BA038 (B777) Thread

[Green-dot]

Dear Oluf,


I newer flew with RR engines, but my P & Ws on a MD-80 once had a very hard time getting up in RPMs with the front end of the engine all iced-up, it first "hesitated" then started to stall, first after three tries and very slow advancement of the throttles it worked normal again. (Eng. Anti-ice was ON)


Was what you describe here on the ground while taxying or in flight?

I remember an accident which was caused by moderate icing, engine vibration and consequent failure of the icing panels on both engines.
Quoting from the report:

"Conduct of flight:
Simultaneously with the icing alert the crew switched the engine ant-icing and airframe anti-icing systems on. The crew attributed the increased vibrations indicated for the RH engine, after reduction of airspeed and entering cloud (icing conditions) in FL100, to ice formation on the fan. The airplane flew, for approx. 6 minutes, with reduced engine thrust under moderate icing conditions which, in all likelyhood, led to icing of the fan blades of both engines.

After a prolonged time under moderate icing conditions and low engine thrust, ice developed on the rotors of the low pressure compressors of both engines. During descent from FL90 to 3,500 ft the engines were running smoothly in a low thrust range and all engine indications were in the normal range. The crew could not recognise the reduced performance of the two engines. Thus the PIC did not hesitate to operate the airbrakes for some time, in order to reduce the airspeed to such a degree that the flaps could be extended. Once flaps and landing gear were extended it became apparent that the engines developed insufficient thrust. There was no malfunction indication at that time because in the flight warning computer monitoring of the N1 to EPR ratio is not intended. EPR indications showed, however, that dispite an RPM increase the engines developed insufficient thrust.

Cause:
The bonded joints of the ice impact panels on both engines failed due to strains caused by ice-induced vibration of the engines and by ice which had detached from the rotors of the low pressure compressor. The loose ice impact panels became trapped in front of the outlet guide vanes of the low pressure compressor and affected the airflow in the by-pass duct in such a way that the engines only produced low thrust."

Now compared to BA38, in the above mentioned case RPM (and EGT) increased but the engines developed insufficient thrust due to air flow restrictions. Increase in RPM and EGT implies that the fuel flow was getting to the engines.

In case of BA38, thrust increased initially, then rolled back even with fully opened fuel metering valves. After the rollback, no mention of increased RPM or EGT, indicating this is not very likely to be an icing problem but that the required fuel flow was not getting to the engines. What is also missing in case of BA38 is that no mention has been made of vibration due to icing or shedding of ice.

For the full report, here is the link (including FDR data, please note RPM, EPR and EGT parameters):

http://www.bfu-web.de/nn_53140/EN/Publications/Investigation_20Report/2004/Report__04__AX001-0__MUC__Fokker,templateId=raw,property=publicationFile.pdf/Report_04_AX001-0_MUC_Fokker.pdf


Green-dot

[stickyb]

I newer flew with RR engines, but my P & Ws on a MD-80 once had a very hard time getting up in RPMs with the front end of the engine all iced-up, it first "hesitated" then started to stall, first after three tries and very slow advancement of the throttles it worked normal again. (Eng. Anti-ice was ON)




A question from a nonpilot trying to understand things.

Oluf, I can see how ice could cause the symptoms you describe above on your MD-80, but I am at a loss to see how core ice could lead to reduction of fuel flow (and we know it was reduction of fuel flow on the BA flight)

Can you explain it to me please?

[barit1]

Turbine engine acceleration is a pretty complex process. Simply dumping in more fuel does not assure the desired results.

The fuel controller (MEC, FADEC, whatever...) has been tailored to produce best acceleration on a reasonably healthy engine - it adds a measured amount of fuel to start the accel process, then as RPM and pressures build, a bit more fuel is added, and the cycle repeats... This is to prevent stalls, surges, overtemp, etc.

However - an ice accumulation effectively "redesigns" the aerodynamics so that the amount of fuel the controller thinks correct is now too much! Ergo, stall, lack of acceleration, etc.

A wise pilot can "milk' the throttle through this stage and not demand so much fuel - he's effectively "redesigning" the accel fuel schedule to match the new compressor aerodynamics. Once a high enough RPM is reached, the ice is either melted by heat of compression, or broken loose by centrifugal force, and things return to normal (unless the impact of ice chunks on downstream blades is significant...)

OK?

[Oluf Husted]

Dear barit1 and other PPruNers,

Thanks for your comments, since I am danish, you will have to excuse some of my word selections, but we seems to agree, that ice is the only "aerodynamic redesign" that melts away.

When I had my experiences with the MD-80 engines, it was on the ground, trying to get T/O power, after some taxiing in foggy weather. And that was my only experience of that kind in a 27 month period.

This was 19 years ago, maybe newer engines are better in trying to protect themselves from stalls, vibrations and ice damage?

Maybe the BA38 engines just never made it to the stall and vibration phase?

Oluf

[tanimbar]

Warning: I'm non-professional; not crew, not engineer - just scientist guest and thanks.

Dear Oluf,
Please stop insisting in this thread that core icing played a role in the crash of G-YMMM.

The AAIB Special bulletin states:

At the point when the right engine began to lose thrust the data indicated that the right engine EEC responded correctly to a reduction in fuel flow to the right engine, followed by a similar response from the left EEC when fuel flow to the left engine diminished.


The crucial words are "reduction in fuel flow" and for both engines.

Also from the AAIB, the fuel metering valves on both engines correctly moved to the fully open position to schedule an increase in fuel flow - but fuel flow did not increase.

The fuel did not flow in sufficient quantities!

Therefore, and unless I've failed to understand how these fuel systems operate, core icing cannot have contributed to the cause of BA038's crash.

Core icing is an interesting subject, probably deserving a thread of its own, but please, PLEASE, stop distracting those of us who are interested in understanding G-YMMM's crash.

Regards, Tanimbar

PS. Apologies to those who have already made these points.

[lomapaseo]

However - an ice accumulation effectively "redesigns" the aerodynamics so that the amount of fuel the controller thinks correct is now too much! Ergo, stall, lack of acceleration, etc

Oh I wish that you hadn't left that so simple and thus feed Oluf with his ever so simple explanations.

It's true that with a significant amount of aerodynamic change (ice accretion) the fuel metering will not match the performance, but the signature is much more likely to be a mismatch in rotor spool RPMs N1, N2, N3 followed by a stall/surge. All of which are sure to be recorded on the various recorders, including non-volatile memory in the FADECS.

Add to this the extereme rareity of such non-mechanical damage events for in-flight scenarios (ground fog at ground idle is not part of this discussion).

[glad rag]

This thread is going in circles, everyone is arguing their pet theory, information, DIS-information, perhaps a temporary lock on it until we ALL get some proper news from the investigators might be an idea?????

GR

[chris weston]

A reference was made in an earlier post to (fuel) chemists seemingly having disappeared from the thread. Well this chemist has not entirely disappeared and has continued to read all the posts on this admirable thread.

I have added nothing, having nothing new to say but a reiteration may be useful – the mods will decide.

My hypothesis (- a careful use of English) remains the same i.e. that either a stratification of the fuel as a function of relative density and / or water scavenging additives such as the friends of epoxy ethane et al, may have fed either low Relative Molecular Mass fuel components or scavenged water at sufficiently high concentrations when increased thrust was commanded by the FADECs, to induce cavitation sufficient to reduce effective flow rates to the donkeys at circa 700 feet.

The hypothesis sort of fits the incomplete I am sure evidence currently available in public domain but only up to a point is the problem and I am very happy to wait for the AAIB final report. The time scale for their reportage is entirely consistent with their SOPs and I have the very highest level of respect for these chaps.

The carefully preserved circa 9 tonnes of fuel are now of course very well mixed and (just one) problem with my hypothesis is the sheer complexity of replicating the conditions. As someone who calls himself an experimental chemist I blanch at the thought of the experimental need to (i) cold soak the necessary tonnes of fresh samples of Chinese fuel (ii) putting said fuel on top of the right amount of LHR fuel left in a 777 cold soaked main wing tank (iii) using the same rigs and (iv) then replicating in full the thermal history of the fuel back to LHR as far as it is known whilst (v) pumping from this tank in line with the data and (vi) constantly measuring the Heat of Combustion of the pumped fuel.

We need to wait.:ok:

[wilyflier]

Bsieker
Thats just how compressor stalls can behave.
...Also,ice by itself can upset the airflow,without necessarily shedding or causing mechanical damage
I do remember a reference to passenger quotations of grinding noises on the approach , but that may of course have been the "landing"
Lom Paseo
,,,,Extreme rarity doesnt rule it out

[bsieker]

Bsieker
Thats just how compressor stalls can behave.
...Also,ice by itself can upset the airflow,without necessarily shedding or causing mechanical damage
I do remember a reference to passenger quotations of grinding noises on the approach , but that may of course have been the "landing"

It may be that compressor stalls can manifest themselves in a way that the engine first accelerates and then rolls back. And also that it may happen on both engines at nearly the same time.

But that is not the point.

The AAIB specifically talks about:

(a) fuel flow reducing, and

(b) metering valve opening to fully open, in response to the low fuel flow.

(Remember that fuel flow is measured between the fuel metering unit and the HP filter, and is also recorded.)

If a compressor stall (caused by core icing/fan icing/cowl icing/whatever) can cause low fuel flow despite high flow being scheduled by opening the metering valve, I'd love to be enlightened about a possible mechanism.


Bernd

[FullWings]

And icing would have affected other aircraft on approach to LHR at that time, including many 777-200ERs, so why weren't they all falling out of the sky on the same flight profile?

[Oluf Husted]

Dear FullWings,

All other flights probably flew "straight and level" in a holding, or at some given flight level, at least three times, thus coming above flight idle for several minutes.

It has been so since 1973, when I began flying into the greater London Area.

Oluf

[Green-dot]

Dear Oluf,


All other flights probably flew "straight and level" in a holding, or at some given flight level, at least three times, thus coming above flight idle for several minutes.


How BA38 performed the holding has not (yet) been published in detail.

But suppose your theory of core icing had occurred:

- EPR, N1 and EGT are the primary engine indications.
- On the RR Trent 800 series, EPR is the thrust setting parameter (with EECs in normal mode).
- The thrust management function of AIMS calculates the reference thrust based on existing ambient conditions and the particular thrust reference mode selected by the pilot on the CDU. The pilot can override a thrust mode by manually advancing the thrust levers (which he did when the engines started to roll back).

Normally, with the thrust levers manually at the full forward position, thrust limit protection allows full rated thrust without exceeding the maximum thrust limits. EECs (in normal mode) provide EPR limit protection and also N1, N2 and N3 RPM overspeed protection.

In case of core icing and/or fan icing (and following your theory, no restriction in fuel flow), advancing the thrust levers fully forward, the EECs (in normal mode) would attempt to accelerate the engines from actual EPR to commanded EPR. With the engines contaminated by ice, the commanded EPR would likely not be reached due to restricted airflow through the engines. The EECs would allow N1, N2 and N3 to accelerate (again, with unrestricted fuel flow available) to attempt reaching the commanded EPR until an RPM reaches an overspeed protection limit. EGT would also rise considerably. Since EPR, N1 and EGT are primary indications, any abnormal indications on the EICAS display would have been noticed by the crew, certainly if an RPM limit would be reached which results in an EICAS advisory message. In this situation, advancing the trust levers will not increase thrust.

If this had occurred, the following would have been recorded on the FDR:

-Low EPR (low engine thrust);
-increased fuel flow;
-Increased RPM;
-Increased EGT.

Of course, this may have resulted in engine thrust stagnation or roll backs but the fuel flow increase would certainly have been observed, if not directly by an FDR parameter, indirectly by RPM and EGT increased parameters against a low EPR parameter.

If it had occurred that way, this investigation would by now have been in an advanced phase with the AAIB completing a final report soon.

However, according to the AAIB reports this did not occur. The AAIB, in the latest report released in February, was focussing on damaged engine HP pumps and the engine fuel system to answer why fuel flow to the engines deminished while the fuel metering valves were commanded fully open.


Regards,
Green-dot

[barit1]

Green-dot - All I can say to you is: please re-read my post #1028.

Simply adding fuel may not produce the desired acceleration, if there is something abnormal going on in the core flowpath. You cannot thus assume a more fuel >> more RPM scenario. :uhoh:

That said, please note that I am not taking a position pro or con icing in the BA038 case. I'm simply pointing that out as one possibility.

[airfoilmod]

I certainly agree that "simply adding Fuel" may not produce the desired result. In Normal circumstances, however, it is basically that simple.

Lomapaseo describes turbojet basics as: Spin, spark, spray. I prefer one I heard from a flight instructor "Suck,Squeeze,Bang,Blow".

Both oversimplified, but to paraphrase your remarks, "Simply adding No Fuel will certainly not attain the desired result". I think you are correct in saying the report does not say, Fuel was/was not supplied in response to commanded level. It is a good catch, and deserves noting.

I vacillate between fascination and impatience regarding AAIB progress.
IMHO all discussion is valuable; I am acquiring knowledge in many fields free of charge, and I appreciate the accessibility of the website. This is a humbling and valuable exercise, to me.

[TyroPicard]

airfoilmod
Surely "Suck,Squeeze,Bang,Blow" refers to the Otto cycle of the four-stroke reciprocating internal combustion engine - AFAIK these were not fitted....

Green-dot's post is very relevant - if core icing were present the engines would have reacted completely differently.
TP

[BOAC]

Suck,Squeeze,Bang,Blow - well, I had a Harrier Pegasus do that once:)

[shawk]

The report notes that the fuel pumps show signs of cavitation damage.
How would engine core icing cause fuel pump cavitation?

[skridlov]

As a know-nothing, I hesitate to venture once again into this astonishingly complex discussion. I can follow the arguments in general terms but do not pretend any more than that. But I previously posted on my own experience of cavitation effects in a centrifugal pump (in a mining context) and would appreciate a layman's-level explanation.

I have seen cavitation cause a 6" Ajax water pump to repeatedly fail to deliver. The cause was restricted flow at the foot-valve which resulted in air coming out of solution and gradually accumulating in the pump casing until sufficient volume was attained to cause cavitation and loss of delivery pressure.

I have seen many references to cavitation in this thread; it seems to be a generally accepted factor in the events leading up to the accident. In my ignorance I keep thinking that if there was cavitation, where did the air/gas come from if not out of solution? And if there was cavitation in a pump, it's hardly surprising that fuel ceased to be delivered (if indeed it did cease...)
When I experienced this phenomenon, there was not a gradual drop in pressure, it was more like a sudden failure (at the head of about 200 ft where the pump delivered).

Could someone please suggest why the cavitation isn't a sufficient explanation in itself? I ask this not as a contentious "stir", merely out of puzzlement. In all the posts I recall cavitation is mentioned in passing as if it is just symptomatic rather than possibly critical - which may well be the case; I simply don't understand why.

Apologies once more for the layman's question.

[cheggersb]

I agree with Tanimbar and others who from time to time try and get the thread back on topic.

The FADEC on modern gas turbines has logic to detect comprssor stall condition.
The FADEC on most gas turbines of that generation has logic to accommodate compressor stall (fuel dipping, blead valve scheduled open etc.).
The a/c QAR catures sufficient data to enable engineers to determine if a compressor stall has occurred.
The a/c QAR and EEC both capture sufficient data for the engineers to determine if the fuel flow into the engine fuel system is less than has been demanded.
The report implies that the condition of the fuel pumps supports the theory that fuel flow from the tanks is lower than intended (suction head too low).
The report also states that despite the condition, the pumps were able to deliver full fuel flow in the correct conditions.
The official report makes no mention of core compressor icing, stall nor an reports of cor compressor damage.

I think before posting pet theories one should familiarise one's self with the facts first. Lets allow the authorities to get on with figuring why the fuel flow from the tanks was lower than it should have been despite sufficient fuel in the tanks. If anyone has new suggestions on this issue great! I think if it was easy they would have told us by now.....:ok:

If anyone wants to talk about "modified aero dynamics" - new topic?

If people want to talk about "turbine compression"....another forum :ugh:

[skridlov]

Just in case it isn't clear, I have no idea what caused this accident, other than ideas stimulated by this discussion. I am completely unqualified to have an opinion, still less a "theory"!

But I'd be interested in hearing someone explain cavitation in this context.
BTW I just spoke to a former colleague who was present at the same event I described and his recollection is that we had repeated reduction in pressure just before we lost it entirely each time this cavitation occurred.

I take the point that there has been a statement that the fuel flow was not impacted in this case. Can someone suitably qualified explain if a cavitating pump can sucessfully deliver at the correct rate. This doesn't imply that I think it can't!

[bsieker]

I have seen many references to cavitation in this thread; it seems to be a generally accepted factor in the events leading up to the accident.

Not so. Although the AAIB report mentions cavitation signs on the HP pump outlet, it is by no means clear that this damage was caused by an event immediately preceding this accident.

In my ignorance I keep thinking that if there was cavitation, where did the air/gas come from if not out of solution? And if there was cavitation in a pump, it's hardly surprising that fuel ceased to be delivered (if indeed it did cease...)

The cavities can be either air or fuel vapors. The fact that cavitation damage was apparent shows that at least some of the bubbles collapsed again on the outlet, causing abrasion. The manufacturer of the pump and experts will know whether or not it was able to sustain sufficient flow under cavitating conditions, all other things being equal. This information does not appear to be publicly available.

Could someone please suggest why the cavitation isn't a sufficient explanation in itself?

Cavitation does not happen under normal conditions in a well-designed and tried-and-tested fuel delivery system with (as the AAIB said) on-spec fuel. It has to be caused by something, which may be upstream flow restriction or excessive fuel aeration. Cavitation may then exacerbate that problem, but will not be a root cause.


Bernd

[woodpecker]

I am sure the AAIB checked other 777 pumps to see if they had signs of cavitation, especially as the manufacturer stated that the pump (even with the cavitation damage/wear) was capable of meeting its design specification.

That information would establish if any deterioration in the pumps performance due cavitation was caused on the flight in question or could have been caused on a previous flight.

[airfoilmod]

It may not be clear to some that cavitation was caused by events just prior to the incident, but that is exactly what Boeing Captain Carbaugh suggested and AAIB is looking for a "restriction in the Fuel system upstream as a cause of the cavitation." See post #662 and the flightglobal.com article (3-14-08) quoting Dave Carbaugh.

It may be a linguistics issue, saying cavitation couldn't be the "cause of Fuel Starvation", but it certainly would show a "restriction upstream" as evidenced by damage to pump lobes. Said restriction might be the "proximate cause of the lack of Fuel". Carbaugh said the cavitation might have been caused BY restriction, making the pump damage a RESULT of restriction. (He also mentioned ice in fuel and made reference to "temperature").

[bsieker]

It may not be clear to some that cavitation was caused by events just prior to the incident, but that is exactly what Boeing Captain Carbaugh suggested and AAIB is looking for a "restriction in the Fuel system upstream as a cause of the cavitation." See post #662 and the flightglobal.com article (3-14-08) quoting Dave Carbaugh.

I agree that it is a likely scenario, and it fits the rest of the scenario just fine, an upstream fuel flow restriction, causing both pump cavitation and restricted flow (the former possibly exacerbating the latter). All I was saying is that we (in the uninformed public) do not (yet) know at what point in time the cavitation occurred that caused the visible signs. This is a matter of level of confidence in the factors in our causal reasoning. If I were to make a Why-Because-Graph of this, pump cavitation would be marked as an "Assumption".

Note that even the flightglobal article talks about a "possible" fuel flow restriction, and that the cavitation signs "might indicate" such a restriction. The assessment that the cavitation occurred "not long before the impact" has not yet been publicly supported by facts. It is so far just an assumption, useful for creating test scenarios, but it should not be confused with an established fact.

It may be a linguistics issue, saying cavitation couldn't be the "cause of Fuel Starvation", but it certainly would show a "restriction upstream" as evidenced by damage to pump lobes. Said restriction might be the "proximate cause of the lack of Fuel". Carbaugh said the cavitation might have been caused BY restriction, making the pump damage a RESULT of restriction. (He also mentioned ice in fuel and made reference to "temperature").

Yes, I almost completely agree. Except that it's not a linguistic issue, but an issue of causality.

My previous comments were in response to skridlov asking (linguistically unambiguous) why cavitation could not be a sufficient cause in itself.


Bernd

[arcniz]

From experience observing complex fluids in experimental systems, I believe cold fuel in the lines leading to the pumps might have a tendency to 'shear' and locally develop some very abnormal physical properties if the supply flow is disrupted upstream and the downstream pumps are still doing their best to push max fuel toward the engines.

Any gasses dissolved in the fuel would increase in volume and come out of solution. Some volatile fractions of the fuel itself might gassify. Whipped around by the under-loaded and possibly over-heating pump impeller blades, the gas plus liquid would flow forward as a frothy foam, still burn-capable but much lower in density and fuel energy than normal.

[Rightbase]

The cavitation damage may or may not be relevant to this incident, but it is incontrovertible evidence that at some stage in its life that pump has had to suck harder than it should. Or put another way, at some stage in its life it has found the fuel supply system under performing.

An analysis of the pump population will indicate how rare or common it is for the fuel supply to under perform.

We know that for it to critically under perform is a rare event. To estimate the probability of this rare critical under performance, it would be useful to know how much non-critical under performance occurs, and how it correlates with aircraft history.

Then we might be able to determine how and why it happens, and how often the under performance becomes 'nearly critical'.

[barit1]

arcniz makes an excellent point re cavitation:
Any gasses dissolved in the fuel would increase in volume and come out of solution. Some volatile fractions of the fuel itself might gassify. Whipped around by the under-loaded and possibly over-heating pump impeller blades, the gas plus liquid would flow forward as a frothy foam, still burn-capable but much lower in density and fuel energy than normal.

Such conditions may well exist at the HP pump inlet, but the pump discharge must still be at a much higher pressure to overcome the burner (air) pressure. Otherwise we would see backflow, starving the burner and an immediate flameout. I'd don't think that's evident on the DFDR.

