Ground and/or air test? Next bulletin already written?
 

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.

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.

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

tanimbar,

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

(my emphasis)

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

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

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.


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

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.

Saturn V:

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:

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?

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.

Fuel
 

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

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...

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


.

"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)

Shawk
 

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.

Generally speaking
 

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

gladrag
 

I think probably the A/C logs are busy.

PG:

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

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.

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

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

Mechanical Deformation
 

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.

S3 in the text:
NoD

Wierd and wonderful
 

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!

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.

Flight Safety
 

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.

(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:


Bernd

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.

My own pet theroy
 

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/showthr...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

Tex37
 

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

Post 1120
 

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........

Energy
 

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

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

Bsieker, a few counter-remarks.

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.

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.

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.

Can someone give me the TRUE FACTS please?

I'm getting bored now :cool: :8

Some water in the fuel?
 

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.

Ice in the mix?
 

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

And on the day there was a

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...

"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....

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

Crazy.

Get a grip please.

U. S. Government AD Notice Search Location
 

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...&search=Search

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... :\

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

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, #461, #467 and #478 for details.

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

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

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

Unusual sound - cavitation?
 

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:
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

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".