What if something was frozen in the system, defrosted and moved to cause an obstruction during the descent ? the aircraft is symmetrical, are the tank layouts ?

Just a thought, as if it was water it would melt and leave no trace.

Before anything I’d like to say I read very interesting comments, from different people, with different views …

I’ll be corrected if I say some stupidities …

In these days of high cost energy, the biggest expense for an Airline is … FUEL !
That’s why Airlines are willing to spend big $$$ in order to hire outside companies that teach them how to save fuel and one of the easiest ways to save fuel is by avoiding to travel unnecessary fuel.
In that purpose, flight planning became very accurate (wind – temp – load – perf …), so much that when parking brake is set at the end of the day, actual fuel burn matches very well the planned one. It could be and is, sometimes, higher depending how precisely flight plan was respected and how much delay was on approach, but below … ?
Personally I don’t remember more than a couple hundred kg … but never ever more than half ton … and I’m also talking on more than 8 hours flight.
BA38 improved its fuel burn by more than 3 tons, and I believe BA Flight Planning Department to be somehow experienced.
In my view, to save 3 tons, you need to be much lighter than flight plan …

It could be interesting to compare planned and actual ZFW … if no unusual mismatch appears … then we’ll have to consider … Take Off Weight !?

I believe (and I wouldn't say I'm an expert) that yes, fuel flow is recorded in FDR. Or to be more specific, I think it is a required parameter - the 777 FDR may record other additional parameters as well, I don't know what they would be.

AAIB will also have the QAR in this case, which will probably record even more data.

Also relevant might be, how, and at what point in the line, the fuel flow is measured - I don't know that on the 777. The AIIB state that cavitation damage might indicate excessive aeration of fuel, in which case your fuel flow reading might be really fuel+air - which probably isn't a lot of help.

fuel burn
 

Report says they flew higher than plan, to "accommodate a request from ATC". Also that they were in hold at LHR for only 5mins, which I would have thought was less than expected. How much fuel saving that would account for I don't know, but it would seem likely they would be under rather than over.

Phil,

The trent 800 FMU is not capable of recording its own positions, however, it will generate a demand discrepancy should the actual vs demand position for the Metering Valve be different. Parameters for the FMU are recorded elsewhere, see post below.

The Spill Valve is built into the FMU, and moves in accordance with the scheduled increases/decreases. As the report says, the FMU's both worked properly on test, therefore, they will have worked properly on the A/C, *if* they were provided with the correct delivery from the pump.

The FMU position is displayed on the EPCS maint. page 1. It is displayed in degrees. It is also recorded on the QAR/FDR.

Fluid
 

Pursuant to earlier post, any fluid flowing more rapidly in a pipe, undergoes a lessening of pressure, and a concomitant rise in Freezing point, even boiling point. This is the essence of "vapour lock", a not uncommon problem in liquid under presure supply systems. Once more, if the Fuel is Homogeneous, and temperature/pressure/flow parameters are met, it is easy to understand how any fluid could boil, freeze or restrict flow in a supply system. An HP pump would be the obvious place to suspect, as its delta P is more pronounced than in other locations/parameters. Throw in a maximum demand for Pressure/Flow (FADEC throttle "slam") on a low pressure stable system at low temp. Just Sayin'.

Airfoilmod,,,,Please ask your friend Mr. Bernoulli,surely the increased flow and pressure drop go together with a temperature drop and a fall in boiling point

Can a supercoold slushy fluid turn solid: or a superheated one aereate when agitated in extreme circumstances? Ive seen superheated water explode , and we`ve all seen freezing rain.

Wilyflier
 

Of course. Open a cold bottle of soda and watch the liquid instantly be entrained with ice crystals, the temperature hasn't changed (functionally), the low pressure merely raises the freezing point of the beverage. Water can be frozen well above its "boiling point", and be made to boil below its "freezing" temperature. By this I mean there is a concomitant increase in associated boiling point for a liquid freezing out of standard temperature and pressure.