[arcniz]

the pump discharge must still be at a much higher pressure to overcome the burner (air) pressure

Seems like a great spot for a backflow check valve, wot? Of course absence of fuel flow for any duration will tend to cause flameout also, but a pulsing flow mode might be the natural result in a fuel choking-foaming situation if some type of check valve is present inline at the outlet side of the pumps. Isn't it likely the igniters were full on at that late stage in the descent?

Even if the foamy froth I described were re-compressed to a higher pressure, the new fluid likely would still have a notably lower density and some different physical properties due to the disruption of the original fuel fluid structure and consequent rejiggering of the intramolecular van der Waal's forces in the resulting froth. The aereated fuel would almost certainly have a lower density than the original JPxx for any given flow pressure.

The phenomenon is rather like those products one purchases at the store which once removed will never again fit into the original box.

[sky9]

Has anyone heard of RR fuel pumps being pulled for strip downs and comparison to ascertain whether this cavitation damage is a one-off or common occurrence?

[HotDog]

Everybody is still trying to outguess the AAIB. Amongst the more bizarre theories is frothing fuel! More like frothing at the mouth, give us a break. Put the lock on this thread.:rolleyes:

[HarryMann]

Everybody is still trying to outguess the AAIB.Not really, as I trust the AAIB are not guessing much at this point... maybe weighing up, but not guessing as in pizzing in de vind!

[PeePeerune]

Was it pilot error?

Whats goin on, the aaib must know at this stage if it was a mechanical problem.

[FullWings]

Was it pilot error?

Always a possibility. However, I can't see a realistic way of duplicating what happened to BA38, even if I had to. How could you cause both engines to 'hang' above flight idle when you're sitting on the flight deck?

[slip and turn]

...Amongst the more bizarre theories is frothing fuel! More like frothing at the mouth, give us a break... Actually HotDog, I think you will find that information was offered by someone who understands more about the properties of pumped fuels at various temparatures and pressures than most of us know about hot dinners or dogs :suspect:

There's a lot you can tell from a laboratory bench that you couldn't possibly guess at from any seat in the cigar tube.

[Milt]

Engine Refresher

barit1 raises an interesting point concerning the pressure to be overcome by the HP engine fuel pumps at the burner nozzles at low thrust. This could be expected to be the addition of the engine core compressor output and the back pressure from the turbine/s which would not be inconsiderable. My guess would be about 100 to 200 psi.

Does anyone have a typical pressure plot through the engine core at low thrust or the range of air pressures at the burner nozzles over the thrust range at low altitude?

[PETTIFOGGER]

BA038

Milt, I do not know the answer to your question, but the following may be of help.

The final delivery path of fuel to a gas generator is designed to be more than capable of overcoming pressure variations in the burner. The only way (that I know of) that the final delivery can be compromised is by a defect or number of defects further up (or is it down) the line. According to the AAIB preliminary report everything between the tanks and the gas generators have been individually tested and pass muster, although cavitation damage on the impellors was noted. This might suggest a restriction in fuel flow, since pumps are carefully designed not to cavitate in the normal and extreme operating parameters that can reasonably be expected.

Hence the suggestion in my only other post (28 Feb #500) that it might be useful to run a computational fluid dynamics programme (CFD) to see where the problem might lie, which may be valuable in any subsequent physical simulation. A CFD programme may not however be able take account adequately of the effect of high frequency sound, and there is a lot of that about. High frequency sound (including structure-borne resonance) can cause disturbance in fluids, including cavitation, which may not be evident after the event, except when the entrained disturbed fluid meets a pump and leaves its mark.

Is this the area in which the AAIB should be looking? Maybe. Why hasn’t it occurred before in identical aircraft? Probably because of the infinite number of temperature/pressure/sound frequency/fuel velocity combinations.

As an aside, the incident happened about 15 minutes before I was due to land (as a passenger) but I only spent another 45 minutes aloft. Very inconsiderate all the same.
rgds

[boaclhryul]

...I only spent another 45 minutes aloft. Very inconsiderate all the same.

You don't make it clear who or what was 'inconsiderate'. Did the BA038 fuel not take your needs into consideration, or was it the engines themselves?

[PETTIFOGGER]

For clarity, I was not (not) on BA038, just following (as pax) from HKG. The 'inconsiderate' bit on which I should have given a fuller explanation was knowing of the incident just after it occurred (announced by pilot) but not knowing the fate of those involved until after we landed. I think we were either first or second to land after the incident/accident.

[cwatters]

I suspect everyone was rather busy working out what to do with the large number of planes delayed and where they were going to go. Watching BBC News during the event I was quite impressed how fast the airport handled such a major incident. I suspect there wasn't spare radio bandwith to give updates on the health of the passengers in those 45 mins. It's busy enough on a good day. Which airport did they divert you to?

[airfoilmod]

Regarding BA038 and to a greater extent, other investigations, it's important to note that there are as many perspectives as there are stakeholders in the "outcome". Perhaps closest to the pure neutral position is AAIB, as folks back away from their point of view, parochial vestings start to appear. I think that is why those who annoyingly post "Wait for the Report, etc." have an inarguable point. Long ago, when media was more limited, had fewer outlets and showed a small amount of judgment, the public was less interested, simply because information and venue were so limited. A friend died at DFW in the Delta 1011 windshear crash, another lost #2 (727) on takeoff, still another landed with left main gear collapsed. Incidents inform as well as titillate, and that should be the goal of AAIB BA038. I don't take this thread too seriously (with certain few exceptions), but the expanse of the web permits, no, encourages amateurish opinion and rant. Having been involved in many investigations, (albeit working for folks with a huge stake in the outcome), I can safely say in my opinion AAIB at this stage is well ahead of anyone here, it is the nature of the exercise. Rant off, but armed.

[alex990088]

I found this in the House of Lords Hansard - Written Answers - 8 May

Lord Bassam of Brighton: The Air Accidents Investigation Branch (AAIB) investigation has identified that when the aircraft was at a height of approximately 700 feet, an increase power demand was received at both engine control units. Despite the correct operation of all engine control functions and sufficient fuel on board the aircraft, the engines did not respond.
The focus of the investigation continues to be the fuel system both in the aircraft and on the engines. Under the direction of the AAIB, extensive and challenging full engine testing continues at Rolls-Royce, Derby and some equally, if not more so, challenging fuel system testing is ongoing at Boeing, Seattle, USA.
These tests are collectively aimed at understanding and, if possible, replicating the system/engine performance experienced during the accident.
Fuel experts and statisticians are also involved in the investigation, reviewing and analysing many thousands of data points recorded on the Boeing 777 fleet of aircraft to see if any trends of statistical significance can further contribute to an understanding of the accident.
The Federal Aviation Administration, the European Aviation Safety Agency and the Civil Aviation Authority are being kept fully briefed on the progress of the investigation.

[FireLight]

Regarding BA038 and to a greater extent, other investigations, it's important to note that there are as many perspectives as there are stakeholders in the "outcome". Perhaps closest to the pure neutral position is AAIB, as folks back away from their point of view, parochial vestings start to appear. I think that is why those who annoyingly post "Wait for the Report, etc." have an inarguable point. Long ago, when media was more limited, had fewer outlets and showed a small amount of judgment, the public was less interested, simply because information and venue were so limited. A friend died at DFW in the Delta 1011 windshear crash, another lost #2 (727) on takeoff, still another landed with left main gear collapsed. Incidents inform as well as titillate, and that should be the goal of AAIB BA038. I don't take this thread too seriously (with certain few exceptions), but the expanse of the web permits, no, encourages amateurish opinion and rant. Having been involved in many investigations, (albeit working for folks with a huge stake in the outcome), I can safely say in my opinion AAIB at this stage is well ahead of anyone here, it is the nature of the exercise. Rant off, but armed.

I entirely agree. The other difference between this thread (and other similar ones) vs the AAIB investigation is that they (naturally) have access to a lot more facts regarding the incident than we do. This thread has expounded on many theories. They range from theories which are nearly or totally fact deficient, to those theories which at least try to fit the known facts into their scenarios. Unfortunately, we don't have enough facts to sharpen the focus much more than it has been to this point. We don't have all of the facts we need to entirely exclude some scenarios either. It does help that there are many here who have important insight on the systems involved. It seems to me that the theories identifying the fuel/fuel system versus environmental conditions are the most likely. The post from alex990088 seems to be acknowledgement that this is the case.

In the end, the facts will determine. :ok: In the meantime, it is a worthwhile mental exercise trying to put the pieces together. Even though we know that the jigsaw puzzle is missing a number of important bits.

[limp_leek]

Lord Bassam of Brighton:


The Air Accidents Investigation Branch (AAIB) investigation has identified that when the aircraft was at a height of approximately 700 feet, an increase power demand was received at both engine control units. Despite the correct operation of all engine control functions and sufficient fuel on board the aircraft, the engines did not respond.


AAIB Special Bulletin: 1/2008

The engines initially responded
but, at a height of about 720 ft, the thrust of the right engine
reduced. Some seven seconds later, the thrust reduced on
the left engine to a similar level.

[precept]

Note the issuance of Special Report S-3 by the AAIB on 12 May.
http://www.aaib.dft.gov.uk/latest_news/special_bulletin_s3_2008.cfm

[Old Aero Guy]

This should answer a few of the theories postulated on this thread.

http://www.flightglobal.com/articles/2008/05/12/223637/aaib-restriction-to-the-fuel-flow-caused-the-uncommanded-power-reduction-that-led-ba-777-to-crash-la.html

The possiblities for the incident are being narrowed.

[phil gollin]

I note the report needs a little editing.

It contains the words :-

"...... The autothrottles commanded an increase in thrust from both engines and the engines initially responded. However, at a height of about 720 ft the thrust of the right engine reduced to approximately 1.03 EPR (engine pressure ratio): some seven seconds later the thrust of the left engine reduced to approximately 1.02 EPR. The reduction in thrust on both engines was the result of a reduced fuel flow and all engine parameters after the thrust reduction were consistent with this. ....."

and

"...... and the engines, in order to understand why neither engine responded to the demand increase in power when all of the engine control functions operated normally. ....."

---------

But seriously, I am amazed that the reduced thrust was so similar 1.03 v. 1.02 EPR !

[bsieker]

Today the AAIB issued a new Special Bulletin (http://www.aaib.gov.uk/latest_news/special_bulletin_s3_2008.cfm), updating information of previous reports.

Of note:

- No evidence of:
. EMI
. Core Engine Icing
. Bird Strike
. Wake Vortex
. Fuel Contamination

- Actual EPR values: Left 1.03 EPR, Right 1.02 EPR.

- Reduced Fuel flow was detected (could be previously inferred, since the previous report stated that the EEC "reacted" to reduced fuel flow)

- Flow stayed low after FMV was fully open (refuting "low-energy fuel" and "foaming fuel" theories)

- Spar valves operated correctly (previously only stated that they had not been operated by the crew, before the accident)

- The cavitation damage on the high-pressure pumps was fresh (I consider this the most interesting new information)

- No recommendations by either AAIB, Boeing or RR.


Findings so far

- Cause of low engine thrust: low fuel flow

- Cause of low fuel flow: Not the faintest clue.


The upshot: complex test rigs are used at Boeing and Rolls-Royce to recreate the conditions of the flight and hopefully stumble upon something useful.

Data recordings of this and other flights are being examined statistically in the hope of finding unusual parameter combinations, since all parameters by themselves appear to be well within the operating envelope of the aircraft.


Bernd

[borghha]

After S3/2008....

Evironmental factors.... fuel flow restriction.... but 'operated within flight envelope'....

as a SLF, I find it puzzling that no SB or general warning to 777 operating companies is deemed necessary: perhaps nature itself will replicate all contributing factors before the AAIB, RR or Boeing manage to do so.

taking account of public interest is a good thing for a public body (that is reading PPrune quite thouroughly and dismissing many theories), but this bulletin is rather worrying: no precautionary measures whatsoever - not very reassuring for a frequentt flyer, neither for some 777 crew I suppose.

[Flight Safety]

Borghha, what would the AAIB recommend? Since the cause is not known yet (and this cause is a real mystery resulting in a difficult investigation), how would you suggest avoiding a repeat?

[borghha]

well Flight Safety,


how would you suggest avoiding a repeat?



the fact that the environmental factors are mentioned, allbeit without implying a direct causal link, but it being made very clear that there were no mechanical/electrical/electronic failures, nor RFI etc suggests to me that they suspect a link between the prolonged cold soak and the incident. So wouldn t it be advisable to avoid these extremely low OAT s, as long as the real cause is not known?

[bsieker]

[...] made very clear that there were no mechanical/electrical/electronic failures, nor RFI etc suggests to me that they suspect a link between the prolonged cold soak and the incident.

As usual, the AAIB chose the words very carefully. The report says there was no evidence of EMI, core icing, electronics or systems malfunctioning.

But since no evidence whatsoever of what went wrong has been found, it may have been one of those things, just without leaving any evidence that could be discovered with the methods employed so far.


Bernd

[bubbers44]

Narrowing it down to low fuel pressure and low flow from the tanks and fresh cavitation to the hp fuel pumps and the isa temp at altitude was -20 that day causing other flights on that route to descend to FL250 that day continues to point to super cold fuel causing fuel waxing to not let the engines come up to commanded power. How often do they check the 777 fuel sensor circuit for accuracy once it is installed? Obviously their low fuel temp light didn't come on in cruise but why did the others?

[Flight Safety]

What I hope the AAIB doesn't eventually issue is a "probable cause" result for this accident, meaning "we don't know what caused it, but we think it was this".

[Flight Safety]

Bubbers44, it's easy enough to test temp probe accuracy in the current configuration. Just instrument the fuel tank at multiple locations and go fly it at cold levels.

[GemDeveloper]

I note with interest the phrasing of the AAIB update:

Begins:

Under the direction of the AAIB, extensive full scale engine testing has been conducted at Rolls-Royce, Derby, and fuel system testing is ongoing at Boeing, Seattle.

Ends.

So, to me that suggests that whatever has been done at Derby now is complete, and that the work at Seattle is continuing.

I can imagine that had the work at Derby shown there to be an issue with the engines' fuel systems, then that would have been reflected in today's AAIB update. But... apparently not. So, it looks as if summats up with the aircraft's fuel system, for whatever reason.

[airfoilmod]

The more similar the results at each engine of a fault in ETOPS the deeper the threat to the concept. By this I mean such "identical" EPR's suggest a "single" fault. Moving in reverse, the further back upstream the fault, the more troubling the implication if it can be shown that it caused simultaneous fuel starvation. Conversely, the more individual the result, the safer the format is from criticism. My first reaction to the report of the short landing was, it's Etops, engine failure is nearly impossible, and dual engine failure is out of the question. Simultaneous dual engine failure? The language in this last report also mentions no "excessive" water in the fuel. That isn't exactly encouraging.

Bsieker- The theory about cavitation being "fresh" was offered by the Boeing Safety Pilot (3-14-08): "not long before the incident". Though it is "fresh" as related to AAIB findings.

borgha- I agree the lack of any useful remedial suggestions is troubling.

[bubbers44]

Yes it would be possible to do that for certification but very expensive to do that for recurring checks once certified. No one is going to spend thousands of dollars to have maintenance go to max altitude to check one light to see if it comes on at an extreme temp especially if you have to cold soak for hours in the right conditions to make it happen. Most polar flights the light doesn't come on. On this day several flights had it happen and descended to FL250 to get it out. If they had a ground check to calibrate I would be interested in how they would do it, sensor being in the fuel tank submerged with fuel and all. It seems they could check it easily now since they could just cut into the tank and test it in a controlled temperature box and see if it was accurate.

[phil gollin]

Apart from the suspiciously similar EPR of the two engines (1.02 and 1.03) there is an interesting line in the report :-

"....... The evidence to date indicates that both engines had low fuel pressure at the inlet to the HP pump. Restrictions in the fuel system between the aircraft fuel tanks and each of the engine HP pumps, resulting in reduced fuel flows is suspected. ....."

I find it almost incredible that two different systems could suffer almost exactly the same restrictions HAVING INITIALLY RESPONDED, but I await the reasons with great interest.
.

[NigelOnDraft]

I can imagine that had the work at Derby shown there to be an issue with the engines' fuel systems, then that would have been reflected in today's AAIB update. But... apparently not. So, it looks as if summats up with the aircraft's fuel system, for whatever reason.What - maybe something like:Restrictions in the fuel system between the aircraft fuel tanks and each of the engine HP pumps, resulting in reduced fuel flows, is suspected. ;)

NoD

[FullWings]

From AAIB report:

No individual parameter from the flight of G‑YMMM
has been identified to be outside previous operating
experience. The analysis is concentrating on identifying
abnormal combinations of parameters

The really interesting thing here is: how often has something similar to what happened to MMM occurred (restricted fuel flow) but gone unnoticed as the restriction was not severe enough to cause engine problems? Could it be that what the industry regards as 'safe' flight profiles are sometimes 'outside the envelope'? We know from this accident that there is little/no evidence left behind, so shouldn't expect too much from records of other flights.

This may not be a specific RR/Boeing problem and could possibly apply to large variety of turbine aircraft that are operated in a certain way...

[soem dood]

An excellent, carefully crafted update from the AAIB.

So, since we are down to a transitory fuel flow reduction affecting both engine subsystems, brought on in conjunction with lower than typical fuel temps, I am at last compelled to point out the following:

Toothpaste:
Toothpaste counterfeited with poison:
Over the years, Chinese counterfeiters have found it profitable to substitute diethylene glycol for its chemical cousin, glycerin, which is usually more expensive.http://www.nytimes.com/2007/06/02/us/02toothpaste.html?_r=1&pagewanted=print&oref=slogin

Antibiotics:
China’s top drug regulator gets death sentence

In one instance, an antibiotic approved by Zheng’s agency killed at least 10 patients last year before it was taken off the market. Zheng Xiaoyu [took] bribes in cash and gifts worth more than $832,000 when he was director of the State Food and Drug Administration...The U.S. Food and Drug Administration also warned consumers not to buy or eat imported fish from China labeled as monkfish because it might actually be pufferfish, which contains a potentially deadly toxin called tetrodotoxin.The warning came days after three southern U.S. states banned imports of catfish from China because they contained traces of antibiotics the FDA says have never been approved for use in aquaculture.
http://www.msnbc.msn.com/id/18911849/

Dog food:
Chinese authorities acknowledged for the first time that ingredients exported to make pet food contained a prohibited chemical, stepping up their probe of two Chinese companies' roles in one of the USA's largest animal-food recalls.

While pledging cooperation with U.S. authorities investigating the recall, the Chinese government in a statement Thursday also disputed that the chemical — melamine, which is used to make plastic — was responsible for harming pets.... "There is no clear evidence showing that melamine is the direct cause of the poisoning or death of the pets," the Ministry of Foreign Affairs in Beijing argued in a prepared statement.http://www.usatoday.com/money/industries/2007-04-26-pet-food-china_N.htm

Heparin:


Federal drug regulators believe that a contaminant detected in a crucial blood thinner that has caused 81 deaths was added deliberately, something the Food and Drug Administration (http://topics.nytimes.com/top/reference/timestopics/organizations/f/food_and_drug_administration/index.html?inline=nyt-org) has only hinted at previously.

“F.D.A.’s working hypothesis is that this was intentional contamination, but this is not yet proven,” Dr. Janet Woodcock, director of the Food and Drug Administration’s drug center, told the House Subcommittee on Oversight and Investigations in written testimony given Tuesday.
A third of the material in some batches of the thinner heparin were contaminants, “and it does strain one’s credulity to suggest that might have been done accidentally,” Dr. Woodcock said.
Two weeks ago, Food and Drug Commissioner Andrew C. von Eschenbach told a Senate subcommittee that the contamination was done “by virtue of economic fraud,”...http://www.nytimes.com/2008/04/30/health/policy/30heparin.html?scp=1&sq=F.D.A.+Says+Drug+Tainting+May+Be+Deliberate&st=nyt

Lead paint:

n 2007, the U.S. Consumer Product Safety Commission issued a recall of 1.5 million Thomas toys made in China because they contained unsafe amounts of lead paint.http://www.chicagotribune.com/business/chi-thu_rc2-settlementmay08,0,7866975.story

Electronic components:
The FBI announced Friday that an investigation into counterfeit network components made in China and sold to the U.S. government has recovered about 3,500 fake devices with a value of $3.5 million. The criminal probe, code-named Operation Cisco Raider, was prompted by concerns that counterfeit network components could give hackers access to government databases. http://www.news.com/8301-10784_3-9941060-7.html?tag=nefd.pop

Bolts and fasteners:
In 2003, more than 66% of goods seized at ports of entry into this country were traced to China. What are Counterfeiting and Piracy Costing the American Economy. National Chamber Foundation, 2005. http://209.85.215.104/search?q=cache:68hfKAREU4YJ:www.efcog.org/wg/ism_qa/docs/EFCOG_Alerts/SCI_Guide_for_Suppliers.pdf+china+counterfeit+fastener&hl=en&ct=clnk&cd=1&gl=us&client=firefox-a
http://www.ebearing.com/counterfeit.htm

Golf clubs, car parts,books, prunes, razors, film, running shoes...
The World's Greatest Fakes -Chinese Copies Are Making Their Way Back To U.S.

"We don't want to ignore counterfeiting, but for those foreign companies, when they enter the Chinese market, I'm afraid they should also pay some cost due to the realities of China," says Fenghttp://www.cbsnews.com/stories/2004/01/26/60minutes/main595875.shtml



Are we so naive that we don't think the Chinese would doctor up some fuel to pass all bench tests, but still not be quite what it seems?

I sure hope a full GC mass spec analysis is being run on the retained fuel samples.

[Hand Solo]

How would you explain every other flight from PEK to Europe that night having no other issues with fuel?

[soem dood]

"How would you explain every other flight from PEK to Europe that night having no other issues with fuel?"

A quite reasonable question, of course. My best SPECULATIVE answer:

1) Their not having followed the same flight regime (including a host of environmental and operational factors)

2) Them not all being B-777, with perhaps a unique vulnerability in the piping configuration.

3) Their not having received the exact fuel load, from the same reservoir , that BA-038 got


In other HIGHLY SPECULATIVE words, I think that substandard fuel was probably a key factor, but one that needed one (or several) other elements to be in place in order to cause the response that occurred -- partial occlusion of the input lines to both HP fuel pumps during decent.

