Fullwings
Quote:
I cannot see any reference to a 15 sec T/D or ELMS auto shut off.

I've had a further browse through the manuals and this appears to have come about through an update in 2006 (ELMS software?), so may not have found its way into all the documentation yet? Service bulletin was 777-28A0040.

Thats why I don't know about it. It is not listed as an SB in the BA B777 AMM.

I just wanted to repost my comment that i made yesterday. It seemed to have been buried under a bunch of posts on EMI and computers that are not even installed on B777s. I just wanted to add a couple of comments regarding the fuel system.
I am not sure if it has been mentioned before, but in addition to the 2 fuel pumps in each main tank, there is a pump bypass valve in the left and right tanks that allow the engine driven fuel pump to suction fuel from the tanks in the event the boost pumps are not working. I believe that this situation would be highlighted by a level B ENG FUEL PRESSURE warning which includes an audio warning.
Additionally, if the boost pumps were not providing flow, they would also give a similar LOW PRESSURE warning. And if all that ice was heading down to the engine, I am sure that the FUEL FILTER clogging warning would have come on.
Even if the crew had no time to deal these warnings, these should have been recorded and the data should have been correlated with system faults in the central maintenance computers. I am sure that these computers have been interrogated already and we would have heard about these problems.
I have not heard of anything along these lines and I was wondering if any B777 guys had any comments.

Green-dot.
"PEDs also have specs but what happens to those specs if the user has dropped his PED once or twice on occasion, perhaps damaging it but is still functioning? "

In my experience PED are normally built to a price, not a spec; as such. A/C, are generally still built to a spec and that includes all the electronics. If that spec is adequate, is a different question. However, the B777 is a very safe A/C.

In a different field, I once had to investigate a problem of a little old lady with voices coming from her kettle! Yes, its was true, voices were coming from her kettle. Quickly traced to a unusual combination of resistance/capacitance/inductance in the kettle was de-coding "our" radio transmissions. We provided a new kettle, end of problem. Just to illustrate PED are built to a price, any spec (unless it adds "bells/whistles") is secondary.


Barrymung.
I concur!
Previous AAIB investigations seem to indicate 3 or more items are required to fail before an accident results. Perhaps, only perhaps, EMI was 1 of the 3?

Correct me where I'm wrong:

During the cruise, the CT pumps are switched of when reaching 900 kgs. Untill the wing tanks are 50 % empty, these 900 kgs remains in the CT, cooling down much faster than in the wings. Some of this fuel might freeze, and not be transfered by the scavenge pumps. During the descent, the remaining untransfered frozen fuel melts in a kind of slush. If for some reason the CT right pump was on, then this mix would have been transfered to both engines (via the X-feed that was supposed to be open according to the previous report and statements of the crew). This would explain the similar reaction of both engines, provided with slush coming from the same location. The 7s delay might be explained by the longer distance to reach the left engine than the right from the CT right pump.
Question, how can this CT right pump be operating during the approach ?

Fuel distribution between wing and centre tanks at t/o
 

The AAIB report states:

The aircraft was serviceable on departure from Beijing
and there were no relevant reported defects. It departed
with 79,000 kg of Jet A-1 fuel on board, and the planned
arrival fuel at London (Heathrow) was 6,900 kg.

My understanding is that the centre tank can hold 80,000 kg of fuel, and each wing tank can hold 29,000 kg.

Doesn't this suggest the aircraft could have completed this flight using just fuel from the centre tank!

Does anyone know how the fuel was distributed between these tanks?

Full wings first and the rest in the belly!

I can answer that at least partially.

The FACs, SECs and ELACs in the A320 do not in all cases control exactly the same control surfaces, and they never control the same surfaces at the same time through the same hydraulic system. There are three different strategies for dealing with failures:

1/ One computer only controls a fixed set of surfaces, with no overlaps:

- SEC 1 controls spoiler surfaces 3 and 4, SEC 2 control spoiler surface 5 and SEC 3 controls spoiler surfaces 1 and 2, on each side.

If one SEC fails, the respective spoiler surfaces can no longer be controlled and are retracted.

