above zero during the final approach.
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Can the experts also explain how the > 138 gallons of water then chokes the engines, causes the plane to crash and leaves not a trace of water in the wing tanks?
If you dissolved 2 cups full of water in the remaining 10.5 tons of fuel it would fail a water test - i.e. it would no longer conform to spec. I dare say even the slowest of air accident investigators could have found some sign of the mythical "ice blockage" on the morning of the accident.
If you dissolved 2 cups full of water in the remaining 10.5 tons of fuel it would fail a water test - i.e. it would no longer conform to spec. I dare say even the slowest of air accident investigators could have found some sign of the mythical "ice blockage" on the morning of the accident.
Last edited by avrflr; 22nd Feb 2008 at 05:04.
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Regarding woodpecker's post. Would the ice in the centre tank really be able to melt enough to cause enough liquid water to make a problem? Or wouldn't it be more likely that it could only melt enough to create a slush? If slush was pumped from the centre tank to the wing tanks, then could this slush have collected on the intake filters or elsewhere in the fuel delivery system and caused a major restriction in fuel flow? It would only take a relatively small amount of slush to cause a problem which would not be detected compared to the major amount of water required to cause a problem which should have been detected.
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I infer from this that the CT extends into the wings, outside of the fuselage profile. An empty tank in the wing will very quickly cold soak. On descent, moist incoming air will deposit ice on the tank walls. Where is the outboard CT boundary?
Rib 8 meets the front spar approximately at the inner end of the engine nacelle and the rear spar approximately in the center of the inboard spoiler.
CT volume is approximately 50% inside and 50% outside of the fuselage (25% on each side). The CT holds approximately 80 tons, which is 58% of the total fuel capacity.
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This seems rather a lot of water. Are you sure?
Re the CWT scavenging system. The flow rates were quoted before. It takes a long time to normally scavenge the CWT because of the diameter of the scavenge pipes, but even if all 138 gallons of ice thawed during the descent and was scavenged, would mixing it with over 3000 gallons of remaining wing tank fuel degrade the engine performance so much?
Note that the wing tank water scavenge pumps would also be operating to scavenge water from the wing tanks as it was coming from the CT.
Regards.
NSEU
P.S. BTW, many thanks to the guys suggesting possible solutions to my own engine hesitation/stalling problems. Spent many hours trying to figure out the fuel tank venting system on my car... Long gone are the days when a simple breather pipe was stuck in the air to vent fumes. Now the system is controlled by the engine management system! What a rat's nest! (miles of tubing, wiring, a charcoal filter, check valve, ECU controlled-solenoid valve, (car) rollover valve, etc). When the engine is hot, the fumes are injected into inlet manifold. A faulty venting system can cause poor idling and engine running rich.. or cause excessive fuel fumes.
Paxing All Over The World
Non-pilot speaking
NSEU
From early on in the various threads, we have been told that the CT would have been empty, long before TOD.
We have been told that the CT is used first in it's entirety. Once fuel is below the level of the main pumps and the engines fed from the wing tanks, other pumps (the scavengers?) drain the tank down and put the remaining fuel into the wings. Thus, no great body of water would exist. If I recall correctly, that is a SOP.
I sit to be corrected.
NSEU
even if all 138 gallons of ice thawed during the descent and was scavenged, would mixing it with over 3000 gallons of remaining wing tank fuel degrade the engine performance so much?
We have been told that the CT is used first in it's entirety. Once fuel is below the level of the main pumps and the engines fed from the wing tanks, other pumps (the scavengers?) drain the tank down and put the remaining fuel into the wings. Thus, no great body of water would exist. If I recall correctly, that is a SOP.
I sit to be corrected.
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138 gallons H20 and stratification
Warning: I'm non-professional; not crew, not engineer - just guest here, thanks.
Interesting posts overnight, many concentrating on the fuel and its state.
Woodpeckers post #280, and others, question whether some 138 gallons of water might have been present in the centre tank and that this, in some form and manner, caused the accident. Problem with this questioning is that the fuel tanks were drained of water at Heathrow before fuelling for the Beijing outboard sector. AAIB Special Bull. 1/2008 states:
"The aircraft's fuel tanks were last checked for water in the fuel on the 15 January 2008 at Heathrow; this was prior to its refuelling for the outboard sector to Beijing."
Additionally, the bulletin states "sump sample taken from the left and right main fuel tanks shortly after the accident revealed no significant quantities of water".
The bulletin further states "Initial results confirm that the fuel conforms to Jet A 1 specifications and that there were no signs of contamination or unusual levels of water content."
We should probably conclude that there was no significant quantity (i.e. something like 138 gallons) of water in the system when it left Heathrow and none when it returned to Heathrow. Even if there had been water in the centre tank before the crash-landing this did not find its way to the wing tanks and then to the engines.
With regard to my earlier posts (195,242), I questioned whether the fuel could stratify due to density and thermal anomalies, in a very cool environment, over an extended period of time, being subjected to high frequency vibration from the engines and so produce layers of liquid that stifle an engine(s) by decreasing the flow rate. Of course, this is a very complex regime, probably one with subtle negative and positive feedbacks, meaning that an initial stratification would progressively mutate into something very different.