Again, a complete GC mass spec analysis should be cheap, easy, and definitive.

[Hand Solo]

There was 3 tonnes of fuel left in the tanks when the AAIB got hold of the aircraft. They've already checked the fuel and found it to be entirely within spec.

[Dairyground]

Where does the fuel for the APU come from? Is the supply from a specific tank; is it from any and all, with its own complex of valves and pumps; or is it take off the fed to one or both main engines? Does the pipework between the supply point and the APU pumps remain full at all times, or do the pipes drain during normal flight?
Hypothesis: if there are empty pipes leading to the APU, and the APU supply is taken from some point in the system downstream of the first pumps in the path to a main engine, then starting up the APU will cause a low-pressure transient in the supply system. The short term effect, possibly for a few seconds, would be for almost all fuel to take the path of least resistance and so starve the main engines.
I understand from some posts too far back to find easily that the APU start sequence was initiated before impact. Was it before of after the start of the final "glider" phase.
On a different sub-thread, would flying through and knocking down the boundary fence, rather than hopping over it, have had any significant effect on the outcome?

[bubbers44]

Probably not. Raising the flaps got them over the fence and closer to the runway and probably kept the sink rate from increasing with the decrease in drag. The crew did everything right and probably the lesser sink rate on impact saved the day. Whatever they come up with as the reason for no throttle response, the pilots did everything they could, thinking outside the box, to get that 777 down the best way they could.

[sevenstrokeroll]

WE are discussing this article and the interim report issued monday, may 12, 2008.





Heathrow crash landing 'caused by fuel freeze'
By David Millward, Transport Editor
Last Updated: 2:32AM BST 13/05/2008


The crash landing of a British Airways Boeing 777 at Heathrow may have been caused by abnormally cold conditions over Russia, air investigators have said.
Image**of*


REUTERS

Experts have homed in on fuel flow as the key issue which led to the crash landing of the Boeing 777





Flight BA38 was forced to make an emergency landing at Heathrow when it lost thrust in both engines at 600ft coming in to land on January 17.
Witnesses described seeing the airline bank sharply to the left and miss the tops of houses by 200ft as First Officer John Coward glided the aircraft to safety.
It missed the perimeter fence by inches before touching down on the grass, leaving one passenger seriously injured and 12 others slightly hurt.

Article continues
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On Monday, in an interim report, the Air Accident Investigations Branch (AAIB) said the drop in temperatures to -105F (-76C) may have caused the fuel in the aircraft to thicken during the flight which meant it was unable to get the additional thrust needed to land.
Further tests will be carried out to establish precisely what happened.
The circumstances leading to the worst aviation accident at Heathrow in more than 30 years will raise some concerns over the safety of the Boeing 777. However, neither Britain's safety regulators, Boeing, nor Rolls Royce, the engine's manufacturers, have recommended operational changes.
There are 667 Boeing 777s in service – with 40 in the British Airways fleet – and it is regarded as one of the safest airliners in the world. None has been lost since the plane first flew in 1995.
In its report the AAIB has focused on the "region of particularly cold air" between the Urals and Eastern Scandinavia during the 10-hour flight from Beijing to Heathrow.
It found that temperatures plummeted far lower than would have been expected for the region.
As a result, AAIB experts are examining what this would have done to the fuel and whether this would have caused a change in its consistency. Although the weather was unusually cold, it was not unprecedented and such problems have never been reported before.
The AAIB has established that the fuel used on the aircraft was of high quality.
While the average freezing temperature of aviation fuel is -53F (-47C), tests showed that the fuel used on the airliner does not turn to ice until -71F (-57C). Tests also found that the fuel temperature throughout the flight never dropped below -29F (-34C).
Even though the fuel did not become frozen it could have thickened to an unusual extent, which could have restricted its flow. Fresh tests on fuel are being carried out both at Rolls-Royce's engine plant in Derby and Boeing's factory in Seattle.
A number of other theories have been ruled out by the AAIB. They include birds flying into the engine or ice blocking the engine intake.
David Learmount, the operations and safety editor at Flight International, said all the evidence was starting to point towards the consistency of the fuel. "There might have been an issue with viscosity – with the fuel becoming thicker and flowing less well," he said.

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[TheInquisitor]

Is it possible for cold-soaked fuel to start depositing wax on the interior of fuel feed lines, in much the same way plaques gradually fur-up your arteries?

Just a thought that occurred - during the cruise phase, a high flow-rate may prevent this from happening. Throttle back for a descent, lower flow rate, less friction, lower velocity=higher effective pressure (iaw Bernouilli). Cold-soaked fuel may start building wax somewhere along the inside of a feed line, hence restriction. After the fact, wax buildup melts, evidence vanishes.

Just throwing a possibility out there - I'm sure somebody much more versed in fluid dynamics than I could critique (read - shoot down!) this theory.

[Green-dot]

Or it was something of an intermittent nature that was not recorded.
Either because something migrated but was restored before reaching a recording parameter. Or it was not on the list of recording parameters at all?

The AAIB states: "The spar valves and the aircraft fuel boost pumps were serviceable and operated correctly during the flight."

This would imply for the spar valves, to make the AAIB reach that conclusion, that both open and closed positions are continuously recorded and any migration from the open to the closed position but not reaching the closed position is also sensed and continuously, accurately recorded.

If so, then the spar valves are ruled out as suspect.

Green-dot

[bsieker]

The APU is fed from the left engine fuel manifold, which is supplied by either left boost pump (main tank aft, main tank forward, center tank left), or by an additional DC pump in the left main tank. When, as the AAIB said they did, the boost pumps are working, there is positive pressure in the fuel manifolds.

Starting up the APU (for which there is no evidence) would not lead to a sufficient pressure drop to starve the main engines. The APU fuel demand is tiny compared to the supply capacity of the boost pumps.


Bernd

[snanceki]

The latest AAIB report has indeed focussed our understanding and has allowed different weighting to be applied to different potential scenarios.

Now the significant new bit of data (IMHO) is the similarity of the actual EPR at WOT (wide open throttle).
What could cause such similar limiting EPR on BOTH engine installations?
I believe we have to be talking of shortcoming in either Design Concept or more likely Standards. i.e. everything was within the agreed operating parameters which is why nothing untoward can be found.

I therefore make the following contention.

As fuel temp decreases its propensity to wax increases but this transition (I believe) is not clear cut being dependent upon the degree of molecular disturbance/agitation.
Now the fuel must have been flowing adequately to maintain cruise requirements which I assume would have been similar to that demanded during the ill fated finals.
However during the descent the reduced fuel demand at flight idle MAY have allowed already very cold fuel to transition to wax due to reduced molecular agitation.
This would fit with the indication from the AAIB that (low temperature?) flow testing is being undertaken.

In addition I have always been "suspicious" about the fuel.
Its wax point has been verified at -57C, a full 10 degrees better than specification. I keep asking. Why?
So maybe the fuel was within specification but does anybody have any idea how this test is conducted (i.e. Is the fuel placed (statically) in a vessel in a lab freezer or is the fuel agitated in someway during the test?)

I also remain surprised that no directive has been issued requiring a temporary tightening of minimum fuel temperature permissible.

I look forward to your response and further debate.

[cwatters]

http://www.paclp.com/product/ISL/lit_isl/FZP%205G2s.pdf

"The freezing point of jet fuel is defined as the temperature at which the last crystal melts when warming a fuel that has been previously cooled until hyrdocarbon crystals form"

http://www.expotechusa.com/catalogs%5CKoehler%5CLiterature%5CCatalog/05-Fuels.pdf

Page 94

The sample is cooled in the test chamber with constantly stirring. The sophisticated dynamic measurement system emits a light pulse every 0.58C from a coaxial fiber optic cable positioned above the test sample. The light pulse is then reflected off the silvered-bottom test jar to an optical sensor. The advanced software package analyzes the response of the light pulse. The initial appearance of crystallization is monitored by light scattering. The sample is then warmed up, and the temperature at which the hydrocarbon crystals disappear is recorded as the freezing point.

[arcniz]

bsieker says (re May 12 report )- Flow stayed low after FMV was fully open (refuting "low-energy fuel" and "foaming fuel" theories)

How does that follow? If fuel were foaming per earlier comments, measured flow would remain low, fuel energy would remain low, and a quick reversal of that condition would not be possible because foam encapsulated vapours would occupy some fair portion of the volume of the flow path, preventing immediate inflow of fuel, even if it were available (given that the foaming would have been caused or initiated by a blockage of some sort to begin with in the tank-HP pump path, and that blockage might well have continued) .

Please explain the logic of your dismissive comment.

[bsieker]

- Flow stayed low after FMV was fully open (refuting "low-energy fuel" and "foaming fuel" theories)
How does that follow? If fuel were foaming per earlier comments, measured flow would remain low, fuel energy would remain low, and a quick reversal of that condition would not be possible because foam encapsulated vapours would occupy some fair portion of the volume of the flow path, preventing immediate inflow of fuel, even if it were available (given that the foaming would have been caused or initiated by a blockage of some sort to begin with in the tank-HP pump path, and that blockage might well have continued) .

Please explain the logic of your dismissive comment.

Sorry if I wasn't clear. I was referring to the theories, that foam was pumped at full volume flow to the engines, but only produced low thrust, because the foam would contain only little energy, compared to normal, liquid fuel.

I assume the Fuel Flow Transmitter measures volume flow, and not mass flow.

I am also unsure foam would remain foamy in the high pressure conditions downstream of the HP pump.

Foam anywhere else, contributing to a fuel flow restriction, and allowing only a minor amount of (liquid) fuel to be pumped to the nozzles, would still be a possibility. I am not a fuel chemist, so I won't comment on the likelihood or mechanics of that.

[hetfield]

I assume the Fuel Flow Transmitter measures volume flow, and not mass flow.

On the GE engines are mass fuel-flow transmitters. RR don't know.

[bsieker]

hetfield,

thanks a lot.

I've been reading up a little and it appears that due to the widely varying temperatures (I'd reasonably expect somewhere between +40 and -40 degrees C) and densities of jet fuel, mass-flow transmitters are commonly used, probably in RR engines, too. Energy content per mass unit is almost constant, but varies per volume unit, so that makes sense.

That would also mean that fuel foam at full volume flow would still register low (mass) fuel flow, contrary to my earlier comments.


Bernd

[r011ingthunder]

A specification is set out by the purchaser or authority over an industry for any given chemical substance (including aviation fuel). The specification sets out for some parameters the lowest acceptable quality of the substance, and for other parameters the widest acceptable range. Having worked in the industrial chemical business for the past 14 years, I know it is not unusual for a chemical substance to be of a far higher standard than that set out on the Specification Sheet.
The reason the spec is put together this way is to allow for variations of product characteristics due to slight differences in manufacturing processes, slight differences in constituent products, slight differences in ambient conditions during the manufacturing processes and various other deviations during manufacture. Someone more familiar with the cracking of hydrocarbon fuels will no doubt provide the specifics of these variances and their corresponding tolerances within the spec.

In this instance, the lowest acceptable quality of the fuel is a freezing point of -47C, but the fuel itself has an actual freezing point of -57C (therefore it's quality is above spec). While this does not rule out the presence of a contaminant, it is certainly not a positive indicator of one.

[OFSO]

This may have no relevance to aviation fuel, however:

I lived in Germany for 25 years and during that time we had some extremely cold surface temperatures - I remember -27ºc on several occasions.

Having always driven diesel cars - from before the time when "winter grade diesel" was commonly available, I occasionally experienced waxing of fuel with consequent inability to start the car. The waxing either builds up in or causes blockage of the extremely small passages in the injector pump, at a time when fuel in the supply pipeline is still flowing easily.

The wax persists for long after the vehicle has been towed into a heated garage: the rule-of-thumb used to be 24hrs of heat-soak before trying to restart the car.

[SaturnV]

The heat exchanger is the only part still being studied as a contributing cause, according to today's Washington Post.

But the recorders quickly disproved that theory, revealing no errors in the computer signals sent from the cockpit to the engines, according to reports and sources. Meanwhile, investigators were looking at the plane's fuel supply. An empty gas tank would have led to a thrust reduction, but the plane's tanks still contained plenty of high-quality fuel, investigators said in reports.

When they took apart the engines, investigators found evidence on pumps that the engines had been starved for fuel in the moments before the crash. That led them to conclude that the fuel supply had become blocked somewhere between the plane's tanks and its engines.

Sources familiar with the probe said engineers suspect that ice collected in or near a fuel-oil heat exchanger on each engine, blocking the fuel supply. The heat exchanger uses cold fuel to cool hot engine oil, and the hot engine oil to warm fuel before it is injected into the engines. The heat exchanger is the only point in the system that engineers have not yet eliminated as the potential bottleneck, the sources said.

But the ice theory is not perfect. Plenty of jets fly through cold weather, and the 777's fuel contained additives designed to prevent it from freezing under such conditions.

Outside experts noted that the ice theory seems implausible for another reason: Each engine should have been drawing fuel from a separate tank. The chances of ice breaking off in separate fuel tanks and blocking the fuel supply in each engine at nearly the same moment is almost too tiny to comprehend, said John Goglia, a former member at the National Transportation Safety Board, which investigates air crashes in the United States.

"This isn't supposed to happen," Goglia said. "These are two independent systems."

http://www.washingtonpost.com/wp-dyn/content/article/2008/05/12/AR2008051202728_2.html

[tanimbar]

Warning: I'm non-professional; not crew, not engineer - just scientist guest and thanks.

Seems Andy Pasztor was right in his article in WSJ, post 908. No surprise, afterall why would David King (AAIB) talk to a journalist if he was not preparing the ground for a further release of information.

Reference has been made in a recent post to dubious Chinese manufacturing practices and related these to the adulteration of fuel on BA038. I think we can safely assume that the fuel has been fully tested, found to be good and was not adulterated. That is, it performs within required standards and does not contain sub-standard substances. As an aside, it is possible the fuel was not Chinese but maybe Russian. In anycase, the question of origin is redundant given that the AAIB are satisified with it.

AAIB satisfaction in the standardisation of the fuel is crucial to understanding the rest of the special bulletin. No doubt initially the AAIB would have been hoping that they could show the fuel was sub-standard. The result would have been - phew!, wipe brows and proceed to the publication of a definitive cause.

As it is, the AAIB, RR and Boeing, are left with a mystery. They strongly suspect the fuel was degraded in some manner, due to the exceptional environmental circumstances of the flight, but cannot yet prove it or, at least, demonstrate that this is the most likely cause of the accident.

So, without a simple cause, e.g. adulteration of fuel or some mechanical defect/malfunction, the authorities are left with testing and research and preparing the public, governments and airlines for operational changes that may be issued in a following bulletin. The AAIB are preparing the ground.

But how to test? Can Boeing fully test the fuel in tanks equivalent in volume and dimensions to those in a 777, at the right temperature and barometric pressure and also push through the test system vibrations generated and experienced by G-YMMM? Does anyone know?

I suspect, but hope to be wrong, that a ground testing system, as described above, does not exist anywhere.

Possibly the AAIB/Boeing et al will commission a test 777. Time is critical: the authorities need an answer before the northern hemisphere winter arrives, but commissioning a test flying 777 will take months and, even assuming they act quickly, the only test area available now is Antarctica! Anyone seen a strange 777 in southern Chile/Argentina??

So, I'm assuming the present testing/research will be inconclusive and the AAIB will issue another bulletin towards the end of 2008 that will contain operational changes (actually, I expect that a draft of this bulletin has already been written). In other words, the precautionary principle will be
applied:-

1) avoid prolonged flight in very cold air masses
2) if 1) cannot be followed then loiter, in some way, until fuel is warmed.

And, if I was a pilot, I'd spool up the engines on descent a couple of times - just to make sure! Better to discover a problem with a few thousand feet to spare rather than 720.

Regards, Tanimbar
PS. I love a mystery and I am enjoying reading and writing on this thread but once in a while I remember, with some embarrassment, that my enjoyment is predicated on the lucky survival of the souls on BA038. In other words, I would not contribute if they had died. Their luck did not run out but what of the next occurence! We here write for enjoyment, the AAIB to save lives. We must remember that.

[rubik101]

I followed a BA 777 across Poland/Germany to London a few nights ago. He asked for descent when over Berlin to FL340. When asked by Berlin for the reason, the reply was, 'Fuel Temperature' so I guess new procedures are already in force. Any BA 777 drivers care to enlighten us?

If the failure had occurred to only one engine, then there could be myriad reasons why this happened.
The fact that both failed at the same time points to the only common factor and that is fuel.
I think the AAIB have it right and spending endless hours and bandwidth on even more speculation and second guessing seems a particularly pointless exercise.

[Basil]

Does anyone have a link to a drawing of the B777 fuel tank vent system?

[bsieker]

tanimbar,

Thanks a lot for your post. It was interesting reading. I'd like to comment on a few minor points.

As it is, the AAIB, RR and Boeing, are left with a mystery. They strongly suspect the fuel was degraded in some manner, due to the exceptional environmental circumstances of the flight, but cannot yet prove it or, at least, demonstrate that this is the most likely cause of the accident. (my emphasis)

To be nitpicking, the AAIB quoted the meteorological services as saying that the conditions encountered by BA038 were "unusual", but "not exceptional".

Possibly the AAIB/Boeing et al will commission a test 777. Time is critical: the authorities need an answer before the northern hemisphere winter arrives, but commissioning a test flying 777 will take months and, even assuming they act quickly, the only test area available now is Antarctica! Anyone seen a strange 777 in southern Chile/Argentina??

I'm curious to see if they're going to go that far.

So, I'm assuming the present testing/research will be inconclusive and the AAIB will issue another bulletin towards the end of 2008 that will contain operational changes (actually, I expect that a draft of this bulletin has already been written). In other words, the precautionary principle will be
applied:-

1) avoid prolonged flight in very cold air masses
2) if 1) cannot be followed then loiter, in some way, until fuel is warmed.

Here we may have a problem. According to the recorded data, the fuel still was relatively warm. So there would have been no need to get it warmed any more.

Lowest recorded fuel temperature was -34C, so 23 degrees above the measured freezing point, and still 13 degrees above the specified freezing point for Jet-A1, and 18 degrees above the specified freezing point for the Chinese fuel type they were actually carrying. If left unchanged the warning would have been triggered at -37C, 3C above freezing point for Jet-A. They were still even above that.



I followed a BA 777 across Poland/Germany to London a few nights ago. He asked for descent when over Berlin to FL340. When asked by Berlin for the reason, the reply was, 'Fuel Temperature' so I guess new procedures are already in force.

Descents for low fuel temperature are normal, when the temperature reaches the threshold (at least 3 degrees above fuel freezing point)

The Manual advises that when that happens, to either descent to a lower (warmer) altitude or to increase cruise mach to increase TAT. Both carries a fuel consumption penalty, and I guess the decision is the PIC's.


Bernd

[GearDown&Locked]

Assuming that the engine fuel feeding system is symmetrical and was isolated between the two of them, I can presume we have only one anomaly that was replicated between the 2 existing engines; IMHO I believe that if this 777 had 4 engines all of them would have suffered the same problem (whatever it may be), and I would guess that the timing between failures would be very close.

This anomaly exists, it’s real, and if it has the ability to affect 2 separate engine/tank systems in the very same time-frame, it’s possible to replicate it for testing purposes. It will happen again.

[soem dood]

Saturn V:

[from quoted source]...the 777's fuel contained additives designed to prevent it from freezing under such conditions...Again, I would say that the Chinese have become quite adept at making their various counterfeits pass known bench tests, such as the stir/optical fuel freeze test mentioned in the thread, so seeing some GCMS results is the point when this concern will finally be put out of my own mind.


GD&L:

Assuming that the engine fuel feeding system is symmetrical and was isolated between the two of them, I can presume we have only one anomaly that was replicated between the 2 existing engines...Exactly. That very slight difference in behaviour of the two engines that was noted in this incident may be a real keystone in the investigation, if the piping is essentially, but not exactly, the same on both sides, with the laggardly engine being plumbed to the most vulnerable routing...



Another thought... what if the water content was 'acceptable', but certainly not zero.... the 777 has some piping that is prone to internal icing, but was not previously known (i.e. a very hard angle, in a relatively more exposed area...).... this area forms an ice 'seed', and agglomeration begins... more ice as water flows by slowly (near idle) and is frozen on contact, and/or 'waxing'... while, at relatively high flows this behaviour is staved off due to more new warmer fuel washing over the area of vulnerability... but with the right odd mixture of fuel... right water content... right pre-cooling... right lower fuel flow volume for sufficient period of time.... could be just the precise but unlikely combination needed...

In reality, what the heck is left?

[shawk]

From the report: "In addition, work has commenced on developing a more complete understanding of the dynamics of the fuel as it flows from the fuel tank to the engine."

This may suggest a restriction due to fluidic resonance in a tuned circuit. If the conditions are exactly right and there is some initiating event, a resonance can occur in a pipe that will restrict fluid flow. Given two near identical tuned circuits and operating conditions, the resonance and the resulting fluid restriction will be virtually identical in both circuits. The report mentions near identical engine thrust restriction.

In a structure with two separate systems with near identical geometry and operating conditions, resonance in one system can induce resonance in the other system by mechanical coupling through the structure. Under these conditions, the typical delay between the initial resonance of the first system and the induced resonance of the second system is on the order of 2 to 20 seconds. The report mentions a 7 second delay between the first and the second engine thrust restriction. Note that very little resonant energy needs to be coupled through the structure to induce a resonance in a near identical circuit.

Such a resonance is likely to be at a very low frequency and it should be relatively audible to the ear. However, it is not certain if a resonance of this frequency would be recorded by any of the flight data recorders. Typical audio recorders in aircraft eliminate low frequencies to improve speech intelligibility and the mechanical vibration of the resonance is too small and too fast to be detected as a variation of flight parameters.

I believe that witnesses reported that the aircraft made an unusual sound as it approached the airport. While witnesses tend to be unreliable, some weight might be given to this observation as witnesses who work at airports tend to have experience with typical aircraft noise.

An investigation to include or exclude this possibility would be an extremely difficult and time consuming process.

