BA038 (B777) Thread
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From shawk, Post 521: " Unusual acoustical standing waves in piping can restrict fuel flow"
I guess such a standing wave would only occur at particular resonant path lengths dependent on air and/or fuel volume shapes. I'm not sure if short wavelength standing waves would restrict fuel flow, but they would cause cavitation, as happens in ultrasonic cleaning.
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Quoting Bill_s, ref. post #527:
and quoting ve3id, ref. post #529:
Bill_s and ve3id:
Reading your posts it seems you are assuming the (possible) source originated from outside the aircraft.
The scenario could be considerably different if the (possible) source originated from inside the aircraft.
To my knowledge certain transmitting PEDs can generate electric fields in the order of 20 Volts (or more) in relatively close proximity to the transmitting PED.
Next question is if something like this had occurred, could it have affected the relays, subject to this discussion? For those "in the know" about where these relays are located and how they "function in the total scheme of things" (Ref. AMM), it would be something to at least take into consideration.
Regards,
Green-dot
The problem is that you would have to induce some 20 volts of stray RF directly into the relay coil wiring, for a significant period of time, for this scenario to work. This RF would have to penetrate both the acft metal hull and any shielding on the wires. I have no idea how much RF power would have to be delivered to the outside of the hull, but my guess is it would be upwards of tens of kilowatts. This massive amount of RF power would probably disrupt other electronics in the acft long before it acted directly on any relay.
(DC = direct current, RF = radio frequency energy)
and quoting ve3id, ref. post #529:
I think you are barking up the wrong tree here! I just did a quick calculation using 28 Volts across a 1k ohm load (the coil at RF) and used 140dB for space loss beyond the e-field.
I came up with 78 TeraWatts!
If someone was using that kind of RF power, you would know. All the lights in and around the airport would dim!
I came up with 78 TeraWatts!
If someone was using that kind of RF power, you would know. All the lights in and around the airport would dim!
Bill_s and ve3id:
Reading your posts it seems you are assuming the (possible) source originated from outside the aircraft.
The scenario could be considerably different if the (possible) source originated from inside the aircraft.
To my knowledge certain transmitting PEDs can generate electric fields in the order of 20 Volts (or more) in relatively close proximity to the transmitting PED.
Next question is if something like this had occurred, could it have affected the relays, subject to this discussion? For those "in the know" about where these relays are located and how they "function in the total scheme of things" (Ref. AMM), it would be something to at least take into consideration.
Regards,
Green-dot
From a recent post:
"This is because air dissolved in the liquid will tend to come out of solution at low pressures, and contribute a partial pressure of air to the contents of any macroscopic cavitation bubble. When that bubble is convected into a region of higher pressure and the vapor condenses, this leaves a small air bubble that only redissolves very slowly, if at all."
I previously posted a description of what may be an analogous event which I encountered when I had a small mining operation. It was evidently disregarded as unworthy of comment
. It may be that I do not understand the above, but it seems to imply a similar phenomenon.
To amplify my original post slightly I'd just add that the gradual accumulation of air in the pump casing had little or no noticeable effect until it suddenly reached a critical threshold, at which point the pump quite abruptly ceased to deliver. This ocurred several times, after repeated re-priming, before the cause of the problem (increased restriction of flow in the suction line) dawned on me.
"This is because air dissolved in the liquid will tend to come out of solution at low pressures, and contribute a partial pressure of air to the contents of any macroscopic cavitation bubble. When that bubble is convected into a region of higher pressure and the vapor condenses, this leaves a small air bubble that only redissolves very slowly, if at all."
I previously posted a description of what may be an analogous event which I encountered when I had a small mining operation. It was evidently disregarded as unworthy of comment
. It may be that I do not understand the above, but it seems to imply a similar phenomenon.To amplify my original post slightly I'd just add that the gradual accumulation of air in the pump casing had little or no noticeable effect until it suddenly reached a critical threshold, at which point the pump quite abruptly ceased to deliver. This ocurred several times, after repeated re-priming, before the cause of the problem (increased restriction of flow in the suction line) dawned on me.
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Don't have time to read the whole thread, so apologies, but does anyone know how the engines responded at 1000'AGL when they would have been at approach power IAW BA SOPs?