2/ One computer controls the surfaces, and there is a reversion priority if that fails. Different hydraulic systems are also used:

- ELAC 1 normally controls the ailerons, if it fails, ELAC 2 controls the ailerons. If both fail, the ailerons revert to damping mode.

- The reversion priority for the elevators is: ELAC 2, ELAC 1, SEC 2, SEC 1.

- for the rudder it is: FAC 1 -> FAC 2.

3/ multiple computers control the same surfaces, but through different hydraulic systems:

- SFCC 1 controls the slats through Blue hydraulic and flaps through Green hydraulic system, SFCC 2 controls slats through Green and flaps through Yellow hydraulic system.

(Source: FCOM 1.27.10, P 5, SEQ 100, REV 24)

So, for the infamous Lufthansa flight with wrongly wired sidestick, only ELAC 1 will control the ailerons with the wrong sese, and although the spoiler-roll-function will work in the correct sense, at low speeds, the ailerons are more effective than the spoilers, and roll-effect will be reversed.


Bernd

Given the mundane nature of the flight and reliability of 777s, the set of circumstances leading to BA038 accident must be very rare.

My favourite, most unlikely alignment (of the Swiss cheeses) would be:

1. Outbound, centre tank fuel used first and remaining centre tank fuel later scavenged to main tanks.
2. Condensation froze on large inner surfaces of centre tank outbound.
3. Freezing surface temperature in Beijing so ice remained as ice on centre tank surfaces.
4. Refuelling melted some ice triggering water alert (138+ gallons) when taxiing but fuel cold and centre tank only 25% full so most ice remained frozen on centre tank surfaces.
5. Inbound, centre tank fuel used first and remaining centre tank fuel later scavenged to main tanks.
6. More condensation froze on large inner surfaces of centre tank inbound.
7. Ice on inner surfaces of centre tank only started to melt late on descent inbound.
8. Newly melted, very cold water scavenged (perhaps after change of attitude from flaps moved water between baffles closer to scavenge suction point) from centre tank to each of main tanks.
9. Main tank fuel well above Jet A1 freezing temp, but well below OAT and water freezing temp.
10. On entry to cold Jet A1, concentration of scavenged water froze forming ice particles suspended in or falling to bottom of each of main tanks.
11. Increased demand on finals sucked (and/or change of attitude from flaps moved) ice particles to inlets causing partial blockage to fuel line inlets from main tanks, exacerbated on right hand main tank by foreign object (scraper).
12. Fuel lines, heat exchangers, bypass, pumps, metering unit and engines continued to work but with a lot less fuel flow than desired.
13. Ice remained trapped in main tanks due to size of particles.
14. No water found in main tank sumps as did not settle there.
15. Only minute % of water found in each main tank as over 5+ tons of fuel in each.

First post – guess I will be on probation for a long time now.

Quoting 3db:

"In my experience PED are normally built to a price, not a spec; as such. A/C, are generally still built to a spec and that includes all the electronics. If that spec is adequate, is a different question. However, the B777 is a very safe A/C."

I agree, the B777 is a very safe A/C. If PEDs are only built to a price and not to a spec, does that mean there is no quality control? To my knowledge, a spec defines what a device is made of and what it should comply with (e.i. items or details included in a description of requirements or plans of a design).

3db:
"In a different field, I once had to investigate a problem of a little old lady with voices coming from her kettle! Yes, its was true, voices were coming from her kettle. Quickly traced to a unusual combination of resistance/capacitance/inductance in the kettle was de-coding "our" radio transmissions. We provided a new kettle, end of problem. Just to illustrate PED are built to a price, any spec (unless it adds "bells/whistles") is secondary."

Same as above, no quality control? Surely there must be procedures in place defining the allowable limits regarding transmitted radiation of PEDs before they are put on the market (and permitted to be used on a plane).

3db:
"Previous AAIB investigations seem to indicate 3 or more items are required to fail before an accident results. Perhaps, only perhaps, EMI was 1 of the 3?"

I agree, perhaps EMI could have been a contributing factor. I am also convinced that the B777 is a very reliable aircraft design.

Therefore, EMI seems highly unlikely. That said, so is a dual engine rollback on a design such as the B777.