[airfoilmod]

Left with several questions following AAIB report. The plumbing of the heat exchanger would be interesting, also if there is bypass or if all fuel must flow through this "radiator".

1. The poor thrust result, being virtually equivalent means both engines experienced poor flow as a result of the same fault.

2. Lack of (starvation) or diminished quality (environmental) would have to have been homogeneous, given separate sources (tanks).

3. From the report, there was some quantity of water in the Fuel. Though not excessive, it isn't soluble in Jet, unless itself dissolved in fluid that is in turn soluble in Fuel (eg Alcohol)

4. The delay in "off-throttle" of seven seconds could be related to a slight difference in temperature of the Fuel, due to its source location.

5. The #1, Port engine draws from the Port Main Tank, which in that flight was on the sun side of the A/C, potentially raising Fuel Temp. slightly.

soem dood-

Your suggestion of Gas Chromatography and Mass Spectroscopy is a good one, I'll wager you're not the only one who's thought of it.

I don't think your questions Re; fuel from China are ill advised at all. Contamination can be caused by substances that "improve" the "quality" of Jet, but in certain circumstances may prove disastrous.

Airfoil

[ARINC]

In the true spirit of PPRUNE I heard that the PM's ECM and or some other source of directed counter measures, was being investigated as a possible cause.

Not sure of his exact whereabouts at the time...

[phil gollin]

Quote :-

In the true spirit of PPRUNE I heard that the PM's ECM and or some other source of directed counter measures, was being investigated as a possible cause.

Not sure of his exact whereabouts at the time...

unquote

Please read the latest special bulletin - it specifically rules out EMI


.

[Human Factor]

...the reply was, 'Fuel Temperature' so I guess new procedures are already in force. Any BA 777 drivers care to enlighten us?

"Fuel Temp Low" has always been an EICAS warning with an appropriate QRH response - namely speed up or find warmer air. Whether the crew in question got the warning or were pre-empting it, I wouldn't like to say. No doubt everyone is a bit more concious of it now than they used to be.

HF (ex-777 driver)

[airfoilmod]

Shawk : Like two champagne glasses filled the same, fingertip tone on one producing a harmonic vibration on the other? Or also, like Flutter?
Ever see the original Engine/Propellor fit on the Electra (P-3)? Add thrust on final and the propellor discs like to take the wings off. Flutter.

Identical mechanical layout and Fluid supply on ETOPS. I brought that up a long time ago. Something like needing to introduce anomalies into "duplicate systems" with ETOPS. Each system marching to a different drum.

However, the engines were on auto-throttle, a very precise system that would command exactly the same thrust from both, a precisely timed move. Neither engine would have necessarily reacted in advance of the other, hence no need for harmonics. In whatever fault mode, the Fuel could be expected to be exactly homogeneous, save for a slight difference in temp., as above. Shaken, not Stirred.

[glad rag]

When was the last time the fuel system (pipework) was rebuilt?

[airfoilmod]

I think probably the A/C logs are busy.

[soem dood]

PG:

I do appreciate what caused you to deadpan a response to a provocative entry, with:

Please read the latest special bulletin - it specifically rules out EMI

And so, I can't believe I have to say this (I am literally gritting my teeth as I type, because I wish this kettle of fish were closed and staying that way) but, actually they technically did not do that. They said they found no evidence consistent with EMI. I doubt they are expending one erg of energy on it at this point, nor should we, but I just wanted to be strictly accurate, where we can.

[ARINC]

Please read the latest special bulletin - it specifically rules out EMI

I found no reference to EMI in any of the bulletins.. (S1-3)

I'd appreciate if you could provide the reference....

[Flight Safety]

What about this possibility.

Pumping high volumes (caused by demand for thrust increase) of very cold viscous fuel (fuel still within specs of course), where some part of the fuel system plumbing (between the tanks and the HP pumps) starts to deform and narrow due to the suction presssure of said cold viscous fuel, thus creating a temporary restriction.

In other words, some part of the pipe work partially (and temporarily) collapses due to the suction pressure and the viscosity of the fuel, causing a restriction. This theory is not unheard of, but does require the pipe work to rebound after the suction pressure is removed (thus leaving no evidence of a restriction). This might also explain how it could happen on both sides of the airplane.

[NigelOnDraft]

I found no reference to EMI in any of the bulletins.. (S1-3)

I'd appreciate if you could provide the reference.... S3 in the text:
There is no evidence of any anomalous behaviour of any of the aircraft or engine systems that suggests electromagnetic interference.

NoD

[Willie Wash]

All these wierd and wonderful conspiracy theories and fanciful senarios will all be proved wrong. I think one should just look a little closer to home. Most of these disasters end up being something really simple.

Lets just suppose that BA engineering staff who are required to stick check the fuel system every 24hrs. What if that check is never done, i'm talking years. The accumalation of water in the fuel will be massive. AAIB have confirmed presence of water in fuel.

I would just suggest the AAIB pull the CCTV at LHR for the times MMM was there. I guarantee that they will never find an engineer performing the required water fuel stick check. Though of course all the sheets will be stamped by their B licence guy.

To illustrate the strength of the thread, immediately following the crash, the whole BA fleet had fuel drain stick checks called. Its rumoured the amount of water found during this operation was enough to float the QE2, but then it must of got there in the last 24hrs!:D

But then this theory is too simple for all you bright sparks in here!

[Flight Safety]

Just to add to the collapse/rebound pipe work theory stated above. If this did happen, maybe there is some evidence such as some small remaining distortion, or paint or other coating with tiny wrinkles or fractures. I'd think you'd have to go looking for this kind of evidence though, to find it.

I also think this theory could explain all facts to date as we (Ppruners) know them, but the evidence of a pipe work collapse/rebound would have to be there to confirm it.

[snowfalcon2]

Not a bad suggestion. I'd guess it would be comparatively easy to do a test to find out where any vacuum induced temporary buckling/collapsing of the fuel piping might occur. Those locations could then be investigated on BA038.


But getting back to the main problem that remains so elusive: Could it be worthwhile to look at the facts from a 180 degrees opposite angle? I.e. perhaps the problem was actually too good fuel? Factually, large numbers of B777s operate sucessfully every day on fuel that freezes already at -47 degrees. Could the fact that this fuel was good all the way to at least -57 degrees cause some indirect effects that eventually led to the fuel starvation problem? What about the water content in the fuel, will it behave differently at such low temperatures?
Just my layman's $0.02.

[bsieker]

What about this possibility.

Pumping high volumes [...] of very cold viscous fuel [...] where some part of the fuel system plumbing [...] starts to deform and narrow due to the suction presssure of said cold viscous fuel, thus creating a temporary restriction.

In other words, some part of the pipe work partially [...] collapses due to the suction pressure and the viscosity of the fuel, causing a restriction. This theory is not unheard of, but does require the pipe work to rebound after the suction pressure is removed (thus leaving no evidence of a restriction). This might also explain how it could happen on both sides of the airplane. (my emphasis)

Interesting theory. However, where I live, it is unheard of.

A few remarks.

1/ With the boost pumps working, the fuel manifold is under positive pressure. There is no suction in normal operation, except in the very short pipe from the suction point to the boost pump. This ensures that no underpressure and thus no cavitation occurs.

2/ The AAIB specifically mentioned that the boost pumps were working correctly.

3/ Even if we did assume suction-feeding, the highest suction under-pressure possible happens when the engines run at high thrust with no boost-pump operating. This is within design specifications, expected, and will not cause a significant deformation of the pipes, sufficient to restrict flow to such a degree as happened in this case. The Flight Manual warns that dissolved air will be released during climb due to decreasing ambient pressure and may restrict fuel flow, and this restriction may cause thrust deterioration or flameout at high altitude. Also see point /2.

4/ The AAIB said that the piping had been examined:


Detailed examination of the fuel system and pipe work has found no unusual deterioration or physical blockages.


Bernd

[Tex37]

Not really one for posting or speculating, but have been more or less following this thread and have been involved in fluid control engineering for several years in my previous occupation.

I would say that given the latest report available that maybe we are all looking at the wrong end of the fuel starvation chain. I am sure that the AAIB are also checking this but it would fit with the known facts that it could possibly be a problem with the venting system to the tanks, if it were the case that ice or other foreign matter were to clog the vents to the tanks then this would give a similar situation to the engines being able to produce thrust at low EPR and when commanded to increase thrust, then the vacuum effect would not allow sufficient fuel flow to the engines and associated pumps, resulting in an initial spool up followed by a reduction in thrust and thus causing the pumps to cavitate, just my 2p´s worth but it figures in my head.

[pls8xx]

Way back on March 3 I posted a theory for this accident. It looks better and better as the investigation advances.
Post #589 Page 30
http://www.pprune.org/forums/showthread.php?t=325095&page=30

1. There was no malfunction of any of the systems, mechanical or electrical.

2. The freezing point of the fuel had nothing to do with the cause of the accident.

3. There was no water ice or physical blockage in the fuel pathway to the HP pumps.

The short version ...

1. A cold spot developed in the fuel significantly below the temperature of the bulk of the fuel.

2. The viscosity of this cold fuel was higher than what the pumps can handle.

3. The cold spot in the fuel did not reach the pumps until the AC was on short finals.

4. The high viscosity made the pumps cavitate and the commanded thrust could not be maintained.


For those who did not understand my March post, the long version ...

Given a suitable input pressure, every pump subjected to increased RPMs will reach a point where cavitation occurs, regardless of liquid type or viscosity. A higher viscosity results in cavitation at a lower RPM. The operation of all pumps can be graphed and though the values may vary the form will be simular.

http://i9.photobucket.com/albums/a93/pls8xx/pump1.jpg

A positive displacement pump as is on the 777 has a predictable and certain output flow for any given pump speed. Displacement per rpm multiplied by the rpm gives output independent of pressure up to the point of cavitation.

Since fuel required tracts power produced, a positive displacement pump can be directly connected to the engine and geared to produce a varying output slightly above that needed at all engine speeds.

A smaller and lighter in weight pump can be used so long as the higher pump speed needed to produce the output does not push the pump into cavitation. In aviation, lighter is better, but in my opinion , the Boeing boys cut things a bit tight with the 777.

At the point of cavitation the percentage of design flow begins to drop and continues to worsen with additional pump speed.

http://i9.photobucket.com/albums/a93/pls8xx/pump2.jpg

The only standard for the viscosity of jet fuel I can find is based at minus 20C, where the maximum viscosity is stated to be 8 centistokes. The viscosity of most liquids increases as temperature drops. Many single component liquids, like pure water, have a linear function of viscosity to temperature up to the point of freezing. Liquids mixtures of hydrocarbons tend not to be linear. Ever hear of multi-viscosity motor oil?

Different formulations of jet fuel can have different viscosity profiles. All that is needed to meet standard is that it be below 8 centistokes at minus 20C. All that is known for certain is that below minus 20C the viscosity will be higher and the greater the difference of temperature from the standard the more uncertain actual viscosity of a particular fuel becomes as illustrated in the graphic below. Note that these viscosity changes apply across the full temperature range and have nothing to do with the point of freezing.

http://i9.photobucket.com/albums/a93/pls8xx/pump3.jpg

If my theory for BA038 is correct, a cold spot developed in the fuel, though the bulk of the fuel did not go below minus 34. (see my March post for particulars) This cold fuel was not mobile in the tank until lower flight levels. Once mobile, the cold fuel, maybe around minus 40C, began drifting toward the boost pump inlet. It arrived at HP pumps on short finals.

In the graphic below I have combined the concepts of the graphics above. One line represents the engine/pump rpm/thrust and the associated maximum viscosity for those values. Added are some possible temperatures that might be associated with a particular fuel viscosity.

From flight idle the engine responded to command with an increase in speed/thrust marked with green dots. Pumping minus 34C fuel there wasn't a problem. When the super cold fuel hit the pumps the viscosity increased above the maximum value and cavitation started. Starved of fuel the engines rolled back. The rollback to lower rpm improved the cavitation problem and when the slightly cavitating pump matched the needs for the thrust level, the situation stabilized at thrust levels of 1.03 & 1.02 EPR.

http://i9.photobucket.com/albums/a93/pls8xx/PUMP4.jpg

[snowfalcon2]

This clogged fuel vent theory seems to be a recurring favourite here on Pprune.
It's certainly feasible in theory. At our airfield we had a case of a totally "imploded" fuel tank on a light airplane where the vent was iced up. Fortunately the forced landing was successful.

However, airliner tank vents are designed to cope with exactly that hazard. And I would also think it highly improbable that if vent clogging were to occur, it would happen in both tanks within seven seconds.
Further, I'd suspect that a vacuum strong enough to restrict fuel flow would also cause visible buckling on the wing skin, noticeable to any passenger who looks outside.

Just my $0.02

[glad rag]

Cavitation, to metallic fuel pipes, did happen on RR Spey engined F4's, but the evidence was obvious. Also on same (USA) AC I remember reading some US articles about flexible TX fuel pipes inside the tanks going porous and causing mayhem........

[airfoilmod]

Sufficient to collapse tanks and Fuel Lines would "Boil" the lights out of the Fuel, not to mention have to be serendipitously synchronized to each "side": they responded, initially (read: simultaneously). The lobes on the pump were Deformed, Pitted (Cavitation). My theory on the "damage" reported by Boeing and AAIB #2. Are we overlooking some rather obvious available energy that can boil some liquids at a fair distance? The vibration and acoustics from 180,000 pounds of thrust made by two 10 foot fans at full go. A closed system, Fuel with no gas present, can be made to vaporize without any mechanical prodding, just vibration and or sound energy. I know, why hasn't it happened before?.

Airfoil

[Tex37]

snowfalcon2

I hear what you say and must admit I may have missed a few pages of this thread. Probably been covered before, but so far as I am aware, the fuel pumps on commercial aircraft (not x feed) are gravity-fed pumps as the aircraft is not expected to fly inverted or in any aerobatic attitude, also as a result, the outflow from the wing tanks is at the lower point.

Not knowing the full specs of the pumps on the 777 would they be able to draw such a vacuum as to result in physical distortion to the wing tanks, given the stresses they are designed to cope with in normal flight?

Give the fact that thrust was commanded on both engines at the same time with a slight imbalance of fuel in each wing tank, if vents were iced then the problem would replicate in each engine at more or less the same time.

Will have to wait for the final report by those who know but if fuel is ok and no apparent systems problems then the fault is either narrowed down to almost nothing or exponentially/impossibly widened to the point of never being identified

[Flight Safety]

Bsieker, a few counter-remarks.

1/ With the boost pumps working, the fuel manifold is under positive pressure. There is no suction in normal operation, except in the very short pipe from the suction point to the boost pump. This ensures that no underpressure and thus no cavitation occurs.
2/ The AAIB specifically mentioned that the boost pumps were working correctly.

Understood about the positive pressure from the boost pumps. However I'm not sure what parameters are recorded in the FDR for the boost pump. An underperforming boost pump (underperforming due to cold fuel viscosity perhaps) could create a vacuum at the outlet, if the HP pump was trying to move more volume. However if the boost pump RPM is monitored and recorded and shown to be within normal parameters, then I agree with you.

Originally Posted by AAIB Special Bulletin S3/2008
Detailed examination of the fuel system and pipe work has found no unusual deterioration or physical blockages.

Just fine parsing of the text, a collapse/rebound event might not necessarily show up in a deterioration/physical blockage detailed examination. I really do think you'd have to specifically go looking for the kind of evidence this type of event would create. But to be honest, I don't even know if the materials necessary for a collapse/rebound event even exist in the pipe work of this fuel system.

3/ Even if we did assume suction-feeding, the highest suction under-pressure possible happens when the engines run at high thrust with no boost-pump operating. This is within design specifications, expected, and will not cause a significant deformation of the pipes, sufficient to restrict flow to such a degree as happened in this case.

You assume that the design of this fuel system is sufficient for all "normal" operation conditions. However it's possible this is not the case for the specific conditions of this flight, and I conclude this mainly on the evidence that the same event happened on both sides of the airplane to 2 independent fuel systems.

Remember also that both engines initially responded with an increase in power, then both rolled back at slightly different times. Could the flow rates between the boost and HP pumps have been different enough that a vacuum developed over a fairly short period of time between the pumps? Might be plausible in theory anyway but could also be a real stretch.

[banana9999]

Can someone give me the TRUE FACTS please?

I'm getting bored now :cool: :8

[Rightbase]

Can anybody work out what the effect of a few litres of water per minute mixed in with the fuel would be? Would it be enough to account for the observed loss of power?

I ask because water could be introduced into the fuel supply of each engine at something like that rate after a few seconds if the initial power increase caused a surge of melt water from the centre tank.

[Doc Strangebrew]

The fuel has been tested extensively; it is of good quality, in
many respects exceeding the appropriate specification,
and shows no evidence of contamination or excessive
water.

'or excessive water.'... that implies that there was indeed water in the fuel they tested...but not a lot...?

These tests are collectively aimed at understanding and, if possible,
replicating the fuel system performance experienced on
the day and the potential for formation of restrictions

And on the day there was a

region of particularly cold
air, with ambient temperatures as low as -76ºC, in the
area between the Urals and Eastern Scandinavia.

Now they then report fuel freezing temps and test results which appear to be within bounds...but not about the possibility of what happened to the water in the system...which apparently was not excessive...that is as maybe but water and temps as low as recorded results in ice ...surely...somewhere in the system...wherever that water is...and no matter how little...

These tests are collectively aimed at understanding and, if possible,
replicating the fuel system performance experienced on
the day and the potential for formation of restrictions.

"and the potential for formation of restrictions"

It does sound as though ice in the fuel... formed from whatever water there was present... might be what they are actively considering as a possible culprit...

Seems there is nowhere else they can go really...all other systems seem hunky dory...

But then again I appreciate there is still a fair way to go in this investigation...but if an extremely rare combination of environmental and system parameters came together in a certain manner...it might explain a good deal...

A pressurised fuel tank might complicate the phase change...but at temps below -20 ...not a lot methinks...you still get ice....:cool:

Before I get thrown to the proverbial...or even in the proverbial...I do not have slightest idea of what actually occurred...but something most certainly did...I offer the above from the prospective of just reading the latest report...tis not a conspiracy theory...just speculation...

But I accept I might be ...possible am... in total error...:uhoh:

And other are conclusions are possible....

[non iron]

Right then mon ami`s, so who first saw the ufo arriving ?

Crazy.

Get a grip please.

[precept]

A good place to look for the latest in U.S. Government actions regarding almost anything, but most importantly for us who are following the BA-38 situation is is the following site. You might want to copy this site to your web browser and check it now and then.


http://regulations.justia.com/search?agency=131&rule=yes&administrative_order=yes&notice=yes&proposed_rule=yes&executive_order=yes&proclamation=yes&registers=all&min-day=1&min-month=1&min-year=2008&max-day=30&max-month=1&max-year=2008&query=777&search=Search

[infrequentflyer789]

Right then mon ami`s, so who first saw the ufo arriving ?


GBs security detail saw it first and zapped it with the secret anti-ufo EMI-RFI-EMP ray thingy.

Unfortunately the UFO got his shields up in time and the EMI-ray bounced back, hit BA038, and the rest we know... :\

[CONF iture]

http://i45.servimg.com/u/f45/11/75/17/84/757_sp10.jpg


Green-dot,
From memory, on 757 there was an indication light for SPAR VALVE status. That one was amber as long as its physical condition was not in total accordance with commanded request.
I believe such indication light does not exist on 777 (?)

My questions:

How long does a spar valve take from fully closed to fully open ?
What kind of electrical input does it need to operate ?
- Is it a continuous input ?
- Is it just a simple impulse which initiate closure / opening ?
- Can it produce an only partial closure / opening ?

Before going in all kind of studies and theories regarding fuel composition and / or pipe structure, wouldn’t it be common sense to simply test the effect a partial and temporary spar valve closure could produce on HP fuel pump ?

Temporary partial closure of that valve, isn’t it the easiest way to restrict a fuel flow ?




Interesting how our mass media is willing to omit or even change wording … and therefore format mind of the masses:

BBC
A now-discounted theory was that radio signals from Gordon Brown's motorcade interfered with the Boeing 777… But the report from the Air Accidents Investigation Branch (AAIB) rules out electromagnetic interference.

When AAIB only says
There is no evidence of any anomalous behavior of any of the aircraft or engine systems that suggests electromagnetic interference

BBC
The plane was operating in what are described as unusually low temperatures, and one theory is that this affected the fuel, though recorded temperatures were within safe limits.

AAIB words
The Met Office described the temperature conditions during the flight as ‘unusually low compared to the average, but not exceptional’

BBC on may Th (http://news.bbc.co.uk/1/hi/england/london/7396899.stm)

[bsieker]

Probably been covered before, but so far as I am aware, the fuel pumps on commercial aircraft (not x feed) are gravity-fed pumps as the aircraft is not expected to fly inverted or in any aerobatic attitude, also as a result, the outflow from the wing tanks is at the lower point.

Yes, it has indeed been covered on this thread. And in quite some detail, complete with drawings. And yes, fuel is drawn from the tank low point (duh!), but no, gravity-feed or suction-feed is not the normal mode of operation, but an emergency.

Please see posts #284 (http://www.pprune.org/forums/showpost.php?p=3929959&postcount=284), #461 (http://www.pprune.org/forums/showpost.php?p=3941558&postcount=461), #467 (http://www.pprune.org/forums/showpost.php?p=3942040&postcount=467) and #478 (http://www.pprune.org/forums/showpost.php?p=3942586&postcount=478) for details.

Not knowing the full specs of the pumps on the 777 would they be able to draw such a vacuum as to result in physical distortion to the wing tanks, given the stresses they are designed to cope with in normal flight?

To the tanks, which really are the wings, perhaps. Tank buckling would also have been noticed by the investigators. Also see below.

Give the fact that thrust was commanded on both engines at the same time with a slight imbalance of fuel in each wing tank, if vents were iced then the problem would replicate in each engine at more or less the same time.

Icing is a known problem, expected and well-understood by the designers of the venting system, and will have been taken into account. But even in the very unlikely event of complete blockage of the venting system, the boost pumps would still be capable of supplying enough fuel for quite some time. Given the large empty volume in the tanks, the under-pressure would only build very slowly.


