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BA038 (B777) Thread

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BA038 (B777) Thread

Old 21st Feb 2008, 08:31
  #221 (permalink)  
 
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Mad (Flt) Scientist,
When a SB is issued by the manufacturer, it must go to someone in the company to decide the priority, in my company that was the Type Engineer. When reading it and it is presumed that they understand the aircraft systems, the person must have wondered why it was issued and the implication that there was a possibility that the spar valves wouldn't be closed when the fire handles are pulled. I would have also thought that the training course for the electrics on the 777 would have shown up the anomaly. Hindsight I know, but having been a TE I know how important it is to realise the implications of SB's etc on Operations.
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Old 21st Feb 2008, 08:31
  #222 (permalink)  
 
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My understanding is that the EEC is the Engine Electronic Control. Now if the system is anything like the tiny little gas turbine on my aircraft, the EEC has very little to do with the Fuel Control Unit, which is a hydromechanical device that functions entirely automatically and is self-powered, using fuel pressure derived from the shaft-driven HP pump as the motive power for its various servo functions.
Sounds awfully like you're trying to compare this to a GE engine, Sven?

The Rolls Royce Trent has a fuel pump and a Fuel Metering Unit. The pump is mechanically driven. The FMU is further downstream and simply meters whatever fuel comes out of the pump (according to EEC electronic input).... and delivers this metered fuel to the spray nozzles in the combustor. It would be too confusing to compare the two engine types

Rgds.
NSEU
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Old 21st Feb 2008, 08:37
  #223 (permalink)  
 
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Ice chips. No doubt about it!

Ah, if BA ain't doing sump checks after fuelling at Shanghai maybe they orta... the fuel there is half water on good days... the icing lights are always coming on. Yes yes yes I know, the truck sample... I like the Chinese.

That 108 knots... um, if we had been hand flying, maybe the old girl would have just sort of naturally wandered on down at Vref or so, then a short transition in ground effect, we would have maybe gone another dozen metres.

I can well see that detail, the speed decay, being overlooked in the mental rushing about caused by the unexpected, inexplicable, sinister and potentially lethal turn of events. It is just that if it had all started say 500 feet higher then with that sort of speed bleed-off we might have had a really high sink rate going by the time we hit the ground.

I mention that only because it is the fashion these days to use the pilot regardless... the chief pilots / fleet managers etc all like it / recommend it / insist on it because they can't trust people to do things they mostly can't do themselves, namely hand-fly an approach on a sunny afternoon. So let's go back to hand flying when hand flying is an option, and keep our motor skills up, and certain piloting-related sub-systems at the back of our brains in the piloting loop.

Just a thought.
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Old 21st Feb 2008, 08:38
  #224 (permalink)  
 
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MiG 15

regarding cavitation, there are two phases, the formation of the bubbles, and their collapse.

They form at low pressure, and collapse at high pressure, so it is straightforward that the cavitation damage is on the outlet (high pressure) side, where the bubbles collapse.


Bernd
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Old 21st Feb 2008, 08:44
  #225 (permalink)  
 
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NSEU
I didn't mean by the simulator, I meant that the drill carried out by the pilots undergoing checks etc in the simulator, could have shown up the anomaly when the shut-down procedure was shared between the two.
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Old 21st Feb 2008, 08:52
  #226 (permalink)  
 
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Originally Posted by Oldlae
I didn't mean by the simulator, I meant that the drill carried out by the pilots undergoing checks etc in the simulator, could have shown up the anomaly when the shut-down procedure was shared between the two.
Only if the simulation included severing of the wiring from the fire handle to the spar fuel valve. Otherwise the valve would have just closed when the fire handle was operated, and no anomaly would have shown.

Is it common in the simulator to simulate such specific damages?

Maybe, just maybe, Boeing should have objected to BA's checklist for unmodifed aircraft, but who knows.


Bernd
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Old 21st Feb 2008, 09:08
  #227 (permalink)  
 
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AAIB: "The engines did not shut down and both engines continued to produce thrust at an engine speed above flight idle, but less than the commanded thrust."
It is interesting that on this topic the latest report essentially repeats the information released earlier and gives no new details about the behaviour of the engines. Was the rpm steady, decreasing or increasing, were there any minor or major surges, did the engines behave identically?

This kind of details may give some clues as to the nature of the fault. For example, I'd expect the signs of a fuel flow impairment to be different than those of a fuel contamination.

My interpretation of this is that the AAIB will use the detail data to simulate any possible fault sources, but wants to do so in peace from any media speculations. And that's OK for me. We'll see the outcome sooner or later.
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Old 21st Feb 2008, 09:14
  #228 (permalink)  
 
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Fuel stratification - follow-up comments

Follow-up to my earlier posting (#195, page 10) on the possibility of fuel stratification.
Warning: I'm not crew nor engineer, just scientist.