The reason i address the EMI issue is that i have had first hand experience with unscheduled closing of fuel shutoff valves in the past due to EMI. This was on another type of aircraft (not any of the Boeing types). I dug up my fuel system manuals for that particular A/C type and checked the wiring diagram which includes the original valve (before EMI became an issue) and the improved valve with an EMI filter installed (lifting restrictions after the original valves were replaced with the improved valves).

I am aware that on the B777 the engine fuel control switching interfaces with ELMS, ARINC 629, AIMS, OPAS and fire switches which are all well shielded systems. The aircraft i refer to (also a FBW design) has none of the above mentioned interfaces. But when comparing the basic electrical control circuit from the fuel control switches to the spar valve actuators, they are functually similar.

Both are fed by 28V dc and with fuel control switch position in "run" or "cutoff", depending on switch position, either the open or close coil in the control relay is energized which controls the valve actuator. Power on the open or close coil moves the actuator motor to the selected position until limit switches are reached which remove power from the actuator.

There is one physical difference between the valves i refer to and the B777 spar valves. The valve i refer to has the control relay (which was sensitive to EMI, hence the filter) integrated in the shutoff valve. In the B777 the control relay and spar valve are separate components. The spar valve control relays (together with the APU fuel shutoff valve relay) are grouped together on a single panel. However, i do not see any EMI filters on the B777 spar valve schematics. Perhaps not required due to other measures taken, making the system imune for such effects, i don't know. Location of other equipment in the vicinity of the relays or valves (or absence of it) is also a factor determining system sensitivity, which obviously varies between aircraft types.

Another point that may be of interest is that (theoretically assuming EMI was a factor) if the spar valve control relays were switched unscheduled from "run" to "cutoff" for a certain period of time, the APU fuel shutoff valve control relay could have switched from "closed" to "open" during the same unscheduled event due to its close proximity to the former control relays. This may perhaps explain why the APU inlet door was observed in the open position?

Since EMI has occurred sporadically in the past as i have explained, why couldn't it happen again on other aircraft designs?

Summarizing, i have factual information that action has been taken to prevent EMI on fuel shutoff valve control relays in the past on another aircraft type. But regarding this B777 incident, evidence (of a source) pointing in that direction would have to be found in order to determine it as a possible cause for a dual engine rollback.

Regards,
Green-dot

Many thanks to FullWings and Swedish Steve...

Again, this shows us that we are only as good as the (blurry) manuals available to us ;)

Cheers.
NSEU

(Edit) P.S. Does anyone find it kind of worrying that fuel pumps can turn themselves off? :uhoh: Won't the crew get an EICAS message anyway if the pumps go low pressure?... and I'm sure the crew alertness monitor will wake them if they are asleep :}

FDR question
 

Coming at this from an IT perspective, can anyone tell me what the FDR actually records?

Specifically, if it records something that was commanded to happen, would it necessarily know that the 'something' didn't actually happen, if there was no failure signal?

I'm thinking that the fuel valves apparently tried to open to allow maximum fuel throughput. Would it be possible for the FDR to record the open-valve event, but have no knowledge of the valve's failure to do so?

I realise that it's very unlikely that this would happen on both engines at the same time, but then it looks like the cause of this accident was something very unlikely.

RoselandNo. Because we have heard in this thread that standard operating procedures on the 777 (for all operators) is use the centre tank FIRST and then the wing tanks.

In these threads we have heard more than once that the centre tank would have been empty by about half way through the sector.

Please correct this statement, if you know better.

It is standard procedure for all the applicable Boeings which I have flown. Wing tanks are filled then the centre tank. Fuel is then used first from the centre tank and then the wing tanks. I believe it is a structural reason for doing so.

Correct. I think one of the reasons is wing bending relief. If you have u/s centre tank fuel pumps and still have fuel in there, on most types it is a) limited to a certain amount and b) has to included as payload, not fuel (and not for the obvious reason!)

Most of the important valves/actuators, etc. have some sort of feedback as to the actual position or on/off open/shut, so that the operating systems can flag up a problem. Many, many maintenance messages that are generated read "XXX not in commanded position"; they often clear but are a sign of possible future problems. If there are no sensors to show success or failure, then this might be able to be logically inferred from other parameters; don't know how much of this goes on as I didn't design it...