Bernd

[DuncanF]

The plane was operating in what are described as unusually low temperatures, and one theory is that this affected the fuel, though recorded temperatures were within safe limits.

AAIB words
The Met Office described the temperature conditions during the flight as ‘unusually low compared to the average, but not exceptional’Actually I think the Beeb is doing the English language a favour in this section and correcting the double tautology in the AAIB statement ...

=> Unusual implies "not average"

=> Unusual is not the same as "exceptional"

Duncan

[tanimbar]

Warning: I'm non-professional; not crew, not engineer - just scientist guest and thanks.

Shawk in post 1110 wrote while discussing fluidic resonance in a tuned circuit:

I believe that witnesses reported that the aircraft made an unusual sound as it approached the airport. While witnesses tend to be unreliable, some weight might be given to this observation as witnesses who work at airports tend to have experience with typical aircraft noise.



If the reports are reliable then it seems more probable that the sound might be due to the high pressure fuel pumps cavitating.

Apologies if this has been mentioned before.

regards, Tanimbar

[infrequentflyer789]

=> Unusual implies "not average"
Duncan

Well, if we're getting picky about correctness, you should note that your statement is only true for some (eg. normal) distributions. If the distribution is highly skewed, then the average value would be unusual, and the most commonly seen values (ie. "usual") would be "not average".

[shawk]

Fluidic resonance in a tuned circuit tends to be low frequency and is far less likely to be attenuated by the aircraft structure.
Interestingly, this noise would be very similar to tire rumble at landing.

Cavitation noise tends to be high frequency and is likely to be attenuated by the aircraft structure.
Below is a spectrum for cavitation noise

http://caltechbook.library.caltech.edu/22/01/figs/fig614.gif

Typical fluid resonance detectors operate between 2Hz and 300Hz.
Unfortunately, I can not find a spectrum for fluidic resonance that is not limited by copyright.
Those interested in further exploring this topic might Google: Bayesian Modelling of Fluid Flow in Pipelines.

[tanimbar]

Shawk,
You say that "cavitation noise tends to be high frequency and is likely to be attenuated by the aircraft structure".

The cavitating high pressure fuel pumps (HPFP), that I mentioned might be the source of the reported strange noise, are integrated with, and driven by, the engines.

Lots of local pipework and other metal objects (nacelle etc) in the vicinity available to resonate with the HPFP, plus, a large hole at one end to let sound out.

Would much attenuatation occur and how would the other resonating objects alter the frequency range?

Of course, if your interesting fluidic resonance idea is correct then the sound is important evidence of causation. If it is the result of the cavitation then it is not important.


Regards, Tanimbar

[lomapaseo]

One would have thought that any significant sounds reaching the ears of ground spotters etc. would have been recorded either as acoustic signals on waterfall plots on the CVR or as forced low-frequency responses on the DFDR from various inputs around the aircraft.

On the other hand less specific sounds (white noise) or those left in the memory of observers have a tendancy to only exist in the minds of theorists.

[shawk]

You may have experience with a water faucet that sometimes makes a loud, low rumble or rattle that is accompanied by a small volume of water that comes out of the tap in discrete spurts of water. That's fluidic resonance in a pipe. Generally, you have to stop the flow to stop the effect. With the resonance gone, water flows normally.

I suspect that high frequency cavitation noise similar to the frequency plot would be inaudible through a typical thickness of aircraft aluminum at 10 meters. A lot depends on the placement of the pump and the shape of the surrounding structure but not much high frequency sound is likely to escape an airframe. From what I've seen, aircraft piping is very well attached and damped to prevent vibration that will cause rapid metal fatigue, so it might not act as a good acoustical conduit for high frequency sound.

If the fluidic resonance theory has much basis in reality, the likely noise is somewhere between 16Hz and 32Hz. This sound frequency range has a very good chance of escaping an airframe and be audible on the ground.

But I'm not certain that a CVR will record audio at 16Hz to 32Hz. They seem to be optimized to record human voice and use brick wall digital filters to not record below 100Hz.
The DFDR may not be sensitive enough to record the airframe effects of a small mass of fuel oscillating at 16Hz to 32Hz.
I hope that experts will add their knowledge to what parameters could be recorded.

[ARINC]

Quote: ARINC
I found no reference to EMI in any of the bulletins.. (S1-3)

I'd appreciate if you could provide the reference....



S3 in the text:
Quote: NoD
There is no evidence of any anomalous behaviour of any of the aircraft or engine systems that suggests electromagnetic interference.
NoD

Thanks NoD

I was chatting to a FADEC "Expert" last week when the subject came up and he was of the opinion that as there are quite a few parameters in the FADEC system that are not logged on the DFDR and that if affected could cause a roll back to flight idle, then EMI still remains a possibility.

[barit1]

Does anybody know for certain the clock speed of the Trent FADEC?

I once knew of a FADEC clocking at 120Mhz, with predictable coupling to VHF comm frequencies. :eek:

[pilotbear]

The unusual sound heard by pax could have been the flaps retracting, which without engines spooled up would probably be the most prominent and "not normally heard sound' at that point on the approach.:E

And considering the obvious technical prowess of most people here I am surprised that you have difficulty with the OFF mode for your PC/MAC when you don't like what you are reading instead of this persistent fascist approach of telling people what they can and can't write, think or do. Get a life eh?:ugh:

Speculation or conjecture no matter how ridiculous is where discoveries and ideas come from, talk out the most ridiculous first and fine tune the way to the truth.:ok:

[HarryMann]

If the fluidic resonance theory has much basis in reality, the likely noise is somewhere between 16Hz and 32Hz. This sound frequency range has a very good chance of escaping an airframe and be audible on the ground.

To whom?

It's fairly common knowledge that GOOD human hearing is in the range 50 Hz to 20,000 the upper frequency decreasing rapidly with age (and usually lower for the male gender)..

It rolls off rapidly below 50Hz to prevent damage from the energy content in low frequency sound.

Bottom string (E) on a bass guitar is around 42 Hz, and it's 2nd harmonic (84Hz) is usually more understood by the ear than the fundamental, which itself is more felt rather than heard.

32 Hz or less? Something by-standers commented upon, amongst the melee of other sounds from an aircraft so unusally close - nope, don't think so.

Neither do I find it credible that cavitation noise within a pump within an engine nacelle would be heard, or commented upon, whatever its frequency... over the mass of aerodynamic and other turbomachinery noise - IMHO of course!

[HarryMann]

Once mobile, the cold fuel, maybe around minus 40C, began drifting toward the boost pump inlet. It arrived at HP pumps on short finals.

From both tanks, a nebulous volume of cold fuel - within 8 seconds of each other?

[airfoilmod]

It sustained damage from Cavitation, but even with that was "able to function normally". The squishiest suspect is of course, Fuel.

1. Ten degrees "off spec.". Not "better than"; OFF SPEC.

2. Fuel contained some amount of water, deemed "less" than "excessive."

3. Water and Jet don't "mix" (non-soluble)

4. Tank Temp., at -34C would freeze water immediately

5. Boeing Captain Carbaugh posits "Ice Crystals", a "Temperature issue"

6. The Heat exchanger has yet to be exonerated from involvement.

7. Fuel and Water could have transited the LP pumps

Possible: Given a seriously cold-soaked engine Pylon and barely warm heat exchanger, ice crystals could accumulate in the exchange matrix of the exchanger, blocking flow to HP pumps.

For this to have happened, the timing sequence of engine thrust loss needs to be seriously considered to satisfy many possibilities.

1. The Fuel/Ice "mix" would have to be inordinately homogeneous between the two main tanks.

2. If the ice crystals had already been formed, further "freeze" may not have been possible anyway.

3. The Fuel/Ice emulsion, at flight idle, transits the Fuel works fine, no loss of supply or even thrust.

4. At Throttle up, the Fuel is moving more rapidly, ice is "collecting"
in restricted spaces, but the engines respond fully before the accumulation can get ahead of the supply.

5. At full flow, the ice crystals gather rapidly in the exchanger and elsewhere, constricting the Flow of Fuel. #2 drops power.

6. Drawn from Port Tank, the Fuel ice "emulsion" is perhaps 1.5C warmer than the Starboard Tank, and #1 drops 8 seconds later. (Port Tank is in sunlight throughout the flight, the inner portion of the Starboard Tank is in the shadow of the Fuselage.)

Why Crystals and not surface accretion? Agitation, and lack of nucleation. Any "Slurry" will shed its solids at turns and pressure foci differentials. It has to be Fuel Fault.

Airfoil

[tristar500]

Lords, Ladies and Gentlemen...

In a nutshell...

How many times has this type of incident happened before?

How many times have aircraft flown through extreme weather conditions (inc freezing temps etc etc) with NO problem at any time during the flight?

Whats so different from this particular flight, to any other commercial flight?

As far as I know, Boeing and BA have issued NO change to operating procedures on the B777 (I stand to be corrected if they have)... Would Boeing have grounded the B777-RR powered, if anyone had lost their lives?

Maybe this is a case for the guys at CSI to solve, since the CAA,AAIB, RR, BA and Boeing seem to be at odds...

[Bis47]

Whats so different from this particular flight, to any other commercial flight?

As far as I can remember early comments :

- Multiday rather cold winter operations
- Fuel tank icing messages
- Actual flight much higher than initial operational flight plan (to satisfy ATC request) => hence extra landing fuel.
- Very low OAT at a given time (- 76°c = ISA - 20). What was the TAT at that time ?Somewhere around minus 50 °c ? Quite a cold spot ... for how long?
- no descent to lower altitude over Germany while many other commercials were requesting FL 250 that day.
- no "low fuel temp" warnings, while other airliners got that same warning ...

So ... each fact not exceptionnal. But the whole set still quite infrequent ...

The culprit might be a deficient fuel temp warning system ... and crew putting too much confidence in that system, instead of "crosschecking" fuel temp information with indicated TAT and met temperatures forecast.

As far as I know, Boeing and BA have issued NO change to operating procedures on the B777 (I stand to be corrected if they have)... Would Boeing have grounded the B777-RR powered, if anyone had lost their lives

I guess (see previous post a few pages above) that crews didn't wait for Boeing or the CAA to take extra margins ... Any captain worth his pay would not want to replay that scenario !

Actually, the operational flight plan was calling for lower altitudes, avoiding very low temperatures ... Is the program already including some margins against limitating low temps? Perhaps ... BA knows the answer.

[PeePeerune]

For Info.

I believe that the B777 fuel system has a suction feed system from tanks so that in the event of a.c fuel pump failure (of anykind),the engines will still receive a flow of fuel via suction ,albeit at a slightly reduced rate.

Suction Feed
-Suction feed line bypasses pumps.
-During climb air from fuel tank may collect in suction
feed line, inhibiting suction feed. Air will deplete
eventually after reaching cruise.
-Once air dissolved suction feed can occur with no
limitation.
-Time to deplete depends on altitude, fuel temp and
type of fuel.

[L337]

- no descent to lower altitude over Germany while many other commercials were requesting FL 250 that day.

- no "low fuel temp" warnings, while other airliners got that same warning ...

One wonders if the fuel temp sensor was reading correctly?

But, seems too easy a solution.

[arcniz]

One wonders if the fuel temp sensor was reading correcitly?

Even if the tank fuel temp sensors used for flight-deck reporting WERE reading correctly, one may wonder if their reported readings were representative of the fuel temps actually in the dir0 ect path of flow to the engines?

As one very experienced in precision temperature measurement, I know uneven distribution of temps in a large volume (of anything) is a near certainty.

If pockets of colder fuel were present in the wing tanks, perhaps already gelled and stratified into layers or globs of denser fuel materially colder than the tank fuel temp sensors could see, then is it not possible that descent-related changes in aircraft attitude and acceleration could possibly shift the coldest and waxiest fuel into the feed path at the critical late stages of descent, with exactly the consequences observed?

A relatively simple enhancement to flight-deck data might be to add reporting of temps IN the actual flow path near the engines. Likely some of this data is already sensed and available within the FADEC -- and possibly available in a usable form.

[Green-dot]

To CONF iture, post #1138:


on 757 there was an indication light for SPAR VALVE status. That one was amber as long as its physical condition was not in total accordance with commanded request.
I believe such indication light does not exist on 777 (?)


The 777 has no transit lights. Spar valve positions are displayed on the fuel synoptic and fuel management maintenance page. These have to be selected on the lower MFD (or left/right inboard MFD) to enable monitoring of the spar valves.


My questions:

How long does a spar valve take from fully closed to fully open ?


15 seconds.


What kind of electrical input does it need to operate ?


28V DC


- Is it a continuous input ?


Spar valve control system (fuel control switches and control relays): yes

Spar valve actuators: no. Limit switches remove power from the spar valve actuators after commanded position is reached.


- Is it just a simple impulse which initiate closure / opening ?


Continuous 28V DC power is provided to the control relay.

Fuel control switch moved to RUN energizes the open coil of the control relay which then sends a power signal to the spar valve actuator. When the spar valve reaches the open position, power through the limit switches is removed from the spar valve actuator.

Fuel control switch moved to CUTOFF energizes the close coil of the control relay which then sends a power signal to the spar valve actuator. When the spar valve reaches the closed position, power through the limit switches is removed from the spar valve actuator.


- Can it produce an only partial closure / opening ?


Yes , if a spar valve actuator jams while in transit. This wil generate a fault indication on EICAS due to a disagree between control relay position and spar valve actuator position.

Or (highly improbable but not impossible):

A temporary reverse logic to the control relay. Example:
With fuel control switch in RUN and temporary reverse logic to the control relay, this moves the spar valve actuator from open to closed position. No alert is generated because there is no disagree between control relay and spar valve actuator. Actuator moves to commanded position.

If the temporary reverse logic is corrected in less than 15 seconds, the spar valve moves [open->close->open] and will be in transit for almost but less than 30 seconds without ever reaching the closed position. Result, obvious limitations to the fuel flow. No alerts are presented and no visual indications if fuel synoptic and/or fuel management maintenance page are not displayed at moment of occurrance.



Before going in all kind of studies and theories regarding fuel composition and / or pipe structure, wouldn’t it be common sense to simply test the effect a partial and temporary spar valve closure could produce on HP fuel pump ?

Temporary partial closure of that valve, isn’t it the easiest way to restrict a fuel flow ?



see my post #590 on page 30 of this thread:
Code One . . . . (http://www.pprune.org/forums/showthread.php?p=3954269#post3954269)


regards,
Green-dot

[airfoilmod]

From an ETOPS perspective, I think some folks have a hard time entertaining how both engines could respond similarly to a single or series of faults. The timing isn't so exclusive of what appears to be reality. There is a "range" of possibilities that could easily have trapped both Powerplants in a "simultaneous" cycle of failure. Subtle changes, or commands, or even trimming, result in subtle responses from the A/C. Large inputs can create their own problems.

Possible: BA038, established in flight, is exposed to fierce low temps.
There is some amount of Water in the Fuel, in some "state".
On descent, Equilibrium is established, the "system" is constant
Engines have established stasis, low operating temp. sipping fuel

1. At low altitude (~800feet) the Autothrottles command significantly increased thrust.

2. The engines respond, accelerate, and make commanded power.

3. Everything impacting the new thrust level is instantly at high level

4. Fuel flows rapidly increase, as does temperature in the engines

5. Stasis collapses, Fault (s) appear and overwhelm the system.

Possible: The isolation of the powerplants from each other works to prevent simultaneous failure, but given a demand in the nature of a sudden command for full power, the scale of the demand can "override" functional isolation. What I'm trying to say, is that though the Fuel was contained in separate tanks, it was of the same make-up, exposed to very similar conditions, and could have easily created unrecoverable circumstances in two engines within 8 seconds.

Back to stasis. The profile of the descent and the extended period of low power and uncommonly cold and perhaps contaminated Fuel created a homogeneous environment for the final hole in the cheese. I think the restriction was closer to the HP pumps than spar valves, LP stand pipes, or system piping upstream from the pylons and heat exchangers. The instantly present Vibration and fuel demand, including sonic energy and increased temp. may have served to melt/dislodge pack up the heat exchangers with the ice crystals that may have collected prior to throttle up. Within 8 seconds of either engine? Possible.

Airfoil

[Machaca]

The Trent 800 FCOC (black) and fuel & oil tubes:

http://img57.imageshack.us/img57/8356/80079bxa3.th.jpg (http://img57.imageshack.us/my.php?image=80079bxa3.jpg)


http://img61.imageshack.us/img61/4224/80071bou9.th.jpg (http://img61.imageshack.us/my.php?image=80071bou9.jpg)


http://img57.imageshack.us/img57/1477/80066bsc7.th.jpg (http://img57.imageshack.us/my.php?image=80066bsc7.jpg)


Cutaway of similar FCOC (fuel flows through matrix of small tubes):

http://img234.imageshack.us/img234/8849/fcoccutawayez9.jpg (http://imageshack.us)

[arcniz]

I fully agree with your assertion that a core vulnerability in ETOPS redundancy is that the top-level systems (i.e. the aircraft, external nav-comm environment, crew) are unitary and therefore not able to be much more redundant.

As you observe, a variant of the same theme is that the recent history of the aircraft is also common-mode for even the redundant systems, esp the power train. When the two halves of the aircraft are somehow identical, certain unanticipated failure modes related to patterns of usage may affect them equally at roughly identical points in time.

Perhaps the logical extension of this observation - especially if the principle eventually is shown to be causative in the case of BA038 - would be a future requirement to ensure that the two halves of an ETOPS aircraft may use many common components but must be deliberately asymmetrical in regard to the form, fit, and function of sustaining flight-critical systems. A logistics nightmare, of course, but probably a reasonable extension of the no-fail-all concept.

Nature handles this problem quite neatly in many living things, including ourselves, by making the redundant critical appendages, sensors, and control systems from the same general parts and pieces, but different in right-left (etc.) symmetry. A substantially trickier design asymmetry might be needed to cover the many plausible common-mode failure symmetries possible with aircraft.

[Oldlae]

Arcniz,
Several hundred posts ago and I have spent some time trying to find them but it was too time consuming, it was explained that the jet pumps in the tanks are designed to keep the fuel in the tanks in motion to prevent cold spots or stratification from forming. As I understand it, the jet pumps were found to be working correctly.

[CONF iture]

If the temporary reverse logic is corrected in less than 15 seconds, the spar valve moves [open->close->open] and will be in transit for almost but less than 30 seconds without ever reaching the closed position. Result, obvious limitations to the fuel flow. No alerts are presented and no visual indications if fuel synoptic and/or fuel management maintenance page are not displayed at moment of occurrance
In normal circumstances, MMM should have been around one minute from touchdown, so I figure out it was only 30 to 40 seconds from impact on this day when right engine reduced to 1.03 EPR
According to AAIB, both EPRs reduced and maintained steady values, 1.03 and 1.02, therefore I don't see as plausible scenario that spar valves were constantly in transit, which would have produced, I think, anything but steady values.

I'd rather go for your previous hypothesis:
Focussing on the T7 dual engine roll back again, just suppose both spar valves had temporarily closed for, say, 80 percent (only 20 percent capacity remaining) with boost pumps on. Could that have resulted in engine roll backs to a stabilized thrust level above flight idle and cause cavitation at the engine pumps as the engines initially responded and then rolled back because reduced fuel flow did not meet engine demand?

And I totally support the following:
I assume similar simulations such as mentioned above would have to be performed to find answers or are there other means (computer simulations?) to observe engine behaviour under such conditions?


I would easily conceive a fuel flow restriction by mechanical means, like a partial spar valve closure, but I don't see how the fuel itself would produce its own restriction in a way that such a restriction stabilizes for the remaining of the flight and don't deteriorate any further ?

[FireLight]

I would easily conceive a fuel flow restriction by mechanical means, like a partial spar valve closure, but I don't see how the fuel itself would produce its own restriction in a way that such a restriction stabilizes for the remaining of the flight and don't deteriorate any further ?


The slush (icy/waxy particle) theory can explain the fuel flow restriction with the following conditions met:

a) That there is a location for the "slush" particles to accumulate. (a screen, a partially open valve, a matrix of tubes (FOHE?))

b) That the particles are of sufficient size and properties to accumulate.

c) That the particles can move from their formative location, or that they form in a location with the right characteristics to restrict fuel flow.

d) That there is sufficient time for the particles to accumulate. (that there is enough time at the end of the flight and during the descent that the engines require no more than low/moderate fuel flow.)

The "slush" would allow fuel to flow through at a steady state as long as the fuel flow demand was low/moderate. At high fuel flow levels, the fuel would be restricted to near the low "steady state" value. (sucking fuel through ice using a straw so to speak)

---------------------------------

There are some hurdles to the theory:

a) That the AAIB report identifies that there were "no significant quantities of water." (Definition of significant?) This can be explained if the "slush" was mostly pulled through the system at the time of the crash. Even at ppm of water, 79,000 kg of fuel would result in a few kg of water/ice, which would be enough to cause mischief. Not to mention the legacy water in the system.

b) That the FOHE were clean of blockage and that the fuel feed lines did not reveal any defects or restrictions. This could be explained also with the "slush" being pulled through the system. There may also have been some loss of the fuel downstream of the HP fuel pump from the crash impact, therefore losing the evidence.

If the "slush" was more waxy components than ice, then they could have redisolved into solution, again leaving minimal evidence.

[r011ingthunder]

Even at ppm of water, 79,000 kg of fuel would result in a few kg of water/ice

For clarity: 79,000 kg of fuel containing 1 ppm of water would equate to 79g of water (w/w)

[johngreen]

In my own realm of fluid dynamics I have witnessed some few examples over many years of systems entering states of very unintended and undesired resonance created by the combination of a free moving valve component and some elasticity in pipework and/or structures.