Earlier I asked if the water scavenge jet pumps in the wing tanks would destroy stratification. Thanks to Green-dot (#267) for telling us how these pumps operate and for pointing out that detected water causes, "water detection message for a particular tank to show". There was no such message shown during the flight, therefore there was no water, greater than 7 gallons, present in the bottom of the wing tanks and the water scavenge jet pumps did not operate. Any stratification would not have been destroyed by these pumps. Hope I've not missed anything.
However, SyEng (#273) says'
"Stratification: no chance. (following para is general, not 777-specific) Apart from the scavenge jet pumps (if they're not full of FOD), the boost pumps cause plenty of mixing, through their own bypass/cooling flow discharged back into the tank, through swirling/entrainment near the inlet and through collector cell feed/overflow. There is also often fuel returned to tanks from other systems heat exchangers. "
SyEng, you say the paragraph is general, not 777-specific, so is it possible to confirm the destruction of any stratification in the wing tanks by the systems you mention for the 777?
Regards, Tanimbar
Interesting posts overnight, many concentrating on the fuel and its state.
Woodpeckers post #280, and others, question whether some 138 gallons of water might have been present in the centre tank and that this, in some form and manner, caused the accident. Problem with this questioning is that the fuel tanks were drained of water at Heathrow before fuelling for the Beijing outboard sector. AAIB Special Bull. 1/2008 states:
"The aircraft's fuel tanks were last checked for water in the fuel on the 15 January 2008 at Heathrow; this was prior to its refuelling for the outboard sector to Beijing."
Additionally, the bulletin states "sump sample taken from the left and right main fuel tanks shortly after the accident revealed no significant quantities of water".
The bulletin further states "Initial results confirm that the fuel conforms to Jet A 1 specifications and that there were no signs of contamination or unusual levels of water content."
We should probably conclude that there was no significant quantity (i.e. something like 138 gallons) of water in the system when it left Heathrow and none when it returned to Heathrow. Even if there had been water in the centre tank before the crash-landing this did not find its way to the wing tanks and then to the engines.
With regard to my earlier posts (195,242), I questioned whether the fuel could stratify due to density and thermal anomalies, in a very cool environment, over an extended period of time, being subjected to high frequency vibration from the engines and so produce layers of liquid that stifle an engine(s) by decreasing the flow rate. Of course, this is a very complex regime, probably one with subtle negative and positive feedbacks, meaning that an initial stratification would progressively mutate into something very different.
Earlier I asked if the water scavenge jet pumps in the wing tanks would destroy stratification. Thanks to Green-dot (#267) for telling us how these pumps operate and for pointing out that detected water causes, "water detection message for a particular tank to show". There was no such message shown during the flight, therefore there was no water, greater than 7 gallons, present in the bottom of the wing tanks and the water scavenge jet pumps did not operate. Any stratification would not have been destroyed by these pumps. Hope I've not missed anything.
However, SyEng (#273) says'
"Stratification: no chance. (following para is general, not 777-specific) Apart from the scavenge jet pumps (if they're not full of FOD), the boost pumps cause plenty of mixing, through their own bypass/cooling flow discharged back into the tank, through swirling/entrainment near the inlet and through collector cell feed/overflow. There is also often fuel returned to tanks from other systems heat exchangers. "
SyEng, you say the paragraph is general, not 777-specific, so is it possible to confirm the destruction of any stratification in the wing tanks by the systems you mention for the 777?
Regards, Tanimbar
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Some observations on the B777 fuel system
The centre tank is a large relatively flat based structure. The centre tank pump inlets are uncovered at about 900kg fuel remaining. The centre tank water scavenge pumps are always running, scavenging liquid from the bottom corners of the tank and delivering it to the booster pump inlets. The centre tank transfer jet pumps are running all the time, but will not deliver fuel to the wing tanks until they have dropped substantially (I posted the figures here a few days ago). There are also water scavenge pumps in the wing tanks that are always running.
Jet pumps have no moving parts. They take motive power from the booster pump outlets and direct it through a nozzle that creates a low pressure and sucks liquid up from the collector pipes. Typically a pump will have about 8 inlet pipes. These pipes are very small, say quarter of an inch dia. I have never been inside a B777 tank, but have good memories of working on Tristar scavenge systems which are very similar. The volume of liquid moved is very small and if all the jet pumps were sucking pure water it would never be enough to affect the flow of fuel to the engines.
Very occasionally an aircraft will land with fuel remaining in the centre tank. This will be because the jet pumps are blocked with debris. Small pieces of tank sealant will clog the nozzles. These are checked on major checks when the tanks are opened up.
Because of the flat bottom of the centre tank, there must be a lot of water in there (138 gallons) before the water sensing system could see it. It is spread over a large surface area. It is the same reason why the centre bank booster pumps stop pumping so early.
Water in tanks does not come from the fuel supplied. In my career I have done thousands of fuel water checks on bowsers, and never had a positive reading. The water comes from moist air that is present in the tanks when they are empty, the moisture condenses on the walls and drops into the bottom of the tank. We sump the tanks regularly to remove this water. Usually a few drops at most comes out of each tank. The reason for so little water is that the water scavenge systems work.