Given the importance of this particular variable, I would wager a serious sum that this information would have been recorded on the FDR, QAR & non-volatile memory in the EECs, to name but a few.

Glad to be of service :)

As I remember the first AAIB report was said to have stated that the Captain after his initial debrief "Thought he had selected crossfeeds open" after the lack of engine response.
This would imply that Xfeeds OPEN would not be initiators of the problem.But also as I remember it was added that one Xfeed was open, but the other was not,(fully open) This led me to wonder out loud if the Xfeed had for some reason been like that the whole time

I mentioned a similar scenario a few days ago, but was shot down in flames....

As mentioned previously:
The scavenge plumbing tubing isn't large. It takes a long time for fuel to be scavenged. I don't think you're going to get a lot of slush through those pipes. Water, maybe, but someone said that even *water* in that quantity is still not enough to affect the engines that much.

To stop (wing) fuel reaching an engine, you'd have to block off both wing boost pump inlets AND the suction feed inlet.

Additional info: Note that the CT fuel scavenge system dumps liquid (fuel or water) well away from wing tank pump inlets (unlike the water scavenge system). If it's dumping slush.. .it's dumping it well away from the pump inlets.

Regards.
NSEU.

Re the discussion of FDR parameters.

Without knowledge of the specific parameters recorded for this FDR, it's very dangerous to assume that "parameter X is important therefore would be recorded".

The FDR parameter list which is REQUIRED for certification, per the FARs and the equivalent other regulations is very much a minimum list. You may be surprised at just what can be missing. For example .... glob99 posted this list. Check item 42, "Throttle/Power Lever Position". There is a Note to that requirement, which states that it only applies to airplanes with "non-mechanically linked cockpit engine controls". Which means that on any non-FADEC aircraft, where the engine is controlled hydro-mechanically, there is no recording of one of the main control inputs by the crew. Logically, throttle is quite important; you'd think an accident investigation would like to know it. But it's not required on some aircraft at all.

Bluntly, if it's not on the regulatory list, it's safer to assume it ISNT there, unless you have the actual spec to hand.

NSEU
 

Even if slush is only transferred into each main tank at rate of 200 kg per hour, then in 10 minutes that is 33 kg in each main tank.

If all, or even only part, of it freezes on contact with fuel that is a lot of potentially clogging debris in each main tank relative to the number and size of inlets. In context, the AAIB have highlighted for consideration four or so foreign objects in whole fuel system which are minute in comparison. I wholly agree that if it does NOT freeze in each main tank and remains as water then it becomes an irrelevance.

I have no idea which main tank inlet would be affected first in this most unlikely scenario which as you say requires the blockage of all inlets at least partially. I expect that eddies particularly from the operation of the the water scavenge pumps might push debris around unless/until it became trapped. Also I expect that the suction feed inlet might be blocked earlier without any noticeable effect - particularly on the right hand side given the foreign object.

It was an interesting decision by the crew to leave the autopilot engaged as the speed decayed.
Perhaps there was a good reason for this. Perhaps they were understandably so preoccupied with trying to work out why there was no response from the thrust levers, that the speed decay went unnoticed.
However, the reason why the autopilot allowed the speed to decay was not because it was attempting to fly at the best speed for the situation presented to the crew. It was because it was attempting to maintain a glideslope that it was commanded to follow. Unfortunately, a decaying speed from about 140 knots at 750 feet to 108 knots at 200 feet, resulted in a severely degraded flight path angle.
If the autopilot/authorottle is not performing what it is commanded to do, (in this case maintaining the commanded speed), then it is best to disconnect, and correct the situation manually.
In this instance, as the speed started to decay, an autopilot disconnect followed by flying at a speed of between VREF and VREF minus 10 would, quite probably, have resulted in a different outcome, and a more controlled landing.

dxzh post #384

Not a bad scenario with commendable contemplation of many inter-relating aspects to line up the holes in your Swiss cheese BUT

Approximately how much water condensation do you contemplate will occur during the venting of an emptying CTank at heights where the vented air has very low specific humidity and which empty volume will be offset to a large extent by almost fully saturated fuel vapour. There will be extra venting with air during descents but my guess is that the most water you might get from the vented volume would be much less than enough to fill a tea cup.