Acknowledging many of the very informative and interesting previous posts on the subject of BA038, I would like to suggest that just such a resonating system as described could very well have propagated involving one of the non return valves in the booster pump and the associated pipework on the right hand side of the aircraft.
The liquid flow through such a system in resonance will be restricted in proportion to variations of the frequency and amplitude these both being influenced by changes to any physical property within the system and most likely very much exacerbated by any increase of differential pressure across the oscillating valve component.
Before any problem becomes apparent, all individual components function perfectly and none are damaged, nothing is dangerously cold, the fuel is within standards, there is no ice present or any EMI, but at some time in the last minutes of the flight a very rare and of course quite unintended combination of factors starts a small harmonic interference in the fuel flow. These might include all or any of a change in fuel demand, the fuel temperature and viscosity, the airframe temperature, the level of fuel in the tanks, the aircraft attitude and normal vibrations within the airframe etc.
Regardless of the exact moment that this resonance started, as the demand for fuel increases the differential pressure across the now oscillating poppet valve hugely exacerbates the resonance. The HP pump firstly manages to draw the bulk of the liquid fuel from the delivery pipe and the engine accelerates as required but the supply of fuel quickly becomes limited by the non return valve now jumping on and off its seat. During the last seconds of the flight this system is in stasis with the HP pump developing its apparent cavitation damage but delivering adequate fuel to the engine to maintain the recorded 1.03 EPR output.

Each side of the fuel system has matching components – and non return valves - and is constructed as a virtual mirror image of pipework and connections. Particularly given the ever increasing perfections and reduced tolerances of mass production is it unreasonable to therefore suggest that the two similar systems will have almost identical resonant qualities? Especially when we know that the left side has almost exactly the same amount of the same fuel at the same temperature etc, etc?
The range of vulnerability to harmonics created by all the variable conditions mentioned could well be adequate for the left side system to independently propagate the harmonic effect exactly as the right side.
Alternately, could it be that the resonance is transferred through the crossfeed pipework which creates a robust physical connection between the two boost pumps on the right hand side to the two boost pumps on the left hand side of the aircraft. Even though the crossfeed valves are properly closed, the resonance can be very effectively transmitted either through the metal of this pipe or the mass of liquid that fills it.
Whichever, within the period of 7 seconds the left hand system imitates the right but the fuel flow on this side reaches stasis with the engine at the slightly lower output of 1.02 EPR.

This scenario seems to me to very adequately explain all the phenomena that are apparent in this incident including the lack of any physical evidence beyond the damage found on the HP pumps.

I can think of two very pertinent questions that need to be asked.
Is it mechanically possible or actually quite impossible that one or other of the non return valves in the booster pumps could be induced to go into a state of oscillation?
Would any of the data recording systems be sensitive enough to identify and record the anomaly of pulsations in the fuel rather than just interpreting a reduced but steady flow?

Further considerations:
Given the very short period of the malfunction, it seems perfectly possible that no evidence of abnormal vibration within the fuel system would be apparent; in normal operational use all pipes and fittings will show signs or wear relative to normal and expected movement. The most likely appearance of unusual wear would be on the poppet valve assemblies but especially if the limits of movement are constrained by resilient materials this could be almost impossible to detect as being any different to normal wear and tear..

While it is very hard to accept the idea that such an event would happen on two such independent systems, very clearly something has happened. The similarity of the components and the mechanical connection of the crossfeed assemblies however do lead towards an illustration that perhaps the two sides are not quite as independent as they appear to be!

The frequency of the harmonics in the fuel system is most likely going to have a relationship to other natural frequencies within the airframe and hence it seems very reasonable that neither anyone on board nor any of the data recording equipment would detect any vibrations or noises created in the fuel systems as being either out of the ordinary or in excess of those to be expected of an aircraft in the landing configuration.


If the fuel delivery, rather than being a normal consistent flow - even though much reduced - is actually arriving at the burners in high frequency spurts, could this not perhaps explain the unusual engine noise suggested by onlookers, this likely to be far more apparent to those outside rather than those on board the aircraft?

Comments would be very welcome….

JG

[Catabolic IBS]

For clarity: 79,000 kg of fuel containing 1 ppm of water would equate to 79g of water (w/w)

Not sure here of units used but!!
1 ppm=1/1 000 000

79 000/1 000 000 gives alot less than 79kg!!

[phil gollin]

Firelight :-

......... There are some hurdles to the theory: .........


But you have to explain how the engines each separately initially responded and then each managed (at slightly different times) to have an almost exactly similar "problem" which limited the thrusts to 1.02 and 1.03 EPR.

.

[airfoilmod]

To abandon ice in Fuel. I have not been 100% comfortable with what water levels in Fuel infer re: AAIB "less than significant" findings. Debating the Authority from a distance with perhaps 1% the information they possess has been frustrating, to say the least.

What the discussion has suggested to me is a different direction; without the data necessary to support it, I'm envisioning a theory that is at least compelling to me.

"contamination" can involve other than chemical or foreign substance intrusion in the Fuel supplied.

Fuel can also be contaminated by SOUND.

Fuel can be contaminated by TEMPERATURE.

Fuel can be contaminated by VIBRATION.

It seems to me that introducing ENERGY into a liquid system can create all manner of mischief. (Witness Cavitation, Foaming, Vaporisation)

Out of my field (by light years), I will suggest that I have seen gasoline "foam" at 1 atm. pressure simply by being subjected to vibration. The corollary, and of the most interest here, would be, given the VERY low temps involved (The lowest flying that day, arguably), what would it take to solidify very cold Fuel? What combination of factors can be entertained that would explain "Bubbling", "Congealing", "Waxing", etc. of tested Fuel?

1. Stasis at descent is demonstrated.

2. Sudden introduction of Massive energy into a balanced system

3. New (Commanded) Thrust level presents dozens of instant challenges to very cold Fuel, not the least of which is acceleration in the lines of the liquid fuel, reducing pressure.

4. Although snubbed against vibration and sound, Fuel passages create several local and isolated environments, each of which must react in its own way to intense vibration (mechanical) and harmonics (Acoustic) while containing extremely frigid Fuel, which has differing resistance of its own to the new "impacts".

5. Flutter. The ancient enemy of powered flight. Any articulating mass in a fluid medium can perform badly at certain levels of tuned energy. This would include, but not be limited to: Fans, Turbines, Pumps, Valves, Filters, Solenoids, and Fuel.

Holding, Airfoil

[r011ingthunder]

Not sure here of units used but!!
1 ppm=1/1 000 000

79 000/1 000 000 gives alot less than 79kg!!

Yes, that is why I wrote 79g NOT 79kg as you did.

So once again, for clarity, 1 ppm of water in 79,000 kilograms (kg) of fuel = 79 grams (g) of water (w/w)

[Pedro 67]

Please note i'm not a pilot, i've just been following the discussion.

Does anyone know what the recommended minimum OAT is for a Boeing 777 and where the fuel temp sensors are located? There seems to be some suggestions that there might have been a localised region of cold fuel. Presumably for that to happen there would have had to be a preliminary failure of the systems in place to mix the fuel up. If the min rec. OAT for a 777 is colder than -76 (which i'm guessing it is), then there should not have been any concern about even localised regions of cold developing unless there was then a secondary failure of another safety system for warming the fuel.

Which seems a little tentative, none of the system failures have been reported yet, but if it is the case it might explain why it's such a rarity. Presumably, then there would need to be a reason why there was a delay of several hours in the effects of the cold....

[airfoilmod]

The Fuel Temp. sensing probe is located between ribs 9 and 10 in the left main tank. It is 12.6 inches from the lower wing skin, 40 inches outboard the aft boost pump inlet.

Credit Machaca, ref. to Boeing manual. (Thx).

[shawk]

Good analysis, johngreen.
A spring loaded diaphragm, such as found in a non-return valve, would make a very interesting fluid motor.
A drawing of the valve would be very helpful, as would valve specifications.

Even if resonance damping was applied to the spring or the diaphragm, a resonance might occur in such a valve due to gradual wear of the damping material or increased stiffness of the spring due to low temperature or some perfect combination of both.
Given the rarity of the event, it may be that some perfect combination of conditions and shock waves from other fuel supply components would need to occur at the valve to trigger this resonance.

Recording a low frequency resonance in a fuel flow probably depends on how often the fuel flow is sampled.
If the sample rate is less that four times the resonance, a fuel flow resonance will probably not be detected.

[soem dood]

Sampling at two times the rate of the fundamental will suffice1, but your point is still well taken -- if the fundamental frequency of the system at resonance is, say 30Hz, and if the sampling rate of the sensor is, say, every second since a profile is not intended to be continuously constructed, but merely the point value sampled, then cyclic perturbations could be missed altogether, or aliased as noise/variance in the discrete samples, but the resonance signal would not be reconstructed.

I think the idea of resonance is interesting, but improbable. I think we will find a rare, but relatively mundane cause of dual fuel supply transitory obstruction.



1 "Nyquist (1928) pointed out that, if the function is substantially limited to the time interval T, 2BT values are sufficient to specify the function, basing his conclusions on a Fourier series representation of the function over the time interval T."

[pax2908]

How is it decided (apart from practical issues) where to install the sensor? Was the a/c once instrumented with tens of sensors in various places, then the worst case was chosen?

[shawk]

Thanks soem dood.
I oversimplified my description of the performance of a switched channel A/D by just combining the Nyquest minimum sample rate and the switching rate to sample two separate fuel system samples.
I plead laziness.
To atone, I looked up the performance of the 777 DFDR.

The DFDR in the 777 appears to record 128 samples / word per second. The parameters of interest to fuel flow and engine performance are sampled once a second.
This gives a maximum sampled frequency of 0.5Hz.
No fluidic resonance recording for you.

http://www.tc.faa.gov/its/worldpac/techrpt/ar06-11.pdf

[Fzz]

The DFDR in the 777 appears to record 128 samples / word per second. The parameters of interest to fuel flow and engine performance are sampled once a second.
This gives a maximum sampled frequency of 0.5Hz.
No fluidic resonance recording for you.

The maximum sampled frequency is correct, but whether you can record something of interest is not so clear. If the sampling isn't low-pass filtered, you will still record aliases of the higher frequency. Basically you record a frequency on the difference between the signal frequency and the nearest integer multiple of the sampled frequency. So if there was a resonance at 21.25Hz, you'd record an alias frequency of 0.25Hz (21Hz being the nearest multiple of the 1Hz sampling). You couldn't tell from the record what the original frequency was, but you would get a recording of something unusual, so long as the sensors really are capable of doing point-sampling rather than one-second averages.

[soem dood]

Awesome, shawk -- I love your concept -- that we atone for our sins here by providing heretofore unpresented information that is potentially relevant to the investigation!

And Fzz is right on, as well! :ok:

(Heck, if everyone did that, we could have this thing wrapped up in a week!) :)

[BOAC]

Excuse the interruption in the jitters/flutters/harmonics/surges and PPMs, but from another thread comes 'confirmation' that a BA internal report says Flaps went 30 to 25. Hope the AAIB read this:p

[airfoilmod]

With a Johnson bar, I learned alot about flaps in ground effect and thereabouts. How to "pop over" an obstacle on very short final, how to make my brakes more effective for short ground roll, how to drop like a stone or float like a moth; this thread is about to get very interesting.

How's YOUR Johnson, Mav?

[shawk]

The profoundly toxic Honeywell Aerospace site lists the Digital Flight Data Acquisition Unit used in the Boeing 777.
http://www.honeywell.com/sites/aero/Data_Management3_C44E2B61A-C7A3-ED10-7145-57B200461FC2_HAE5AA5AC-BA3F-61C5-2E66-9A74EACA8348.htm
To read the specification sheet, you need to register for a Honeywell ID. This will take several days to get.

The Honeywell unit is a 12 bit A/D with a single 12 bit sample per second for fuel and engine parameters.
A sub-harmonic that is 7 octaves below the fundamental is quite unlikely to be resolved with a 12 bit A/D and a single sample.

A better bet might be to analyze the audio track of any amateur video shot of the landing.
If anyone has a pointer to these, post it here and I can take a look at the audio track.

[cwatters]

Shawk,

> This gives a maximum sampled frequency of 0.5Hz.
> No fluidic resonance recording for you.

I agree it wouldn't accurately record higher frequencies but that doesn't mean the samples would all appear "normal" - unless the resonance was somehow synchronised to the sampling.

[airfoilmod]

You may be overlooking the CVR. The cockpit voice recorder.

[shawk]

I'd speculate / assume / guess that the CVR has a steep low frequency cutoff to improve voice intelligibility.

[dxzh]

A few questions related to cavitation and one on water content:

1. Have there been any other reports at all of cavitation damage in any HP pump used in 777 or any other similar Boeing 7 series aircraft? I assume that fuel which has not been degassed never normally causes cavitation in HP pumps during takeoff or climb.

2. From before TOD but particularly below flight level 170, what volume of air would be entrained (or possibly dissolved) in solution in the fuel in each of the main tanks as a result of the continued operation of the centre tank fuel scavenge pumps after the 900kg rump of fuel was scavenged?

3. Would degassing be inhibited by relative coldness of fuel or its relatively high boiling point (corresponding to relatively low fuel freezing temp)?

4. Given no cavitation damage in LP boost pumps, should cavitation be assumed to be due to either (a) the higher rotational speed and/or design of the HP pumps coupled with something causing nucleation, (b) the formation of a restriction downstream of the LP boost pumps but upstream of the HP pumps or (c) a restriction upstream of LP boost pumps insufficient to cause LP pump cavitation damage?

5. Would cavitation in the HP pumps, once developed, release sufficient gas that the flow downstream would be reduced without any other restriction upstream of the HP pumps, or is the cavitation merely a secondary effect, possibly exacerbating but otherwise only symptomatic, of restrictions upstream of the HP pumps?

6. Might the suction feeds be blocked in descent (as they can be in climb) through degassing?

7. What amount of water is “excessive” in the 5.1/5.4 tons or so of fuel in each main tank ... 5, 50, 500, 5000 grammes? I guess 134 gallons or so in the 80 T centre tank is “excessive” giving rise to an alert, but does that mean that less is not?!

[barit1]

4. Given no cavitation damage in LP boost pumps, should cavitation be assumed to be due to either (a) the higher rotational speed and/or design of the HP pumps coupled with something causing nucleation, (b) the formation of a restriction downstream of the LP boost pumps but upstream of the HP pumps or (c) a restriction upstream of LP boost pumps insufficient to cause LP pump cavitation damage?

Once again, the discharge of the HP pumps is indeed at high pressure ( :ooh: ) to overcome the compressor discharge air pressure and feed the burner. Thus any air or vapor content would be compressed to a very small volume, if not redissolved completely.

And the HP and LP pumps are quite different in design. The HP is of necessity a positive displacement type - gear type or vane type etc. Since it is such, the inlet can readily suck enough vacuum to cavitate if the LP pump isn't performing.

The LP pump is designed to tolerate a certain degree of gas/vapor/air in its inlet, which is its assigned job in protecting the HP pump. I think it may be a axial or centrifugal, non-positive displacement type -- but I'll defer to someone with more detail.

[johngreen]

Barit1

The low pressure pumps are something very much like this....

http://www.eatonaerospaceltd.com/fuel/Resources/031_2_Type9602boostpump777_300.pdf (http://www.eatonaerospaceltd.com/fuel/Resources/031_2_Type9602boostpump777_300.pdf)

I beleive there are non return valves fitted in both the inlets and outlets and it is one or more of these that I am suggesting as being complicit in my post above.
There are two such pumps on either side of the aircraft and a smaller very similar one (on the left side I think) which is specifically for the APU.

JG

[RevMan2]

Tucked away 2 items below the Lufthansa HAM incident report in the BFU document (http://www.bfu-web.de/nn_41542/DE/Pu...tin2008-03.pdf) is the following item

12.3.2008 13:10 CET
B777-300 foreign registration (operator not specified)

The thrust levers were moved backwards and forwards very rapidly on approach to Frankfurt/Main.
According to the pilots, there was a delay in thrust increase.

An analysis of the flight recorder did not indicate abnormal turbine response.

[Green-dot]

To CONF iture:


In normal circumstances, MMM should have been around one minute from touchdown, so I figure out it was only 30 to 40 seconds from impact on this day when right engine reduced to 1.03 EPR
According to AAIB, both EPRs reduced and maintained steady values, 1.03 and 1.02, therefore I don't see as plausible scenario that spar valves were constantly in transit, which would have produced, I think, anything but steady values.


That depends on the timing between the moment the spar valves were in transit toward their most restrictive (but not fully closed) position and the moment of engine thrust increase, demanding more fuel flow. If engine acceleration coincided with closing spar valves, the engines could have rolled back before the spar valves reached their most restrictive positions.

Auto throttle commanded the thrust levers to increase thrust to which the engines initially responded. When the engines rolled back and no longer responded to auto throttle, the crew intervened and moved the thrust levers forward manually. This could theoretically have occurred at the moment the spar valves were near or at their most restrictive positions.

This would also have been the most damaging moment to the HP pumps with regards to cavitation. With the fuel flow now drastically reduced and the fuel metering valves fully open, this could have resulted in an imbalance to the fuel flow in the engine fuel circulation circuit (any excessive fuel circulating between FMU to the HP pump and again to the FMU). Technically speaking, manually moving the thrust levers forward could have prolonged this imbalance compared to if the thrust levers were retarded, giving the system time to restore to a stable situation with spar valves transiting back to the full open position. In this scenario, the crew would not have sufficient time to diagnose the situation and did what any crew would have done which is to squeeze as much thrust from the engines as possible so close to “terra firma.”

The EECs (in normal mode) control EPR for engine thrust on a RR Trent 895-17. A stable 1.03 and 1.02 EPR does not necessarily imply that the other parameters, N1, N2, N3 and EGT were stable as well. When the thrust levers were advanced, the EECs would be attempting to reach commanded EPR but N3, N2, N1 and EGT might well have been at varying values while stable EPR values were retained. The AAIB only mentions the EPR values but not the other engine parameters and confirms fuel flow reduction:

“The reduction in thrust on both engines was the result of a reduced fuel flow and all engine parameters after the thrust reduction were consistent with this.”

Question remains what those other consistent engine parameters were indicating.

I stress, the spar valve scenario is purely theoretical. The AAIB reported that “the spar valves were serviceable and operated correctly during the flight.” This implies that the AAIB has evidence in the form of non-volatile memory and/or recorded data, in this case data parameters which would have to have continuously monitored the spar valve positions, in open, closed or in-transit. However, no explanation is given as to how the AAIB reached this conclusion and if indeed the spar valves are monitored and recorded during all circumstances. The text is nicely formulated, so that the public perception is such that sufficient data has been obtained without the need to explain the details.


Regards,
Green-dot

[CONF iture]

Green-dot,
I personally find your technical analysis viable, but I like to go further on your other comments:
The AAIB only mentions the EPR values but not the other engine parameters ...
Question remains what those other consistent engine parameters were indicating.
... However, no explanation is given as to how the AAIB reached this conclusion and if indeed the spar valves are monitored and recorded during all circumstances. The text is nicely formulated, so that the public perception is such that sufficient data has been obtained without the need to explain the details.
Thanks to Internet, we can see a huge interest from general public for this specific event as well as others, but still, access to datas is very controlled, limited, and restricted. Give full access to all these datas, and we've got a much better chance to solve this enigma.
But no, everything is confined to an official Authority, with the only participation of the Big corporations, the ones directly involved in this bizarre event ...
And what does Joe public ... ?
Listen ... comply ... and :oh::oh::oh:
Do we really want to trust and follow a few in Power without questioning ... Will we ever learn ?

[bsieker]

The low pressure pumps are something very much like this...

http://www.eatonaerospaceltd.com/fuel/Resources/031_2_Type9602boostpump777_300.pdf

I beleive there are non return valves fitted in both the inlets and outlets and it is one or more of these that I am suggesting as being complicit in my post above.
There are two such pumps on either side of the aircraft and a smaller very similar one (on the left side I think) which is specifically for the APU.

JG

Thanks for the link!

These are the boost pumps, sucking directly from the tanks, located in the wings, and supplying positive pressure to the fuel manifolds.

There are in fact 7 such pumps, two in each wing tank, supplying their respective manifolds, and two in the centre tank, each supplying one manifold. There is an additional, DC-powered, pump in the left wing tank, feeding the left manifold, that can be used to start the APU if both main generators are offline.

I do not know if it can create enough flow to supply one or both main engines.

When one refers simply to "low-pressure pump", one usually means the engine's LP pump, which is part of the engine, and directly driven by the engine, and not the (electrically driven) boost pump.

As far as I know, the engine LP pump is also of centrifugal/impeller type, but whether or not it does have a dedicated check (no-return) valve I do not know.


Bernd

[airfoilmod]

The mechanically driven Fuel Pump mounted on the engine is "Low Pressure"? This is counter-intuitive to this mechanic, the HP pump must be by definition "last in line". Driven by a steel shaft which derives its power from the engine gearbox suggests High Pressure. Also, it is my understanding that the engines will run fine on "suction" alone. Back to school for me. (Read: suction/gravity). My impression also is that the high pressure pump is "geared" or lobed; impellors ("vanes") are more susceptible to cavitation.

Airfoil

[barit1]

Often the gearbox-driven pump assembly contains both an LP pump along with the HP pump. The purpose of the LP pump is essentially the same as the tank boost/transfer pumps, i.e. to keep a positive pressure head on the HP inlet.

Any positive-displacement pump (whether gear or vane type) can cavitate under the right conditions. In fact a marine propeller can cavitate as well if driven hard enough.

And yes, gravity feed will work just fine if the flow demand doesn't exceed the plumbing capacity/vertical head/viscosity limitations. I know of one application that is always started by gravity feed, but switches to a positive LP boost before reaching idle. (There are probably many such aircraft...)

[Nigel_the_Normal]

I seem to remember a post soon after the crash about Peter Burkhill retracting a stage of flap at the last minute and this was criticised by the usual gob s h i t e know alls on here.

BA have concluded that this action gained the aircraft a number of feet and almost certainly saved it from the ILS aerial bank, thus saving everyone.

Would the know all idiot who posted some nonsense about negative lift now care to come back on and apologise to Peter?

No?

[bsieker]

airfoilmod,

apologies for the misleading wording.

There are two pumps in the engine, one low pressure, that ensures uninterrupted feed to the high-pressure pump, and often, as shown in the diagram posted by Jet II (http://www.pprune.org/forums/showpost.php?p=3942040&postcount=467) here a while ago, also pumps the fuel through the fuel/oil heat exchanger and the low-pressure fuel filter.