Hand Solo, you wrote:
"I would think stratification would be unlikely given the aircrafts frequent manouvring and encounters with turbulence. The fuel would have been well and truly mixed up for most of the flight and certainly for the last 30 minutes."

I think you might be mistaken given that the g forces applied to the aircraft during manoeuvers are very low and the resulting force is typically normal to the wing surface (apologies, being very simplistic). The g forces would not be sufficient to disturb a stratification to cause it to break-up, i.e. the fuel becomes fully mixed.

Turbulence could cause a break-up of stratification if violent and with the imposed g forces being applied largely parallel to the wing surface. Moderate turbulence, i.e. the aircraft falls and rises rapidly, and applying g forces normal to the wing surface, might cause disturbance of stratified layers, even cause disruption, but I don't think it would take long for the full stratification to return in calm air. Plus, there are no reports of such events on this flight.

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?

Bsieker also wrote:
"A minor slip: the imbalance was 300kg, not 30kg, which makes it still less likely that both engines would be fed the same undigestible type of stratum at roughly the same."

Sorry, a typo. When considering the imbalance, and its possible implications, I had in mind the 300Kg figure. A 5.8% weight imbalance equates to a few centimeters in depth of fuel (for example, 2.9 cm for a fuel depth of 0.5m). Of course, I don't know the dimensions of the fuel tank and so cannot calculate the exact difference in fuel depth between the two wing tanks but, I'd be surprised if the actual difference is greater than 2-3cm. This would possibly suggest that stratification, if it exists, in
both wing tanks would be essentially identical and might be fed into the tank outlet ports at the nearly the same time. Fascinating!

Further, I note now, having missed the text before, that the AAIB Special Bulletin 1/2008 concludes with:
"In addition, comprehensive examination and analysis is to be conducted on the entire aircraft and engine fuel system; including the modelling of fuel flows taking account of the environmental and aerodynamic effects."

Also, someone elsewhere on this thread mentioned that it had been previously thought that the mixing of fuels, provenanced from different sources, was not likely to cause any problems but was now being investigated.

And finally to politics, the elephant in the room: the fuel was sourced from Peking in an Olympic
year.

Regards, Tanimbar
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Old 21st Feb 2008, 09:16
  #229 (permalink)  
 
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How can one believe that ice obstruction can lead to such a result ?!
First we have 4 LP pumps in the tanks, meaning a quadruple simultaneous obstruction, reducing the fuel flow to a SIMILAR value on both sides (resulting in this just above flight idle thrust). This looks unrealistic to me.
In case of ice obstructions, the fuel restrictions wouldn't have been exactly similar, and thrust values would have been different on each engine.
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Old 21st Feb 2008, 09:57
  #230 (permalink)  
 
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"Further, I note now, having missed the text before, that the AAIB Special Bulletin 1/2008 concludes with:
"In addition, comprehensive examination and analysis is to be conducted on the entire aircraft and engine fuel system; including the modelling of fuel flows taking account of the environmental and aerodynamic effects.""

It seems to me this is the most likely way the AAIB will uncover the cause.

I dont know how much data they have recovered from the flight recorders, but must be enough for some detailed modeling. If the fuel flow is the common mode failure, it was asynchronous by 7 seconds between the engines. Is it not possible the initial response to a demand for thrust was using the fuel in the pipework and any other volumes after the restriction blockage.

So if we know the diameter of the pipework, the speed of the engines, caculated quantity of fuel used in the 3 seconds one engine and 7 seconds the other, cant one calculate the distance back along the fuel lines when the fuel stopped coming? Is there an offcentre pump, tank, point or junction that fits the data?

Just an idea
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Old 21st Feb 2008, 12:23
  #231 (permalink)  
 
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Quote (NSEU) :-

......... The Rolls Royce Trent has a fuel pump and a Fuel Metering Unit. The pump is mechanically driven. The FMU is further downstream and simply meters whatever fuel comes out of the pump (according to EEC electronic input).... and delivers this metered fuel to the spray nozzles in the combustor. ........

unquote

So, the Fuel Metering Unit should have its own data recorded - and hence shouldn't the AAIB Special Bulletin have comented on its staisfactory performance ? The bulletin comments that the correct commands were given by the EEC, but not what data was being recorded by either the FMU or spill valves as to what actions they were performing.

.
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Old 21st Feb 2008, 12:42
  #232 (permalink)  
 
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phil gollin.. did you miss this bit?
Data also revealed that the fuel metering valves on both engines correctly moved to the fully open position to schedule an increase in fuel flow. Both fuel metering units were tested and examined, and revealed no pre-existing defects.
Regards
TP
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Old 21st Feb 2008, 13:07
  #233 (permalink)  
 
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No, it says the units worked when tested, it did not state that the meausurements recorded by those units (and the spill valves
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Old 21st Feb 2008, 13:22
  #234 (permalink)  
 
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Originally Posted by Mig15 View Post
Like I said, it was just a thought!