Now there is a challenge to our meteorological boffins. Is my reasonably well educated guess better than yours?

And while you are about it how close might it be to a vacuum when jet fuel at sub zero temperatures C starts boiling? This is significant to the onset of cavitation at the HPressure fuel pump inlets.

overthewing asks
Modern FDRs record a few hundred parameters. However, as Mad (Flt) Scientist points out, the parameters *required* to be recorded are comparatively few. In other words, you need to know the specific FDR installation to know what's on it.

One interpretation of the question: if the FDR records a command signal, does it necessarily record the reaction, if there is no equivalent sensor or other similar feedback?

The answer: obviously it doesn't.

Next interpretation: Are there command signals recorded, such that the reaction of the kit to that signal is not recorded?

Answer: Undoubtedly so.

Rephrase: is it possible for the FDR to record an open-valve command signal without recording the sensed position of the valve?

Answer: sure it's *possible*, but whether anything like that happened is something that I would think the AAIB knows already.

As FullWings notes, actual fuel flow is so fundamental a flight parameter that one assumes that both command and reaction are recorded. However, one does needs to know how something is sensed and what the failure modes of sensing are in order to judge whether record is a reliable indication of reality.

PBL

Fuel System Design Question.
 

Re: dxzh post #384

IMHO I believe that it would be extremely unlikely that both engines could have rolled back within 8 secs of each other resulting from an independent (ice or whatever) blockage in each wing tank pick up.
However I believe that you may not be far off target.

Before posting an alternative I would appreciate an answer to the following question.

How does fuel get from the CWT to the engines?

a. Via the wing tanks (i.e.pumped from CWT into the wing tanks then out again using the wing tank pumps) OR
b. direct from the CWT to a rail / manifold etc common with the output from the wing tanks, with the higher boost pressure from the CWT pump giving it priority to supply the engines?

Anybody got a pic?

How many boost pumps (not water scavenge) are there in the CWT. 1 or 2 (1 LH and 1 RH)?

JFI is this a 777 specific or standard approach to engine fuel supply management?

Just when - if ever - did the crew plan to disengage the Autopilot?
 

Post #395 from Worn Out Rubber is very interesting -as is Topslides's post 101.

Presumably the AP just disconnected itself at some point - maybe at an altitude of around 40 feet - when it felt it could render no further assistance?

The AAIB reports states clearly that there was no fuel contamination or water present in the fuel from the main tanks. This however does not exclude waxing, since it was likely to be gone at the time the examination started at a ground temp of + 10°C.

The cold soak this a/c went through was extreme and prolonged. Pilots on the earlier thread said they descended to lower FL. On the BA38 flight, TAT dropped to -45° and fuel temp -35°. But that is, fuel temp as senses by he fuel temp probe in one of the main tanks. It seems possible that somewhere down the fuel path, the actual fuel temp might have dropped further, as there it would not be protected by the inertia of the large volumes in the tanks. Pressure drops caused by suction at pump inlets would have made the temp drop even further. (Temp directly proportionate to pressure)

All of this could have caused local waxing and restriction, until an equilibrium was created during the cruise between fuel flow and temp drop somewhere along the path. at TOD, the thrust decreased and so did the flow, thereby increasing the restriction until a new equilibrium was created.

When more thrust was demanded, this was at first supplied to a certain extent thanks to the fuel volume in the lines between the restriction and the throttle valves, this creating a vacuum in the rest of the line, with possibly a very turbulent flow or even a shock wave and cavitation as a consequence. As the restriction prevented a higher flow than the one created by the last thrust reduction, thrust went back to that equilibrium, somewhat above flight idle. The time differential between engine 1 and 2 returning to idle could be explained by the difference in distance between the restriction and the valves.

Once on the ground, temp there would rapidly make the fuel in the restriction return to its non waxed state, leaving no trace? or would it leave a trace in the composition of the fuel??