Some mentioned that the AAIB was concentrating on the FOHE as the location of a possible fuel blockage; this would be consistent then with the engine LP pump not showing cavitation signs, because (at least in the diagram) the FOHE, and thus the blockage, would have been downstream of the (engine) LP pump.


Bernd

[BOAC]

Does anyone know how BA deduced that the flap change was beneficial? Is this an 'official' performance calculation, sim trial or just subjective? Most of us are certainly 'uncertain' of the balance of benefits and disadvantages at such a low height, and it would be useful background info to carry in our mental flight bags.

[Bis47]

Generally speaking ... (I'm not qualified B777 ... by far).

There is a flaps setting called "approach flaps setting" that allows a N-1 go-around with 2+ % climb gradient.

That "approach" flaps setting causes much less drag than the "full landing" flaps setting while still providing a lot of lift (81% - and generally more - of the full flaps setting).

That approcach flaps setting is generally very close to the maximum take-of flaps setting.

So, reducing from landing flaps to approach flaps is a normal move in case of thrust problems. I would say : a normal, immediate move. The stall margin is reduced by less than 10%. Second immediate move : lower the nose, keep the speed.

In short : Know your aircraft, fly your aircraft.
Whatever the aircraft type.

[BOAC]

Yes - I am aware of that, butThe stall margin is reduced by less than 10%. Second immediate move : lower the nose, keep the speed.
- herein lies the dilemma. You don't give any clue as to what you are 'quailfied on' (or indeed 'if'), but would you suggest doing that at 200AGL/400/600 etc? It is not that simple! Lower the nose and increase speed by up to 10% you say. I wonder where they would have 'landed' doing it that way? What they did worked out. It would be nice to have a little more than that from a qualified source. Indeed on 10 and 9 April you called for the same. Do you now have the answer? M Mouse suggests that BA 'trials' prove there would have been a better way (9 April).
reducing from landing flaps to approach flaps is a normal move in case of thrust problems. - is a new one to me. Can you substantiate that SOP for any carrier, and perhaps amplify 'thrust problems'? Also I am uncertain what "a N-1 go-around" is and I am also used to a 2.5% 'approach' gradient.

[michaelmarsh]

Thanks for some common sense BOAC- what a load of uninformed nonsense we read on this forum!

[Flight Safety]

Regarding water in the fuel. It's not at all uncommon for aircraft on long duration flights to land with fuel temps below 0C (or 32F). I stand to be corrected, but I don't see how a "not significant" amount of water in the fuel could have effected this flight any differently than many other "ordinary" long duration flights.

[IO540]

What type are the fuel flow sensors?

It has been stated they read mass flow and not volume flow, but mass flow is not trivial. Are they coriolis meters?

[stickyb]

An internal British Airways probe into a crash-landing at Heathrow has revealed that the captain and first officer prevented a far worse accident.
It states that both officers tried to keep the plane in the air after it suffered a double engine failure.
The report also suggests an emergency evacuation alarm was too quiet for some of the cabin crew to hear.
The BA probe into the Boeing 777 crash in January will not be made public, but details have been seen by the BBC.
Flight BA 38 from Beijing was extensively damaged when it touched down on 17 January.
Some 136 passengers were safely evacuated from the plane.
The report states that the captain, Peter Burkill, changed the angles of the plane's wing flaps in order to reduce the drag on the plane.
This action helped the plane to maintain speed.
And Senior First Officer John Coward pushed the plane's nose towards the ground in order to increase speed before levelling out just before the Boeing crashed on to the grass. Concerns about the volume of the evacuation alarm is being investigated in the official inquiry into the crash

http://news.bbc.co.uk/2/hi/uk_news/england/london/7409625.stm

[Pinkman]

The BBC segment referred to above was really rubbish "BA report reveals heroics". It referred to the internal BA report (where the flight deck crew cleaned up the aircraft to get a few extra yards). And that was it. Didnt talk about the POSSIBLE CAUSE or STATUS of the AAIB investigation and the experiments that are ongoing to establish the cause. It would have taken 15 - 20 seconds.

The BBC website is hilarious....It states that "both officers tried to keep the plane in the air after it suffered a double engine failure" and on the bullets on the same page: "Engines running on crash plane"

Wake up BBC!

[sky9]

Pinkman,
I think that you have got a little mixed up. As I see it there are 2 issues.
1 Why was there a double engine "failure" in so far as the engines failed to respond to inputs from the thrust lever or autothrottle. This has become such an involved investigation that the AAIB with the help of both RR and Boeing have yet to come up with an answer. It is interesting to note that there have been no AD published that would provide a clue as to where the investigation is going and why.

2 What were the crew reactions during the 50 secs or so that they had after becoming aware of the situation that was presented to them where there was no checklist for or they had previous experience of.
If as reported the Captain raised the flaps to the G/A position so reducing drag I would suggest that the 2 crew on board did everything that they could to retrieve the situation other than get out and push it. The fact that they all walked away from it is down to their actions and a big dose of luck.

[BOAC]

Is this indeed just crass reporting by Auntie, an attempt by BA PR to re-write history or are the AAIB on their way to changing their report? Perhaps the Beeb are looking at PPRune and taking some of the crap as a fact?

I was under the impression that the report stated that the autopilot was 'left engaged' until it disengaged itself near to the point of stall as the speed washed off (presumably no more tailplane trim/elevator authority available to it). I grant that a 'lowering of the nose' at that point would be a good idea, but hardly the stuff of heroism that gained those extra yards? BA appear convinced that the flap reduction was beneficial, but nothing still from the AAIB on that.

Can one of our moles explain how the nose was lowered to stretch the glide (edited to add 'as reported' on the Today programme this am. I DO see that there is no implicit claim to this effect on the linked item)?

[TyroPicard]

Pinkman et al...
The BBC seem to have updated their report to read..
" after it suffered a loss of engine power."
The power of PPRuNe..
TP

[woodpecker]

BA appear convinced that the flap reduction was beneficial, but nothing still from the AAIB on that

Hopefully this post will not be swept up and deposited in some other area of pprune as we are talking about the selection of 25 flap.

Surely the job of the AAIB is to establish what happened to cause the situation and 100% of their efforts should be focused on that. On the way they may come across items that need immediate attention in the form of directives (as was the case when it was realised that the order of operation of the fire handles and fuel cut-offs could effect the closing of the spar valves) but their job is not to feed titbits to the many flightless birds around the bird table here on Pprune

They are aware of what other actions the crew took in those few seconds, and the effects those actions had on the flight profile, and I am sure they will come out in the full report.

Their interim reports have been factual and (correctly) have not focused on whether the crew "allowed" the aircraft to stall or, as the A/P cutout held onto what speed was still available by lowering the nose.

Lets not forget, the crew had less than 50 seconds, no "electronic checklist", no simulator training, no ops manual guidance, very little EICAS information, attitude increasing to that of the dear old Trident (with rapidly dissapearing visual sector) but still managed to stick to the basic BA procedure of the handling pilot flying (initially in this case using the autopilot) while the other two (remember there was an extra crew member on the flight deck) pilots trying various actions (not all reported so far) to restore the power to "normal levels" and at the same time accepting the worst and retracting the flaps to 25 which has (again not been officially quoted) been calculated as giving them an extra twenty eight feet, allowing them to clear the localiser array by seven feet.

Lets focus on what a good job they did and allow the AAIB (without criticism of not giving us the "total" picture) to find out why it happened.

[J.L. Seagull]

I'm exceptionally surprised that the contents of an internal BA investigation or report into the crash have been published or even released outside the company at all. AAIB have taken charge of the investigation and executorship of the MOR, and that normally means no details will be published until they conclude. A little like being sub-judice.
[That of course excepts any directly related safety issues required to prevent re-occurrence.]
Someone is playing fast and loose inside BA methinks, I'd love to know what AAIB thought about this latest information!

[Bearcat]

the big question is why did those engines fail....was the aircraft flown outside its temperature envelope ??

[Super VC-10]

Which raises a question:-

Is there such a thing as a "temperature envelope"? :confused:

[woodpecker]

Someone is playing fast and loose inside BA methinks

All airlines have to have investigation procedures in place following any reportable accident/incident.

These procedures are approved by the CAA. One of the requirements of BA's (and I am sure many other airlines) procedure is that "interested parties" are kept in the picture by regular updates.

Whether it's a left hydraulic loss or a total hull loss the procedure is the same.

[NigelOnDraft]

Bearcat... just a quick read of the AIIB bulletin would answer your question the big question is why did those engines fail....was the aircraft flown outside its temperature envelope ??The aircraft was operated within its certified flight envelope throughout the flight.
NoD

[BOAC]

Surely the job of the AAIB is to establish what happened to cause the situation and 100% of their efforts should be focused on that. - absolutely, but while we have this 'unexplained' occurrence it is important that crews have an idea of what happened, what was done and the results of such, and this should not be confined to BA crews. If retracting flap gained 28 feet, then tell everyone. Who 'showed' the BBC the report?

Remember also that the appetite of the flightless birds has largely been stimulated by BA's PR activities - and your and your colleagues' posts.

I personally think you and the other BA posters were wrong to feed titbits yourselves, especially where presumably confidential company information is concerned. To quote JL, "Someone is playing fast and loose inside BA methinks".

Few of the flighted birds here have cause to level any criticism at the crew at this stage. All of them need answers as they come.

[Bearcat]

Super VC-10 says


Which raises a question:-

Is there such a thing as a "temperature envelope"?

should have said environmental envelope? SAT-70 is the limit for the airbus 320/1.....what is it for the 777?

[130.4]

Who 'showed' the BBC the report?

It may not have been the BBC. The Daily Mail/Evening Standard has the same story almost word-for-word. I don't know who's plagiarising whom, or if they independently copied the same words from the report, though I would be surprised if the report had worded it that way.

[Sunfish]

Pinkman:

"BA report reveals heroics". It referred to the internal BA report (where the flight deck crew cleaned up the aircraft to get a few extra yards). And that was it.

....and that was apparently enough to save all on board.

Pinkman, could you and your fellow non flying colleagues please try and be more offensive next time? The standards of your posts seem to be slipping lately.

[atakacs]

"BA report reveals heroics". It referred to the internal BA report (where the flight deck crew cleaned up the aircraft to get a few extra yards). And that was it.


....and that was apparently enough to save all on board.

Does anyone have any firm evidence of this ?

To be honest given the timing of the incident and (thankfully) very unique circumstances there wasn't much they could actually do.

Reducing flaps might have saved the day, or it might have them miss the runaway. I'd love to see some hard evidence either way.

[Notso Fantastic]

To be honest given the timing of the incident and (thankfully) very unique circumstances there wasn't much they could actually do.

Reducing flaps might have saved the day, or it might have them miss the runaway. I'd love to see some hard evidence either way.
You evidently don't have much experience of actual flying. Who are you to decide? Why don't you have the courage to state from what basis of experience you speak? (like put something in your profile?)

[woodpecker]

Sunfish,

I think atakacs should be added to your list! What does he mean by
might have them miss the runaway

[Machaca]

http://img238.imageshack.us/img238/5950/777temphb3.jpg (http://imageshack.us)

[atakacs]

You evidently don't have much experience of actual flying. Who are you to decide? Why don't you have the courage to state from what basis of experience you speak? (like put something in your profile?)

Sorry if it sounded otherwise but I was merely asking. It might possibly be a trivial question, in which case I would be more than thankfully to read a factual analysis of the 777 performances under those very unusual circumstances with various flap settings.

I believe to have read every single post in this thread and if this was already unequivocally addressed then I overlooked it, in which case my sincere apologies.

[Bis47]

Reading the chart, - 80° sat is the lowest temp limit.

Since this is not a frequent occurrence, I wonder how much testing was done to substantiate that ...

[armchairpilot94116]

ATW Daily News




777 Heathrow accident remains a mystery

Thursday May 22, 2008 Investigation into the Jan. 17 crash landing of a British Airways 777 on final approach to London Heathrow has not revealed an anomaly that could have caused the flow of fuel to the engines to be reduced as the pilots called for more power, according to the UK Air Accident Investigations Branch, although restricted fuel flow was certainly the cause of the nonfatal accident (ATWOnline, Feb. 20) (http://atwonline.com/news/story.html?storyID=11787). In an update released yesterday, AAIB noted that the engine control system detected the reduced flow and "commanded the fuel metering valve to open fully." The valve responded, "but with no appreciable change in the fuel flow to either engine." Some kind of "restriction" in the fuel system between the fuel tanks and the pumps is suspected.
Examination of the aircraft and analysis of recorded data "have revealed no evidence of an aircraft or engine control system malfunction," nor is there any evidence of "a wake vortex encounter, a bird strike or core engine icing. There is no evidence of any anomalous behavior of any of the aircraft or engine systems that suggests electromagnetic interference. The fuel has been tested extensively; it is of good quality, in many respects exceeding the appropriate specification, and shows no evidence of contamination or excessive water. Detailed examination of the fuel system and pipe work has found no unusual deterioration or physical blockages."
Investigators are examining whether "particularly" cold temperatures along the flight path may have had something to do with the restriction, although the temperatures "were not exceptional" and tests revealed that the fuel onboard had a measured freezing temperature of -57ﾺC, well below the minimum recorded lowest fuel temperature of -34ﾺC.

[soem dood]

<tap tap tap>

"Is this thing on?"

Use a GC/MS analysis to characterize the [Chinese] fuel.

That is all.

[barit1]

The fuel system engineers will follow this train of thought, I trust, based on the difference between volume-unit and mass-unit measurements. If I follow that AAIB reading of the data correctly:

1) Both fuel metering valves opened per (initially) autothrottle, and later manual command. Thus I'd exonerate all upstream control hardware and software.

2) The flow metering valve is typically a volume metering device, calibrated such that when a fuel has known density and BTU content, then a calibrated mass flow (and thus BTU rate) is delivered to the burner.

3) However - fuel flow, measured typically by mass-flow devices (someone update me if this is not the case on 777s), in this instance shows little or no increase in mass flow.

The only conclusion I can draw is the properties of fuel reaching the burner were quite abnormal, likely a high volumetric percentage of air or vapor, with drastically reduced BTU delivery.

And the next question I must ask is: What was the indicated metering valve position, and the indicated mass flowmeter signal, during the latter part of descent before the throttle increase was commanded?

[Bearcat]

NigelOnDraft Bearcat... just a quick read of the AIIB bulletin would answer your question
Quote:
the big question is why did those engines fail....was the aircraft flown outside its temperature envelope ??
Quote:
The aircraft was operated within its certified flight envelope throughout the flight.

Thanks NoD.

[GearDown&Locked]

Would the oil temps that go into the heat exchange systems be recorded anywhere? What impact would it have in the fuel heating process if the oil wasn't warm enough?

If the overall engine temperature was bellow average and stayed that way throughout the reported idle descent, oil temps could have been also bellow expected, thus not helping the heat exchangers to do their thing in reheating the fuel properly, developing a "cold loop" between the engines and the fuel in the tanks.

[arcniz]

Use a GC/MS analysis to characterize the [Chinese] fuel.

Well & good to identify spectra for the chemical constituents, but not necessarily a predictor of the behavior of the fuel when uncommon components and unfamiliar proportions are present (if happens to be the case in this instance - which is likely, given the inherent variability of fossil fuels and refinery processing methods.).

Consider the analogy of running a cat through a blender, then running spectra on the resulting goo to see if Sylvester was a good mouser.

There's also the matter of analyzing performance "at temperature" - for the very cold region of interest. Not so easy as putting a coin in a vending machine & having all the answers slide out the chute. Could turn into a full-scale research project to completely characterize the fuel in relevant circumstances. Probably is being done as the clock runs.

One lab test that would be very instructive would be to evaluate the relative frothing propensities of the fuel samples taken, as compared to "normal" JPxx.

Commercially available additives exist to virtually eliminate foaming in aviation fuels. Likewise, additives exist to greatly amplify fuel frothing and foaming behavior in airborne usage, typically as a method for limiting fire or explosive flame propagation in the event of tank ruptures & punctures - such as might arise from coming too close to a cannon during a military engagement. The possibility that high-grade fuel formulated for military use could become mixed with fuel for commercial use is not too far-fetched. Conceivably the mixing of military and commercial fuel batches might even be a common practice in some places when extreme cold-weather performance is known to be required.

[Flight Safety]

Surely early in the investigation, someone thought to subject some of the recovered fuel to controlled progressively colder temperatures, to see if it developed any unusual properties that might result in a restriction.

[seekayess]

Folks, let me apologise in ADVANCE if this one sounds dumb, but . . . .


Many, many eons ago, I was on a cross-country flight in a Gnat. After the entire 750 nm was done at about FL 400, with the minimum/no movement of the throttle needed, I commenced my descent, throttling back and a/b out.

As I finished my descent and put the u/c down, I had to now open the throttle.

Bloody hell! No response from the engine!!

Had to do a belly landing on the side of the runway (am cutting the story short!).

Investigations found that there was sufficient fuel on board.

One of the technical items in the engine was an AFRC -- Air Fuel Ratio Control. This, as it's name suggests, was supposed to automatically open/ close an aperture in the fuel flow to ensure that too much fuel did not go to the engines etc.

With no movement of the throttle for an almost one hour flight at FL 400, icing had taken place in this aperture. So, when I opened the throttle, the extra fuel required could not flow because of the icing, which ahd not had the chance to melt away due to the rapid descent.



I know technology has changed immensely in the last 40 years and this was a fighter aircraft (PS: I have ONLY flown fighters -- the reason for my apology in the first line!), maybe such a thing may not have happened.

. . . but, I thought why not post this one!!


:O

[ChristiaanJ]

"...Detailed examination of the fuel system and pipe work has found no unusual deterioration or physical blockages."
IIRC there was a mention at one point of 'cavitation-type' damage to at least one of the fuel pumps. Am I wrong, or has that now been discarded as one of the probable causes?

CJ

[soem dood]

Thanks, arcniz --

Cat in blender analogy is a good one! So good, in fact, that I'll hijack it a bit -- what if we do in fact have 14 specimens of FelineFrosty™ aka cat-in-a-blender, and four were terrific mousers, nine were so-so, and one was an unabashed mouse-sympathizer.

While there is no guarantee that an analysis of the spectral signatures would establish a high correlation of any feature with mouse-aggression, IF (a speculative if) such a relationship did emerge, wither with or without a pre-existing hypothesis to explain it, surely that is probative, and at least could indicate a direction for future inquiry!

BTW -- T. gondii could account for odd cat-mouse interaction behavior if it were the mice we were looking at, but would likely not show up using such a crude method as GC/MS, so your point is somewhat reinforced in my mind -- physical tests that demonstrate properties important to the material's suitability as a fuel while in the conditions observed, are probably the best hope.

Thanks again!

[misd-agin]

Repeat after me, "their performance was heroic." That seems to be the common mantra against anyone wondering what happened during the flight portion of the accident.

Could someone please explain what actions they took that were heroic?

Oh, that's right, the investigation is still being completed. All we have are the tidbits the AAIB has given us, which didn't mention any heroic actions.

So I'll ask again, what were the heroic actions that folks keep bringing up?

[arcniz]

Could someone please explain what actions they took that were heroic?

here's a stab at it:

By the result alone, without any further looking, about the crew of BA038 one can say:

In a very difficult situation, that occurred totally unforeseen...

with only seconds to make observations, reach conclusions and set the actual final plan for dealing with it...

they did their jobs as well as any persons could - and likely better than many would.

They kept their cool, collaborated to fly the aircraft for her last mile steady on the balance of a feather between too short and too slow, knowing full well their own lives and many others depended on the actions of that moment.

Their presence of mind and professional focus continued after the probably unexpected safe arrival, largely intact, on the earth. They did the job to render the damaged machine as safe as was possible with the electrical, engine, and fire controls available, monitored the evacuation, climbed out of the wreck and then likely started on the paperwork while waiting for a ride to the terminal. That was the heroic part.

[Basil]

seekayess,
I'd say yours is very reasonable comment.
Did you get a Green Endorsement for the Gnat incident?

misd-agin,
I think that the change of configuration demonstrated very quick thinking, involving, as it would, a rapid assessment of whether any immediate sink would be more than offset by the higher L/D ratio for the remainder of the flight. 'Heroic' ? Well, I think we're arguing semantics here. If they are heroes then the resulting attention and expectation will probably make their lives more difficult. Quick witted professionals? I'd say so.
I've certainly learnt from having weeks to consider actions which took place in seconds and, although having retired, I am unlikely to put any conclusions into practice, there just may be the occasion when the crew flying Basil to visit the kids, having read all this, will save the day :ok:

[TheShadow]

The theory I prefer for BA038 is related to the long-term cold-soak.

Forget the fuel having "frozen". Think moreso the cavitation damage to the fuel-pumps (and their restricted throughput/fuel starvation) having been caused by the treacle-like consistency of the fuel brought about by waxing (which occurs a lot earlier than the fuel's freezing point).

Haven't seen any arguments to the contrary, although there has been much reference to waxing. You have to extrapolate the waxing consequence to flows and that being related to the effect upon viscosity of a preponderance of wax globules in the fuel.

Paraffinization of oil has always been a problem for oil-pumping from great depths for ocean oil-rigs. It's likely to be little different for any oil-based fuel. Low temperature hydrocarbon metamorphosis in the presence of various catalysts (such as fuel additives and water) is a whole branch of organic chemistry.

[Belgique]

The Shadow
I'd tend to agree:

Microcrystalline wax:
is a petroleum based wax that has very different physical properties to paraffin wax. Micro wax is more dense (weighs more), has a different molecular structure (closer packed molecules), is thicker when liquid (higher viscosity), is more flexible (bends easier), has more adhesion factors (is sticky), tougher (scuff resistant), and has a higher melt point than paraffin wax.

Viscosity:
the Ability of wax to resist flow. The viscosity of wax is represented by a number that defines how a particular wax will flow at a certain temperature. The temperature and the viscosity of wax have opposing relationships. The higher the temperature, the lower the viscosity, and the lower the temperature, the higher the viscosity. This means that a hot wax has a low viscosity and will flow much better than a cold wax with a high viscosity.