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Fuel - "excessive aeration"
Fuel "excessive aeration" theory might be explained, in part, by a significant difference in more recent Boeings (post TWA800). Recent Boeings use centre tank fuel scavenge jet pumps with motive power provided by main tank fuel from the main tank booster pumps. Moreover in 777s such jet pumps remain on, once main tanks reduced to half full, for remainder of flight; whereas in 737s (other than more recent NGs) they are powered on for just 20 minutes (and link only to main tank No1) before a shut-off valve removes motive power.
Accordingly, here is another slightly different, ridiculously remote Swiss cheese line-up:
1. Jet A-1 in main tanks was saturated with air, either when uplifted and/or as result of prior centre fuel scavenge tank jet pump operation on fuel remaining from previous sectors. (Kerosene has much greater propensity to become saturated with air and retain it even at altitude compared to say water.)
2. Centre fuel scavenge tank jet pumps operated to entrain and dissolve air into fuel in each main tank thereby supersaturating fuel in each main tank during last hour of flight (by then last of centre tank fuel has been scavenged). Increasing pressure on descent helped increase the level of saturation.
3. LP main tank booster pumps were unaffected by supersaturated fuel (i.e. no cavitation) and no LOW PRESS alert as a result.
4. Cavitation induced in HP pump as flow increased by order of magnitude on finals.
5. Relatively cold fuel made cavitation much more pronounced (see http://naca.central.cranfield.ac.uk/...rc/cp/1128.pdf) and, once started, cavitation worsened preventing increase in flow rate as more and more air was released into system. Some flow was maintained, just insufficient.
6. Suction feed as alternate to HP pumps was ineffective and/or became ineffective to increase fuel flow to level demanded. Perhaps due to:
(a) inherent weakness of suction feed (for inadequacy of suction feed in certain flight conditions, see article on UAL flight 767 which in climb at altitude suffered rollback when boost pumps turned off and suction feed alone turned out to be insufficient - http://findarticles.com/p/articles/m..._73578123/pg_4).
(b) secondary effect of release of air at HP pumps as a result of cavitation impeding operation of suction feed bypass.
(c) reduced flow at suction feed inlet from main tanks due to (i) FOD blockage on right hand side at least and/or (ii) perhaps (if sufficient newly melted water, derived from ice formed in surge tanks or centre tank, could have entered and frozen in main tanks) ice particles.
Accordingly, here is another slightly different, ridiculously remote Swiss cheese line-up:
1. Jet A-1 in main tanks was saturated with air, either when uplifted and/or as result of prior centre fuel scavenge tank jet pump operation on fuel remaining from previous sectors. (Kerosene has much greater propensity to become saturated with air and retain it even at altitude compared to say water.)
2. Centre fuel scavenge tank jet pumps operated to entrain and dissolve air into fuel in each main tank thereby supersaturating fuel in each main tank during last hour of flight (by then last of centre tank fuel has been scavenged). Increasing pressure on descent helped increase the level of saturation.
3. LP main tank booster pumps were unaffected by supersaturated fuel (i.e. no cavitation) and no LOW PRESS alert as a result.
4. Cavitation induced in HP pump as flow increased by order of magnitude on finals.
5. Relatively cold fuel made cavitation much more pronounced (see http://naca.central.cranfield.ac.uk/...rc/cp/1128.pdf) and, once started, cavitation worsened preventing increase in flow rate as more and more air was released into system. Some flow was maintained, just insufficient.
6. Suction feed as alternate to HP pumps was ineffective and/or became ineffective to increase fuel flow to level demanded. Perhaps due to:
(a) inherent weakness of suction feed (for inadequacy of suction feed in certain flight conditions, see article on UAL flight 767 which in climb at altitude suffered rollback when boost pumps turned off and suction feed alone turned out to be insufficient - http://findarticles.com/p/articles/m..._73578123/pg_4).
(b) secondary effect of release of air at HP pumps as a result of cavitation impeding operation of suction feed bypass.
(c) reduced flow at suction feed inlet from main tanks due to (i) FOD blockage on right hand side at least and/or (ii) perhaps (if sufficient newly melted water, derived from ice formed in surge tanks or centre tank, could have entered and frozen in main tanks) ice particles.