And that is what I'm getting at. Restricted flow from solid particles blocking orifice(s) that starve the engines. I won't quote my former posts regarding how this can happen to two engines at almost the exact same time. It is highly possible. Perhaps even unavoidable. Post #252

The EEC commands the FMU Fuel Metering Valve to a certain position and a feedback resolver on the metering valve sends a signal back to the EEC (closed loop).

I assume the "FMV" position data shown on the EICAS EPCS page is from the feedback (Torque resolver) rather than the EEC command.

This has been mentioned before. Apparently it's part of normal QAR ops. It's not a continuous recording system like the CVR and DFDR.
The recording system collates data and dumps it onto the QAR "tape"(PC memory card) at specified times.

Nothing would have worked after the crash (QAR, CVR or DFDR).

With reference to post #242, posted by Tanimbar:

quote:
"Bsieker, you wrote:
"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."

Thanks for that. Can anyone comment on how these pumps operate and to what 'depth' in the fuel their effect is propogated? Are they designed to fully mix fuel within the total volume of the tanks and so prevent stratification?" unquote

The water scavenge jet pumps take fluid from the lowest points in the tanks (1 in each main wing tank and 2 in the center tank) and send it to the fuel pump inlets. This prevents water from collecting at the bottom of the tanks. The water scavenge pumps are jet pumps and operate automatically when the fuel pumps are on. They use fuel from the fuel pumps as motive fuel. The flow of the motive fuel through the jet pump causes suction that takes fluid from the lowest point in the tanks.

If a water detector finds water in a tank, the fuel quantity processor unit (FQPU) causes the water detection message for a particular tank to show. The main tank water detection messages show at the bottom of maintenance page 1. The center tank water detection messages show at the bottom of maintenance page 2. The amount of water required to show the L MAIN WATER or R MAIN WATER message is approximately 7 gallons. The amount of water for the center tank message is approximately 138 gallons.


Regards,
Green-dot

138 gallons?
 

Green-dot
This seems rather a lot of water. Are you sure?

nickeyjsmith,
 

You wrote:

What if something was frozen in the system, defrosted and moved to cause an obstruction during the descent ? the aircraft is symmetrical, are the tank layouts ?
Just a thought, as if it was water it would melt and leave no trace.

I have had the same thoughts for a while - expecially after having read the latest report several times.
No discrepancies of serious nature has been found on the aircraft - all systems seem to have been working - all regulations with regard to keeping correct fuel temperatures are adhered to - etc - etc.

But it crashed anyway!

I am afraid that this accident may never be fully explaned:confused::confused::ugh:

airfoilmod,
...I still dont follow you.
... Fall in pressure is accompanied by a fall in boiling point and an increase in evaporation (or even gassing) ,I presume youve made tea in the galley at 8000ft cabin alt?
...When you open the soda bottle, pressure drops sharply and the gas expands , temperature goes down, producing sufficient drop in the water to freeze it into ice slush .But dont tell me we suddenly find the freezing point of the water going up
....No ,the bottle temp goes down.Try it with the CO2 bottle ! Or the choke of my Rotax carburetter
Chillyflier

Martin2116, that is exactly what I was thinking. Furthermore the obstruction was not complete because the engines still had enough fuel to idle after the incident which means the obstruction could have occurred / formed at different times but being the engines were only requiring minimal fuel prior to throttle up it was not noticed. Once more power was requested, the result of the obstruction became noticeable after the excess fuel in the lines downstream of the obstruction was used up.

If both fuel paths are not symmetrical but one has the ability to store more fuel in the lines then it could explain the 4 second difference. If the obstructions happened at different points that would also explain the 4 second difference though not as plausible. A third explanation for the 4 second difference would be that one side's lines downstream of the obstruction weren't as full of fuel as the other, ie. half as full, that would also explain the difference. A final explanation would be that both engines weren't burning the same quantity of fuel per second on throttle up, but I doubt that would be the case.

