nickyjsmith

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.

CONF iture

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 !?

infrequentflyer789

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.

infrequentflyer789

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.

RO13ERTS

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.

gas path

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

airfoilmod

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'.

wilyflier

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.

airfoilmod

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

NSEU

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.

NSEU

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).

Green-dot

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

SyEng

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

grebllaw123d

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

wilyflier

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

GordyOZ

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.

peebs24

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.

SyEng

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

airfoilmod

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.

Green-dot

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.