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From Message #561: The 777 uses a 'new' acoustic based fuel volume measurement technique. Presumably a pure ultrasonic tone is far more likely to set up a standing wave than the white noise from a vibration source, unless of course the designers had already thought of the potential problem and use frequency agility based on prime numbers?
I guess such a standing wave would only occur at particular resonant path lengths dependent on air and/or fuel volume shapes.
I guess such a standing wave would only occur at particular resonant path lengths dependent on air and/or fuel volume shapes.
Thanks Jetdoc. So if the suction bypass valves are simply check valves, at any time the pressure of air trapped in the suction pipe exceeds the fuel manifold pressure, they should open and allow the pressurised air into the engine fuel supply.......?
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777fly
The suction pipe is really not all that long. Just imagine the pipe with a screen on the end of it similar to the boost pump pickups. Its attached directly to the engine feed manifold. The check valve is in the pipeline. The pick up point is somewhat low in the tank about 3 or 4 inches above the tank bottom. Its location is the 4th manhole cover outboard from the wing root and probably less than 10 feet inboard of the engine pylon. One would hope that it is still covered by fuel and not full of air even at that point in the flight.
The suction pipe is really not all that long. Just imagine the pipe with a screen on the end of it similar to the boost pump pickups. Its attached directly to the engine feed manifold. The check valve is in the pipeline. The pick up point is somewhat low in the tank about 3 or 4 inches above the tank bottom. Its location is the 4th manhole cover outboard from the wing root and probably less than 10 feet inboard of the engine pylon. One would hope that it is still covered by fuel and not full of air even at that point in the flight.
Last edited by Jetdoc; 2nd Mar 2008 at 17:21. Reason: correct spelling
Blame the snowman ...
or:
Why I fly Warriors, in nice warm weather.
Have you noticed how dirty a snowman gets as it melts?
(The situation on the approach to LHR was worse than worst case design spec, so look at worst case design spec. Any numbers are very approximate, but spurious precision has been retained so that their origin is recognisable)
900Kg of 'unusable' fuel in the centre tank - recovered to the outboard (high) end of the main tanks by the fuel scavenge system when main tank pumps are working. That's 2000 lbs or 250 UK gallons.
Up to 138 (US?) gallons of water trigger the water-in-fuel warning - 115 UK gallons. So the centre tank dregs to be scavenged could be half water, half fuel. It gets scavenged 'water first' in the cruise.
The water (mostly) comes in with ambient air replacing fuel used. At altitude the air is cold and even if saturated has relatively low water content. In cloud, the air is saturated and carries suspended droplets as well. Climbing uses fuel at the greatest rate. Climbing on the centre tank through cloud brings water into the centre tank at the greatest rate - but it is a relatively short phase of a long haul flight.
The water droplets, either carried in as cloud or condensing with adiabatic expansion of the air in the tanks, collect in the fuel, and slowly settle at the water scavenge pump inlet (at cruise attitude) and get 'burned off' when the scavenge pump discharges them adjacent to the inlet of the pump supplying the engine. Water only accumulates at the water scavenge inlet if it arrives faster than the scavenge pump sucks it away. Nothing can go wrong ...
But what if it is not water? What if the local condensation and ingested cloud is ice?
Ice granules will not coalesce to form droplets, so the layer of ice granules at the bottom of the centre tank will only be scavenged near the scavenge pump inlet. Instead of flowing to the lowest point as liquid water would, the granules will roll down a local embankment of granules, under the gravitational influence of their small density difference. So whilst the scavenge pump will keep the local area clear, ice crystals will settle like snow everywhere else. The scavenge pump will only start to clean the whole tank when the tank temperature rises above freezing, and the crystals melt into droplets and globules of water that run down to the scavenge point. If the whole take-off, climb and cruise has been in sub-zero temperatures, the centre tank water scavenge pump will be off before this happens. Centre tank clearance of water will start when (and if) the centre tank temperature rises above freezing in the descent, when it will be cleared to the main tank by the fuel scavenge pump, and burned off from the main tank by the main tank water scavenge.