It's also noteworthy that waxes have a differentiation between their congealing and freezing points and can be "precipitated out" of fuel. A jet fuel's "pour point" is taken to be 4 to 20 degrees below its freeze point. Fuel will not flow at all when it's below its pour-point. However, in contrast to "pourability", once the physical characteristics of a pumped fuel changes towards a higher viscosity, the pumpability of a particular system will very much depend upon whether the fuel is being pushed or pulled (i.e. sucked or blown).

Cavitation damage to pumps and a failure to throughput a programmed volume will be a logical consequence of a much higher viscosity (than allowed for in the system's design flows). Fuel is used as an hydraulic fluid in HP fuel system servo valves and the proper function of these servo-systems is dependent upon the fuel remaining in a design viscosity range.

The rate at which fuel temps decline (and the temp to which it will drop) is a function of:
a. airplane geometry
b. total (TAT) and ambient air temps
c. fuel management
d. exposure time
e. the fuel's initial temperature

(tinyurl.com/6jxpfn)

There's no guarantee that a fuel load will remain anywhere near a uniform temperature throughout its tankage.

[777fly]

There appears to be an impression amongst some that the environmental conditions experienced by BA038, prior to the accident, were unique or unusual. Rare, I agree, but not unusual. BA operate a daily service to Beijing, so presumably the PEK services flown in the days before and after operated through a similar air mass, as did dozens of 777's of other operators. I have flown 777s through almost identical flight profiles and temperatures several times, as have many others on hundreds of occasions, without a problem. The question has to be why this has not happened before and the task for the AAIB is to determine what made this particular flight unique. Given that all engine and fuel systems appear to have been functioning correctly and that the fuel was never near it's freeze point, the only variable factor, barring EMI, is likely to be the quality, composition and properties of the fuel uplifted in Beijing on that day. That may be stating the obvious, but the flight conditions that day were certainly not unique.

[ChristiaanJ]

777fly,
I agree that we hope the AAIB will manage to get all the pieces of the puzzle in place.

I doubt there will be one single clear-cut cause, or it would have happened far earlier in the life of the aircraft.
I'd rather think they will find that enough holes in the Swiss cheese lined up for the first time.

Rare weather conditions (not unique, but rare just the same), the aircraft maybe staying longer at FL410 than advisable (didn't others descend to FL240 and lower?), a continuous-descent approach (CDA) with no call for increased thrust until about 2 miles out, and then the fuel characteristics (I would not want to use the word 'quality' since from what we hear the fuel was 'within spec', but even so the exact fuel composition can vary considerably).

Maybe the AAIB will add another couple of holes to the Gruyère. An obvious one would have been a faulty fuel temp sensor, but I have seen no mention of that.


As to the discussion about 'heroes'....
'Heroes', to me, are people who do something incredibly brave and often incredibly stupid, with total disregard for their own lives, and bring it off.

To me, those two pilots were true professionals, who in those few seconds, thanks to their knowledge and experience, drew the right conclusions, did the right things, and turned what could have been a fireball into a remarkable short-field landing...
Nothing 'heroic' in my book, but true professionalism, for which I have a far deeper respect.

CJ

[Pinkman]

Soem Dood I know where you are coming from and I would hope that GC/Mass Spec would have been the first thing that was done for the post incident investigation. Well, possibly the next thing after the traditional Jet tests (Freeze point, Distillation Range, T90, Final Boiling point etc etc.)

If you had access to the results I am willing to bet that what it will show you is that while Chinese Jet fuel is compositionally a bit different to typical kerosene fractions in the west (hence the -52 deg FP) it met specification. The question in my mind long ago ceased to be what caused the fuel starvation, (a couple of us have repeated endlessly our prediction that it was due to fuel stratification caused by unusual temps leading to a mush of ice and wax at inlet) but whether existing Jet specs and automated test methods are fully protective of safety in all conditions.

The only way this will ever be proven is through the simulations now being done at Boeing and RR (challenging!). If it is ever established without doubt, it will lead to the biggest shake up in Jet fuel testing in two decades.

Pinkman

[PickyPerkins]

Belgique said:
Fuel is used as an hydraulic fluid in HP fuel system servo valves
Fuel full of cavitation bubbles isn't likely to work very well as a hydraulic fluid.

What exactly do these servo valves control? http://pickyperkins.home.infionline.net/pi.gif

[UNCTUOUS]

Picky Perkins said: Belgique said:
Quote:
Fuel is used as an hydraulic fluid in HP fuel system servo valves
Fuel full of cavitation bubbles isn't likely to work very well as a hydraulic fluid.

What exactly do these servo valves control?


Just GOOGLE HP fuel system servo valves - and all will be clear.....

[DC2 slf]

I understand that the engines were idling when a command to increase thrust produced a brief increase followed by reduction in both engines.
Has it been reported if the engines were reduced to idle followed by a successful command for increased thrust at any earlier point in the descent? Or was each thrust reduction to a low level without in intervening idle period?

[soem dood]

Thanks, pinkman!

Nothing to add to your insightful post, except that if someone wants to eyeball the actual chromatograms for some varieties of fuels, including raw crudes, as well as Jet-A, JP-5, Diesel #1 gasoline, AV-Gas, etc. then this little 6 page PDF has them displayed (caution - link is directly to a PDF file).

http://www.image-train.net/products/papers/CM1_alimi.pdf

General method of analyzing kerosene w/GC (from NIOSH):
http://www.osha.gov/dts/sltc/methods/partial/pv2139/pv2139.html

This caution caught my eye: "Different kerosenes may have similar constituents but not have similar concentrations of these constituents..."

and it seems to reinforce the likelihood that we are probably looking at a very subtle issue, that until all the swiss cheese lined up for BA038, might have manifested itself only in ways far below overt detection/specific identification. Once the chain of cause and effect is determined with finality (and unlike some, I feel it will be), I'll bet there will be an opportunity to look back at some 'oddities' in past, and finally go "Ah HA! there you are, you little son of a gun!" :8

[777fly]

Pinkman Re your post:
' a couple of us have repeated endlessly our prediction that it was due to fuel stratification caused by unusual temperatures leading to.....'

If this were so, the problem would have shown up long ago. There is no need to reproduce the accident flight profile, as it has been flown many times without incident already. There is some other factor which will emerge in due course....

[FullWings]

gerryfoley,

I understand that the engines were idling when a command to increase thrust produced a brief increase followed by reduction in both engines.
Not quite. According to the AAIB, BA38 had complied with BA stabilised approach criteria, i.e. it was in the landing config. by 1,000'AAL with approach power set. As there was some wind that day, the power would have been varying around the normal datum during gusts and lulls; at around 700' one engine stopped responding to commands and ran down to a low EPR, followed seven seconds later by the other one.

Has it been reported if the engines were reduced to idle followed by a successful command for increased thrust at any earlier point in the descent? Or was each thrust reduction to a low level without in intervening idle period?
Following the normal LHR speed control involves staged reductions from 220Kts-->180Kts-->160Kts-->Vapp. The last two or three reductions usually occur after being established on the GS and for a 777 on an average day you need to have the engines at idle plus some drag from gear or speedbrake to make the change happen. I don't think the actual thrust levels during the intermediate/final approach have been reported but I would be surprised if the engines weren't at flight idle for a significant time before the power came up prior to the 1,000' approach 'gate'.

777fly,

...There is no need to reproduce the accident flight profile, as it has been flown many times without incident already.
Without incident, defined as a discovered anomaly, yes. What we don't know is how many times there has been a restriction in the fuel supply system that has gone unnoticed because it has not been severe enough to affect the engines.

If it was, in fact, quite common, how would we know about it? Imagine a blockage forming that only became evident over 90% of rated power: how often do we use full TOGA power after starting the descent? Not very often, IMHO. If the restriction reduced the delivery capacity of the fuel system from 200% of maximum to 110% it would remain undiscoverable by normal means but still exist... :ooh:

[Rightbase]

Pp:
Fuel full of cavitation bubbles isn't likely to work very well as a hydraulic fluid.
Cavitation bubbles are full of low pressure gas or vacuum, and they occur as the fluid is torn apart by suction. The vacuum filled bubbles collapse implosively when pressure is restored and the shock waves cause local damage. This happens in a very localised area as the fluid passes from the low pressure side of a pump to the high pressure side.

Used as a hydraulic fluid under pressure, cavitation bubbles would not be present.

[wilyflier]

Just bubbles then.

[soem dood]

Wow. Well, we finally hit the wall -- it's been a couple of days with NO new posts on this thread.

I do not want to post in order to prattle on and just hear myself talk, but I will make one observation, which others can respond to or not. I know I previously mentioned the idea of using GC/MS to test/characterize the Chinese fuel, and it was properly pointed out that such a test may be far from definitive, and that other test measures may be better. Fair enough, and point well taken.

But let's review an article that gives on overview of the observations to date, as well as the latest tack the inquiry is taking:



[Looking at restrictions in piping]
...engine test cell... altered to enable... calibrated restrictions at various locations in the engine and aircraft fuel feed...


[looking at environment]
...primary challenge... to create the environmental conditions experienced... to test a representation of the aircraft fuel system... replicating the fuel system performance experienced on the day...

[reviewing/re-accomplishing system design characterization]
... developing a more complete understanding of the dynamics of the fuel as it flows from the fuel tank to the engine...

[flight data review]
...a data analysis team... reviewing and analyzing recorded data from a large sample of flights on similar aircraft.... concentrating on... abnormal combinations of parameters...

[recap]
"Recorded data indicates that an adequate fuel quantity was on board the aircraft and that the Autothrottle and engine control commands were performing as expected prior to, and after, the reduction in thrust... work includes a detailed analysis and examination of the complete fuel flow path from the aircraft tanks to the engine fuel nozzles," the second AAIB report stated.
http://www.aviationtoday.com/asw/categories/commercial/22017.html

IMHO, the ONE potentially relevant factor that this article, and the AAIB updates themselves are completely silent on is the MATERIAL, i.e. the fuel itself, whether it be chemical composition, or physical properties, or additives, or other attributes.

The only data presented so far on the physical material responsible for combusting and being converted to the required energy was that it's freezing point 'far exceeded' the spec. for that parameter.

Super.

However, I find the lack of ANY further information on that front, even if it is just to close off the fuel itself as possibly being a contributor (with evidence to back it up), to be very odd.

Am I off in the weeds here?

[Green-dot]

However, I find the lack of ANY further information on that front, even if it is just to close off the fuel itself as possibly being a contributor (with evidence to back it up), to be very odd.


Same can be said about the spar valves.
The AAIB states: "The spar valves and the aircraft boost pumps were serviceable and operated correctly during the flight."

No reference has been given as to how the AAIB came to that conclusion. Logically this would mean recorded data is available, at least that is the perception when reading the report . . . . .

Perhaps the final report will explain how the AAIB came to this conclusion.

Also, i wonder when approach idle was activated over minimum idle.
Was this activated by engine anti-ice or during landing flap selection?

If it was activated during landing flap selection, the thrust increase from minimum idle to approach idle (if the aircraft was indeed following a CDA profile at minumum idle) must have occurred during intermediate/final approach, sometime before the final thrust increase and rollback?

If it was activated by engine anti ice, there would have been a cycling between minimum- and approach idle several times, each time engine anti ice activated and de-activated, before landing flap was selected. In both cases, with landing flaps selected, the engines would have been in approach idle when the problems started.


Green-dot

[ChristiaanJ]

Am I off in the weeds here?
I would say so. It's a magazine article, remember, not an AAIB report.
"Ice collecting in or near an engine component has emerged as a prime suspect."This is mentioned early in the article.
It's "a prime suspect", but until more testing is done, it cannot be listed as the "probable cause", and the AAIB has not yet done so.

[Mariner9]

IMHO, the ONE potentially relevant factor that this article, and the AAIB updates themselves are completely silent on is the MATERIAL, i.e. the fuel itself, whether it be chemical composition, or physical properties, or additives, or other attributes.

That would not be my interpretation of the AAIB reports.

Special bulletin 3/2008
The fuel has been tested extensively; it is of good quality, in many respects exceeding the appropriate specification, and shows no evidence of contamination or excessive water.

GCMS must have been done to confirm that fuel did not contain any contaminants, for as Pinkman pointed out, the standard Defstan/ASTM tests can only confirm or otherwise that the fuel meets spec. Whereas only GCMS can show whether the fuel consists "Solely of hydrocarbons and approved additives" as it must do and apparently did.

[airfoilmod]

With a fully open mind, the Fuel will attract attention, however.

Reading a release of information can be subject to premature conclusions, especially when very little further data is supplied. Nothing of what has been said by AAIB totally eliminates its consideration, obviously, since AAIB have Not provided a causal chain.

"Exceeding" the appropriate specification is a troubling statement. In what way? A Spec is a range of points that contain a compliant sample in all respects. Is a freezing point of -57 "better" than -49? Perhaps, but in what way; what about the Asian Fuel causes the odd (Though lower) temp? What about its composition makes it "different"?

BA038 flew at much higher altitude than other A/C in a similar profile. The 777 has but one Fuel Temp sensor, and it is located in the Port wing, which basked in the sun for much of the flight. Was 38C an accurate reading? Was the Sensor tested for performance? What of the Temp. elsewhere?

Was there some attribute of the Chinese Fuel related to its lower FP that ennabled it to contain water in solution as well as in emulsion?

In Soem Dood's defense (not that it is necessary), Fuel (Lack) is the proximate cause of loss of Thrust; Saying the Fuel played no part in a result that has remained publicly unexamined (completely) is very premature, very. Fuel temperature is a Fuel attribute. To be blunt, the Fuel on Board BA038 came from a supply that is not widely used in similar flights elsewhere, drawing attention simply by virtue of its "rarity".

Airfoil

[phil gollin]

Maybe very stupid, but,

Can the AAIB issue a full report with no conclusions, i.e. "this is all we've done, here are the things we've ruled out, but we don't know what actually caused the accident" ?

And would Boeing and Rolls-Royce have to "sign-off" on that ?

.

[tanimbar]

I suspect the AAIB will be thinking, "let's not take our fences until we have to".

Meaning, there is a long way to go before a FINAL report is issued - possibly years. And during that time the AAIB and others will seek the answer to the riddle. And they know they must find the answer/cause.

Meanwhile other bulletins will probably be issued which will include operational changes to high altitude, low temperature flights. Expect the next before the northern hemisphere Winter.

Regards, Tanimbar

[slip and turn]

As airfoilmod suggests, the exact physics of the fuel must surely be where the best minds are concentrated.

A friend of mine joined me and the kids at the weekend for a Chinese 'cook-in'. You know, root ginger, garlic, chillis stir fried, sweet and sour sauce from scratch ... superb smells from the kitchen and the kids just loved getting involved right from the initial shop we all did together at WalMart, to individually coating the chicken and prawns for deep fry, and then scoffing the lot :)

WalMart were right out of cornflower. My friend said potato flower worked just as good, and so it did.

But as the kids were coating the prawns and chicken in the water/flower mix, some of the physics emerged...I still find what we saw quite astounding all these years after I first saw the starch experiment at school.

One moment you are mixing the water and flower and saying "Its not quite right, shall we add some more? It's too runny"

Next you are saying well it fries good.

Then you return to the bowl and put your finger in and say "Wow, lookey here - it's gone hard!"

And you stir your finger and it becomes runny again before anyone can see what you meant.

And if you play further with it, and let it settle again, and remove just a tiny amount of the surface water, you can prize out the mixture in one bowl shaped solid, with the consistency of those bbq firelighters ... but you ain't gonna light no fires with it ...

[Mariner9]

Of course the fuel is the source of on-going investigation, I neither stated nor implied it wasn't, and nor do the AAIB. I was merely pointing out that those assuming that GCMS/FTIR or whatever had not been undertaken were probably mistaken. Rest assured, the properties of that fuel are being thoroughly evaluated. (I investigate contaminations in jet fuel for a living by the way :8)

[chris weston]

Slip and turn nicely describes the thixotropic (http://en.wikipedia.org/wiki/Thixotropy) behaviour of his starch/water/oil (?) cooking mixture in a mouthwatering way in 1256 but I doubt that the likely BA038 fuel problem lies in this direction.

I am pretty sure that the kind of molecules (yes I know about the ionics that may or may not be there too!) you need for this sort of thixotropic behaviour, will show up big time on the standard battery of NMR/MS/IR analytical weaponry available to AAIB.

We'll see!

CW

[Rightbase]

the standard battery of NMR/MS/IR analytical weaponry available to AAIB.

Or a humble rheogoniometer.

[chris weston]

The plot thickens......

Sorry.

Seriously, in my view if it was one or more thixotropes at work, truly the mother of all Sinoadditives, the fuel would have been out of spec and we would have heard by now.

CW

[Smilin_Ed]

Do I interpret this discussion correctly? Is it being said that whatever caused the freezing point of the fuel to be much lower than typical caused the fuel to not be properly combustible?

[Bunk-Rest]

Bear in mind that the whitewash being prepared by Rolls & Boeing will be taking into account of all the points raised in these posts...........

[airfoilmod]

If AAIB are that sinister, they certainly wouldn't need hints from the likes of PPRuNe. And if they're straight-up (obviously), conjecture doesn't hurt. What you seem to be suggesting isn't helpful, unless you're trying to be funny. Every venture undertaken by Human beings requires a little Faith.

[chris weston]

Smilin Ed, in respect of your last (1261), I think not.

"Do I interpret this discussion correctly? Is it being said that whatever caused the freezing point of the fuel to be much lower than typical caused the fuel to not be properly combustible?"


I'm not a fuel chemist, just a pure chemist of sorts and hope to be corrected if wrong by those in this thread more knowledgeable than me, but simply shortening the average alkane chain length in the fuel will be the basis of the lowering of the freezing point/waxing point temperatures, not the addition of non combustible additives.

Technically such changes will alter the Heats of Combustion a little but never so as to render the fuel non combustible. That the fuel was within spec is key.

The arguments for and against the effects of water scavenging additives (which are usually ether like molecules derived from ethoxyethane and inherently themselves very combustible) and / or fuel having become stratified, have been rehearsed at length in earlier posts on this thread.

CW :)

[precept]

The BA 038 accident facts published to date make it one of the most significant events in aviation history. Those who respect aviation must keep the investigation front and center. Whether grey beard or shave tail, all of us must stay the course and do what must be done to bring this event to a conclusion which benefits each of us who dare to expand aviation.

[Ex Cargo Clown]

Smilin Ed, in respect of your last (1261), I think not.

"Do I interpret this discussion correctly? Is it being said that whatever caused the freezing point of the fuel to be much lower than typical caused the fuel to not be properly combustible?"


I'm not a fuel chemist, just a pure chemist of sorts and hope to be corrected if wrong by those in this thread more knowledgeable than me, but simply shortening the average alkane chain length in the fuel will be the basis of the lowering of the freezing point/waxing point temperatures, not the addition of non combustible additives.

Technically such changes will alter the Heats of Combustion a little but never so as to render the fuel non combustible. That the fuel was within spec is key.

The arguments for and against the effects of water scavenging additives (which are usually ether like molecules derived from ethoxyethane and inherently themselves very combustible) and / or fuel having become stratified, have been rehearsed at length in earlier posts on this thread.

CW

Not as simple as that, works for short length C1, C2, C3 molecules as they are all around the same MP (Gotta love Carbon Bonding), but as the chain gets longer the melting point increases quite markedly.

[GMDS]

Some T7 operators sent out a bulletin stating that they had some EAI valve malfunctions lately and Boeing is working on it. In the mean time crews are requested to test the system in descent.

In view of my early posting (#75), i am just wondering if this is not some coincidence.

But the visor is set on fuel and i concurr with Bunk-Rest's post of the 30th.

[CONF iture]

Some T7 operators sent out a bulletin stating that they had some EAI valve malfunctions lately and Boeing is working on it. In the mean time crews are requested to test the system in descent.
In view of my early posting (#75), i am just wondering if this is not some coincidence
GMDS, I'm not able to see your early post ... Would you tell us a little bit more on that bulletin ?

[GMDS]

CONF iture

The bulletin does not say a lot more than what i posted.

Sorry for the ref to my post, it was on a early but different thread about BA38.
Here is what I wrote. (I have to put it in the right timely context though: It was only three days after the accident, well before the AAIB report this thread refers to.)

If i go about the RR Trents on the T7, there is a possibility of no malfunction but still having the effect described. The engine anti-ice is mostly switched to "AUTO". I have witnessed its reluctance to go to "ON" more than once. Lets assume the inlet PT probe was iced up, apparently a ice warning was out that day and prolonged holding in the FL80 to 120 is a ice prone region. As the EEC uses EPR as parameter for thrust setting, in conjunction with the thrust lever angle, a demand either by the auto throttle or the pilots might have been ignored by the EEC, simply because with a PT inlet iced up and PT outlet not, the difference would fake "differential pressure", thus thrust, to the EEC. It would say "I am already giving you what you're asking for", not increasing FF and the result would be the low thrust setting persisting with a functioning and happy EEC.
Far fetched?

[CONF iture]

Ok GMDS, but this doesn't fit with AAIB Bulletin S3/2008 SPECIAL
The reduction in thrust on both engines was the result of a reduced fuel flow and all engine parameters after the thrust reduction were consistent with this. Parameters recorded on the Quick Access Recorder, Flight Data Recorder and non-volatile memory from the Electronic Engine Controller (EEC) indicate that the engine control system detected the reduced fuel flow and commanded the fuel metering valve to open fully. The fuel metering valve responded to this command and opened fully but with no appreciable change in the fuel flow to either engine.

[airfoilmod]

How would that explain balking at Manuals, indeed no apparent change after cancelling A/T? Are you saying EPR would override FF in MAN.?

[GMDS]

CONF iture:

Absolutely, that's why I backed off with my suspicion (I mentioned that my theory was very early in the debate). In my re-contribution just above I also clearly stated " ... I wonder ... ", as apparently ice was not involved. However, i still wonder ..... but backing off a second time.

airfoilmold:

in a situation as i described, it would make no difference if with A/T or in manual. In both cases the command to the EEC would be the same -> "gimme more diff press" and in both the EEC would say "you already have max".