Just remember, the information recorded and available to the AAIB is recorded electronically. Could an EMP not cause erroneous data to be recorded which appears to show all systems were operating "normally"?

For environment, don't just think of weather!

For this to be an interference (or software) fault, with the information the AAIB have released now, it would have to be a fault which:

- reduced engine thrust to below that commanded
- simultaneously faked the data to the FDR etc. to show that everything right through to the fuel valves was responding to commands
- did all this without leaving any other trace of interference in the recorded data

To say that is not credible (for software bug or RFI) is putting it mildly.

Yes it could still have been software failure, or RFI, just as it could still have been aliens. I note the AAIB haven't explicitly ruled out alien involvement either...
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Old 21st Feb 2008, 14:20
  #235 (permalink)  
 
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I don't think mixing of different fuel grades or fuel from different sources should be an issue: refineries routinely blend to produce jet fuel in the first place and transfers of different grades between storage tanks are common place.

I'm not convinced by arguments based on fuel freezing - if the TAT was -37 and the fuel freeze point -57 how did it freeze? The freeze point is the appearance of crystals, not the point at which the fuel goes solid, and the margin was 20C, not the 3C required.

Stratification of products in storage tanks (caused by density differences) is well known in the liquid gas industry. However, stratification does not occur in ships, and this is attributed to motion. Note the design of the ships minimises sloshing at the liquid surface, it is the induced motion in the bulk fluid that prevents stratification occurring. I would suggest the continuous motion of an aircraft in flight would have a similar effect.

The odd bit in all this is that whatever happened affected both engines at virtually the same time, though the systems are supposedly completely independent. Is there some kind of (hidden) master- slave relationship between the engines buried in the software?

If I understand correctly, there are two pumps per tank - even with a master slave relationship between the engines this seems to rule out obstruction of the pump inlet screens by a foreign object or pump failure.

I think if the investigators knew the answer it would have leaked by now - which makes the whole affair ever more curious.
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Old 21st Feb 2008, 16:57
  #236 (permalink)  
 
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I'm not convinced by arguments based on fuel freezing - if the TAT was -37 and the fuel freeze point -57 how did it freeze? The freeze point is the appearance of crystals, not the point at which the fuel goes solid, and the margin was 20C, not the 3C required.
Indeed, but how about the fuel temperature in the pipework? could it, somewhere down the line, get below the temp measured in the coldest spot of the tank? could part of the fuel, during the descent towards LHR, when idling, begin to freeze and cause an (temporary) obstructon?
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Old 21st Feb 2008, 17:14
  #237 (permalink)  
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borgha

Fuel, leaving the Tankage, is under ever increasing pressure from LP pumps toward the HP pumps not to mention, it is moving, both actions would increase its resistance to solidification by introducing friction due to transit, and pressure, which lowers the Freezing Point. At the Low Side of the HP pumps, given a restriction upstream, the Pressure could be drastically reduced, raising the Freezing Point and potentiating fuel solidification or, more likely, transient emulsified water turning to ice which could have blocked critical pathways. This is my current theory, notwithstanding AAIB testing of several samples of collected Fuel.

Again, with a "homogeneous Fuel supply" isolation/separation of Fuel source might have not "protected" either powerplant. The restriction would be timed emphatically on temperatures, demand, and pressure, all of which may have been so close, the isolation would be "overridden" by parameters that could not be prevented by merely "separating" the Fuel sources. The five second lag actually supports this eventuation.

If my theory has weight, it might be better to Fuel the Engines differently.
One from a Wing the other from the Center, given that the location and other factors of the different tanks increase the heterogeneity of the Fuel source.

Last edited by airfoilmod; 21st Feb 2008 at 17:28. Reason: add
 
Old 21st Feb 2008, 17:32
  #238 (permalink)  
 
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Thanks airfoilmod, I see your point, but what would be the fuel pressure/freezing point gradient? this might be of some importance on the LP side of any of the pumps. is there any expert out there in fluid dynamics?
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Old 21st Feb 2008, 17:34
  #239 (permalink)  
 
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PLEASE PLEASE can we stop talking about BA doing sump checks after refuelling.
I am sure 95% or more do not after refuelling, if l understand correctly the final fuel figure is worked out after final pax & cargo figures are known.
You would then have to wait 2 hours or more for the fuel to settle before the suspended water sinks to the sump drain points.
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Old 21st Feb 2008, 17:42
  #240 (permalink)  
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Restriction

I think we may be going in the same direction. Additionally, it might be more harmful than not to be so focussed on precision in FADEC. I think the system probably worked perfectly so I use it only to suggest that in ETOPS it may actually be helpful to introduce anomalies of benign nature into the control system. If a Fuel source is Homogeneous, sourcing it in different locations in the A/C creates a tolerable "discontinuity" that may introduce a higher level of safety, rather than a lower, albeit predictable, one.

The authority I would suggest, Daniel Bernoulli, dead, but his work lives on.
 

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