If 7 seconds calculates to be the point of the fuel intakes at the fuel tanks, then being there are two intakes on each tank is it possible that one of the intakes was completely blocked, meaning the lines associated with that particular intake were dry of fuel, while the other three intakes were only partially blocked, thus allowing about double the fuel in the lines on one side as opposed to the other.

Hopefully the investigation team already has or soon will be doing all the calculations necessary to find these points in the fuel supply as they will be key points to examine for possible causes.

I used to run the company that made most of the oil/water separators used in a/c refueling. The units are designed to deliver fuel with less than 5ppm water. There is also (supposed to be) a shut off on the delivery side which stops delivery if fuel with more that 5ppm water is detected. Of course, 5ppm of water can accumulate over several refuels but it was always felt that the scavenging pumps would take care of most of the build up and tank draining during routine maintenance would clear the rest. I am now retired so I am not sure if this is still the standard for aviation fuel. I am a PPL so I am not familiar with the details of commercial a/c fuel systems.

re. my post 216
 

According to the AAIB special report, the right MLG punctured the centre tank. 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?


Where are the pickups? Fwd or Aft of cruise attitude lowest point? How high above tank floor?

From the AAIB special report: "Samples from the

I follow you
 

At 8,000 feet,the water boils at lower temp.(lower pressure). This system is entropically related, all variables change to accomodate a conservation of energy in the system.
My Bottom Line is that Fuel, Emulsion, whatever is in the Line in the aircraft can Freeze simply by experiencing a sufficient drop in Pressure.
If you can grant me that, the rest isn't germane. Are you saying that Water always freezes at 0 degrees C.? The soda bottle can be below 0 Degrees with no ice, open the cap, instant ice, same low temp. The freezing point has migrated, but not because of Temperature.

SyEng,

Quote:

"This seems rather a lot of water. Are you sure?"


Yes, i am sure. G-YMMM is a B777-236 ER with a large center tank. The centertank carries a huge amount of fuel, approx. 176,400 lb (26,100 gallons).

The original B777-200 has small center tanks (near left and right wing root only, not in the fuselage/wing center section). Its capacity is approx. 83,800 lb (12,400 gallons). In the B777-200, the center tank message is shown at approximately 14.5 gallons.

Water detectors are ultrasonic devices. The FQPU sends signals to the water detectors. The water detectors send a puls to the bottom of the tank. If there is water at the bottom of the tank, the pulse reflects off the boundary layer between water and fuel. I think there is a minimum layer of water required before water can be detected. Since the large (ER) center tank has a large relatively flat surface area, it is not surprising there is a larger amount of water present before it can be detected.

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.

Airfoldmod, I don't think you paid enough attention during physics lectures. Does altitude alter the freezing point of water?

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

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.

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.

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.

At rib 8.
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.

This also surprised me when I read it in the Boeing 777 Maintenance Manual. I quoted it in an earlier forum posting, but I was hesitant. I really wanted to get more information from a higher source.

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.

777-200 ER Tanks
 

http://img61.imageshack.us/img61/620...anksvh4.th.jpg


http://img57.imageshack.us/img57/574...nks2dm2.th.jpg


http://img57.imageshack.us/img57/675...nks5lm6.th.jpg


http://img57.imageshack.us/img57/989...nks3zm6.th.jpg

Non-pilot speaking
NSEUFrom 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.

Cowled Environment
 

Are temperatures in the pylon and under the cowling much above OAT during cruise? Descent?

http://img57.imageshack.us/img57/1477/80066bsc7.th.jpg

for the fan cowl prob not too much after extended periods - why do u have a "theory"?

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

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.

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.:confused:

From a previous post in another thread on this issue....

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.

I was simply speculating on what would happen if there was ice in the bottom of the tank. You can't scavenge ice.

Rgds.
NSEU

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?

What would happen is that it would stay in the tank as ice, until it melted when it would stay in the tank as water. It would then be found there (as either ice or water), at the point that the investigators looked at the tanks and, in fact, found "no significant quantities of water".

One point to mention.......after the first engine lack of power, was a cross feed valve opened before second engine lack of power occured???