If the centre tank does not collect enough heat to provide the latent heat needed to melt the ice slush before engine shutdown, the slush will melt on the ramp and collect in the sump, potentially causing a water warning at next start-up. This will clear when the water scavenge pump burns it off. If sub-zero ramp and take-off temperatures freeze this water, it almost certainly stays frozen until the next descent. There is no scavenge whilst burning fuel from the centre tank because the inlet is frozen solid. There is then a double dose of solid and powdered ice to be cleared by the fuel scavenge pump when it melts during the next descent.
Provided there is a big enough safety gap between bottom level of useable fuel and the top level of water, and assuming the scavenge pumps can't pump water quickly enough to bring the aircraft down, nothing can go wrong ....
But the snowman effect spoils the party. As a snowman thaws, the atmospheric dirt and dust collected by the falling snow (and, admittedly, dirt collected by the youngsters who built the snowman) is caught and concentrated by its receding and shrinking surface. This is not just because the dirt cannot evaporate. Even a melting snowman seems able to cling on to his dark coat whilst shedding melt water. The dirt is efficiently concentrated on the surface. So after several journeys in freezing or near freezing ground temperatures, or maybe a couple of such journeys through a particularly dirty industrial atmosphere, it is easy to visualise the dregs of the centre tank water and slush, with all its accumulated micro-muck, being slopped into the fuel scavenge area by a change of trim or even by the unstable progression of the melt process.
So water with concentrated micro-dirt surges into both fuel scavenge areas of the centre tank, to be promptly fed to the main tanks, draining 'fairly promptly' to the water scavenge points on the main tanks, to be then fed to the engines - affecting both engines within a few seconds of one another.
The dirt nucleates cavitation, killing throughput of the LP engine pump.
Fire away ...
Why I fly Warriors, in nice warm weather.
Have you noticed how dirty a snowman gets as it melts?
(The situation on the approach to LHR was worse than worst case design spec, so look at worst case design spec. Any numbers are very approximate, but spurious precision has been retained so that their origin is recognisable)
900Kg of 'unusable' fuel in the centre tank - recovered to the outboard (high) end of the main tanks by the fuel scavenge system when main tank pumps are working. That's 2000 lbs or 250 UK gallons.
Up to 138 (US?) gallons of water trigger the water-in-fuel warning - 115 UK gallons. So the centre tank dregs to be scavenged could be half water, half fuel. It gets scavenged 'water first' in the cruise.
The water (mostly) comes in with ambient air replacing fuel used. At altitude the air is cold and even if saturated has relatively low water content. In cloud, the air is saturated and carries suspended droplets as well. Climbing uses fuel at the greatest rate. Climbing on the centre tank through cloud brings water into the centre tank at the greatest rate - but it is a relatively short phase of a long haul flight.
The water droplets, either carried in as cloud or condensing with adiabatic expansion of the air in the tanks, collect in the fuel, and slowly settle at the water scavenge pump inlet (at cruise attitude) and get 'burned off' when the scavenge pump discharges them adjacent to the inlet of the pump supplying the engine. Water only accumulates at the water scavenge inlet if it arrives faster than the scavenge pump sucks it away. Nothing can go wrong ...
But what if it is not water? What if the local condensation and ingested cloud is ice?
Ice granules will not coalesce to form droplets, so the layer of ice granules at the bottom of the centre tank will only be scavenged near the scavenge pump inlet. Instead of flowing to the lowest point as liquid water would, the granules will roll down a local embankment of granules, under the gravitational influence of their small density difference. So whilst the scavenge pump will keep the local area clear, ice crystals will settle like snow everywhere else. The scavenge pump will only start to clean the whole tank when the tank temperature rises above freezing, and the crystals melt into droplets and globules of water that run down to the scavenge point. If the whole take-off, climb and cruise has been in sub-zero temperatures, the centre tank water scavenge pump will be off before this happens. Centre tank clearance of water will start when (and if) the centre tank temperature rises above freezing in the descent, when it will be cleared to the main tank by the fuel scavenge pump, and burned off from the main tank by the main tank water scavenge.