You cannot sump the tanks on the ramp during refuelling. It takes a long time for the water to separate out and accumulate on the bottom of the tank. It is done on layover checks every couple of days.
The centre tank is a large relatively flat based structure. The centre tank pump inlets are uncovered at about 900kg fuel remaining. The centre tank water scavenge pumps are always running, scavenging liquid from the bottom corners of the tank and delivering it to the booster pump inlets. The centre tank transfer jet pumps are running all the time, but will not deliver fuel to the wing tanks until they have dropped substantially (I posted the figures here a few days ago). There are also water scavenge pumps in the wing tanks that are always running.
Jet pumps have no moving parts. They take motive power from the booster pump outlets and direct it through a nozzle that creates a low pressure and sucks liquid up from the collector pipes. Typically a pump will have about 8 inlet pipes. These pipes are very small, say quarter of an inch dia. I have never been inside a B777 tank, but have good memories of working on Tristar scavenge systems which are very similar. The volume of liquid moved is very small and if all the jet pumps were sucking pure water it would never be enough to affect the flow of fuel to the engines.
Very occasionally an aircraft will land with fuel remaining in the centre tank. This will be because the jet pumps are blocked with debris. Small pieces of tank sealant will clog the nozzles. These are checked on major checks when the tanks are opened up.
Because of the flat bottom of the centre tank, there must be a lot of water in there (138 gallons) before the water sensing system could see it. It is spread over a large surface area. It is the same reason why the centre bank booster pumps stop pumping so early.
Water in tanks does not come from the fuel supplied. In my career I have done thousands of fuel water checks on bowsers, and never had a positive reading. The water comes from moist air that is present in the tanks when they are empty, the moisture condenses on the walls and drops into the bottom of the tank. We sump the tanks regularly to remove this water. Usually a few drops at most comes out of each tank. The reason for so little water is that the water scavenge systems work.
You cannot sump the tanks on the ramp during refuelling. It takes a long time for the water to separate out and accumulate on the bottom of the tank. It is done on layover checks every couple of days.
The Reverend
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My dear God, from aliens to EMI to RMI to running out of fuel, to icebergs in fuel tanks, Gordon Brown elctronic shields. What else can you mob helpfully suggest to the official, professional investigation team to put an end to this stream of absolute nonsense ragarding this accident.
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My dear God, from aliens to EMI to RMI to running out of fuel, to icebergs in fuel tanks,
Quote:
Many, many years go, when I was a young pilot, there was an incident in where I live, with Airbus A300's. Company was Karair, at that time a charter subsidiary of Finnair, they had two A300's, with highest utilisation of the type then, basically flying non-stop between Finland and the Canary Islands. About 18 hours airborne per day per aircraft, if I recall correctly. At some point, it was discovered that there was a huge chunk of ice floating in the fuel tank of one of the aircraft, as the condenced water never had time to melt, and thus had never been drained from the system between the flights. The aircraft had actually been very close to fuel starvation because of this. Of course, about 20 years have passed, lessons have been learnt, and this could not happen again, couldn't it...?
Now this rings a bell.
I think about 10 years ago Continental had an issue with their early 777s in that during winter ops from NWK the fuel temp never dropped sufficiently to allow any suspended/solid water to thaw and therefore be drained during normal routine checks. Result was a very large, thick sheet of ice in the base of the fuel tanks which would often give erroneous tank qty readings.
I have no idea if this has anything to do with recent events. I just wanted to comment on the above quote.
NSEU
Quote:
even if all 138 gallons of ice thawed during the descent and was scavenged, would mixing it with over 3000 gallons of remaining wing tank fuel degrade the engine performance so much?
From early on in the various threads, we have been told that the CT would have been empty, long before TOD.
We have been told that the CT is used first in it's entirety. Once fuel is below the level of the main pumps and the engines fed from the wing tanks, other pumps (the scavengers?) drain the tank down and put the remaining fuel into the wings. Thus, no great body of water would exist. If I recall correctly, that is a SOP.
Quote:
even if all 138 gallons of ice thawed during the descent and was scavenged, would mixing it with over 3000 gallons of remaining wing tank fuel degrade the engine performance so much?
From early on in the various threads, we have been told that the CT would have been empty, long before TOD.
We have been told that the CT is used first in it's entirety. Once fuel is below the level of the main pumps and the engines fed from the wing tanks, other pumps (the scavengers?) drain the tank down and put the remaining fuel into the wings. Thus, no great body of water would exist. If I recall correctly, that is a SOP.
Rgds.
NSEU
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Is there any truth in Private Eye's observations on p29 of issue 1204? Is it normal to have redundant software written by different companies in the commercial aerospace industry?
Machaca
Thanks for the drawings, would you have a schematic drawing of the aircraft fuel system from tanks to engine you could show us?
Thanks for the drawings, would you have a schematic drawing of the aircraft fuel system from tanks to engine you could show us?
Last edited by Oldlae; 22nd Feb 2008 at 12:24. Reason: spelling
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