If the centre tank does not collect enough heat to provide the latent heat needed to melt the ice slush before engine shutdown, the slush will melt on the ramp and collect in the sump, potentially causing a water warning at next start-up. This will clear when the water scavenge pump burns it off. If sub-zero ramp and take-off temperatures freeze this water, it almost certainly stays frozen until the next descent. There is no scavenge whilst burning fuel from the centre tank because the inlet is frozen solid. There is then a double dose of solid and powdered ice to be cleared by the fuel scavenge pump when it melts during the next descent.
Provided there is a big enough safety gap between bottom level of useable fuel and the top level of water, and assuming the scavenge pumps can't pump water quickly enough to bring the aircraft down, nothing can go wrong ....
But the snowman effect spoils the party. As a snowman thaws, the atmospheric dirt and dust collected by the falling snow (and, admittedly, dirt collected by the youngsters who built the snowman) is caught and concentrated by its receding and shrinking surface. This is not just because the dirt cannot evaporate. Even a melting snowman seems able to cling on to his dark coat whilst shedding melt water. The dirt is efficiently concentrated on the surface. So after several journeys in freezing or near freezing ground temperatures, or maybe a couple of such journeys through a particularly dirty industrial atmosphere, it is easy to visualise the dregs of the centre tank water and slush, with all its accumulated micro-muck, being slopped into the fuel scavenge area by a change of trim or even by the unstable progression of the melt process.
So water with concentrated micro-dirt surges into both fuel scavenge areas of the centre tank, to be promptly fed to the main tanks, draining 'fairly promptly' to the water scavenge points on the main tanks, to be then fed to the engines - affecting both engines within a few seconds of one another.
The dirt nucleates cavitation, killing throughput of the LP engine pump.
Fire away ...
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Not quite simultaneous thrust reduction
So when does this happen (Rightbase)?
5LY in post 326 had an answer:
Doing the conversion gives 13,500kg. The aircraft landed with roughly 10,500kg on board (AAIB). So rough calc suggests the scavenge from the Center to BOTH Main tanks started about half an hour before landing (72,000kg/12 hours = 6,000kg/hr of flight). At what point did it reach the possible contamination - depends on rate of flow of scavenge system... - but the contamination reached both main tanks at roughly the same time... thrust reduction 7 seconds apart ... (pure speculation of course!).
5LY in post 326 had an answer:
The center tank pumps are manually turned off when the tank reaches 2000 pounds (900Kg.). The scavange system then operates automatically to draw out the remaining fuel when the total fuel remaining reaches 29,000 pounds (do your own conversion).
Last edited by Leodis737; 2nd Mar 2008 at 23:01. Reason: completion!
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For both engine LP pumps to fail to deliver sufficient flow (and failing in such a way as to prevent suction feed) within a few seconds of each other due to prior events (wear/damage) or instantaneous fault is as unlikely as to be unimaginable.
For the fluid diet of the pumps and engines to be the cause, fed from different tanks, and again with that vital proviso - within a few seconds of each other - the common centre tank and its associated functions at that stage of flight must be involved ?
This seems to be the guts of what we are hearing now...
Sorry, just thinking out loud to focus my feeble ability to keep up with some of the earlier posts, but the Dirty Snowman has put nucleation and cavitation into contextual focus for me... Any more ..ations needed to complete this picture?
For the fluid diet of the pumps and engines to be the cause, fed from different tanks, and again with that vital proviso - within a few seconds of each other - the common centre tank and its associated functions at that stage of flight must be involved ?
This seems to be the guts of what we are hearing now...
Sorry, just thinking out loud to focus my feeble ability to keep up with some of the earlier posts, but the Dirty Snowman has put nucleation and cavitation into contextual focus for me... Any more ..ations needed to complete this picture?
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Leodis - don't lose any sleep over purely speculating on this thread. Subtract 2 AIB report posts from the total and the rest equals the number of other speculators sharing the beach with you 
It happens when ...
melting slush in the otherwise empty centre tank collapses and slips down into the fuel scavenge intake areas - both at about the same time.
The fuel scavenge pumps (powered by pressure from the main tank pumps) then start to transfer the muck to the main tanks. With one wing slightly higher than the other (was the cross wind from the South on that day?) this purged muck slides down one main tank faster than the other, so the right engine gets it first, followed a few seconds later by the left.
Only an idea, but as a jigsaw puzzle piece it looks to me like a better than average fit.
But what do I know? I only fly singles with gravity feed.
The fuel scavenge pumps (powered by pressure from the main tank pumps) then start to transfer the muck to the main tanks. With one wing slightly higher than the other (was the cross wind from the South on that day?) this purged muck slides down one main tank faster than the other, so the right engine gets it first, followed a few seconds later by the left.
Only an idea, but as a jigsaw puzzle piece it looks to me like a better than average fit.
But what do I know? I only fly singles with gravity feed.
On advice ...
Just to stop others making money out of my fabulous (good word?) idea:
Suggestions - published so that they are freely available, not patented:
Refuel the tank to be used for climb out with warmed fuel, so that the water build up can be purged before it freezes. This will also prevent legacy build up from previous flights. It might even thaw legacy frozen muck so that maintenance can drain it off before engine start.
Lag the tank so that it stays warm.
Warm tanks in flight so that the water purge works in the cruise, instead of leaving it to the warmer descent phase.
Disable the fuel purge from at least one side before you are committed to needing power to make the field.
Suggestions - published so that they are freely available, not patented:
Refuel the tank to be used for climb out with warmed fuel, so that the water build up can be purged before it freezes. This will also prevent legacy build up from previous flights. It might even thaw legacy frozen muck so that maintenance can drain it off before engine start.
Lag the tank so that it stays warm.
Warm tanks in flight so that the water purge works in the cruise, instead of leaving it to the warmer descent phase.
Disable the fuel purge from at least one side before you are committed to needing power to make the field.
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"The fuel scavenge pumps (powered by pressure from the main tank pumps) then start to transfer the muck to the main tanks."
Note that muck in the CT has to pass through an inlet screen before it is scavenged, then through a narrow opening at the jet pump, then, flowing from the outlet of the jet pump, has to jump over baffles, pass through wing boost pump inlet screens, then has to pass through one or two engine filters (depending on where the blockage is), one of which has an impending bypass alert..... and almost simutaneously affect two engines.
Not saying it couldn't happen, but....
The CT fuel scavenge rate is 400Kg/per hour minimum. By your calculations the centre tank would have 700Kg remaining on landing 
Note that muck in the CT has to pass through an inlet screen before it is scavenged, then through a narrow opening at the jet pump, then, flowing from the outlet of the jet pump, has to jump over baffles, pass through wing boost pump inlet screens, then has to pass through one or two engine filters (depending on where the blockage is), one of which has an impending bypass alert..... and almost simutaneously affect two engines.
Not saying it couldn't happen, but....
So rough calc suggests the scavenge from the Center to BOTH Main tanks started about half an hour before landing (72,000kg/12 hours = 6,000kg/hr of flight).
Still barking up the wrong tree
The fact that there is a measurement of 20 Volts per metre does not mean that the voltage will be developed across a coil. The problem is that the relay coil has a very low impedance compared to air, even looking like a short circuit. There is no way a PED would create a field strong enough to drive the current.
Sorry, it is out of the question.
If a PED had interfered with the high-impedance parts of a circuit that controlled a relay, then I could see it, but by the fact that we are talking about relays acting on their own I assume that has been ruled out
Sorry, it is out of the question.
If a PED had interfered with the high-impedance parts of a circuit that controlled a relay, then I could see it, but by the fact that we are talking about relays acting on their own I assume that has been ruled out
Rightbase: "Climbing in cloud brings water into the centre tank"
How does that happen? During the climb the air in the centre tank will be venting overboard as it expands due to reducing external atmospheric pressure. How does cloud vapour overcome that, to get into the tank? Even if it could, the volume of tank occupied by the 4000kg or so of fuel used in the climb could not contain enough 'cloud' to hold a signficant amount of water. Also it is very rare to spend more than a few minutes in cloud during the climb, if at all.
How does that happen? During the climb the air in the centre tank will be venting overboard as it expands due to reducing external atmospheric pressure. How does cloud vapour overcome that, to get into the tank? Even if it could, the volume of tank occupied by the 4000kg or so of fuel used in the climb could not contain enough 'cloud' to hold a signficant amount of water. Also it is very rare to spend more than a few minutes in cloud during the climb, if at all.