It seems to me that serious thought should be given to prolonged 777 operations in very cold temperatures and the effects it has on the fuel temp along its path.

Was the autopilot an automatic disconnect, or did the pilot disconnect?

Think about how long there was between the onset of the problem and touchdown; then take away the time for overcoming the initial shock and coming to some understanding of what has happened... Not much left, really.

We'll have to wait for a more detailed report to come out but, IMHO, having the AP engaged down to 200' or so (whether by decision or omission) may prove to have been beneficial.

That's an interesting assertion. I believe most people think about gliding performance with the aeroplane in a 'clean' configuration, where max. L/D and the stall speed are usually quite widely separated. This is not the case when using drag flap (30 in this scenario) combined with extended gear; you will find the best speed to fly is much closer to Vs than might be apparent on first inspection.

Vref is calculated using a fixed multiplier and checked against limits (Vmca, etc.) - it doesn't necessarily have anything to do with glide performance. Without access to the exact residual power settings and a simulator/modelling program to try it out on, I don't think it's possible to make a call on this one. Could have been better, could have been worse...

I've flown aircraft with best glide ratios ranging from about 6:1 to 60+:1. The common factor has been that the *usable* speed range has shrunk with reducing performance, to the point that an attempt to increase speed results in a higher rate of descent and not much else.

From the AAIB report:

I take it to mean that it disconnected itself, as opposed to "was disconnected", although I don't like to question the semantics of interim reports too closely as you can end up going round in circles over the exact meaning of a single word. Could easily have been taken out by the pilot.

Remember the 777 is a FBW aircraft and so the distinction between 'automatic' and 'manual' flight is subtly blurred. With the autopilot out, the envelope protection would have still been active (airspeed, angle of bank, etc.) and the automatics (even though nominally disconnected) would still be capable of making control inputs. The handling pilot would have been able to override these, albeit using higher-than-normal forces.

I seem to remember that the autopilot will continue to remain engaged with a reduction in speed (whether in say Alt Hold or in this case locked onto the glidepath) and continuing to trim the stabiliser for this new speed until the stall is approached when it stops trimming. No-one explained on the course whether the autopilot cuts out as a function of speed or due to the "out of trim situation".

Flying manually is exactly the same, the stabiliser can be trimmed until approaching the stall when further trimming is inhibited. Thereafter the fly by wire restricts (but can be overridden with excessive force) further rearward movement of the control column.

With regard to the fuel system and the possibility of any ice in the centre tank melting during the descent (and being scavenged to the wing tanks) my question to the fuel/liquid property experts is....

How would a small amount of water (now just above zero centigrade) react with being introduced into a wing tank containing about 6000kgs of fuel still well below zero. Could/would it form small ice pellets.

Fullwings

See Post #115 from Misd-agin:

cavitation
 

While not saying it is the cause and with all due respect "cavitation" IS fuel disruption to pumps, at least introduction of vapour or air. The report stated suction damage to pumps. http://en.wikipedia.org/wiki/Cavitation

So perhaps get off pilots' backs about flap settings and other procedures and think about the engineering/fluid dynamics issues that evidence of cavitation on pumps in the report suggests.

Cheers
Mike


"I think the previous poster who suggested fuel starvation is reading something into the report that isn't there. There's nothing I can see in this report that gives an indication of the cause"

Snanceki..
How does fuel get from the CWT to the engines?

a. Via the wing tanks (i.e.pumped from CWT into the wing tanks then out again using the wing tank pumps) OR
b. direct from the CWT to a rail / manifold etc common with the output from the wing tanks, with the higher boost pressure from the CWT pump giving it priority to supply the engines?

Anybody got a pic?

How many boost pumps (not water scavenge) are there in the CWT. 1 or 2 (1 LH and 1 RH)?

JFI is this a 777 specific or standard approach to engine fuel supply management?

The engine fuel feed line is split into two. Left and Right. These are connected by Crossfeed valves, two in parallel, normally closed.
There are two CWT Boost pumps. They feed into this line,one each side of the crossfeed valves. These pumps have a higher supply pressure than the wing tank boost pumps and so are the preferred supplier of engine fuel when they are running.

This system of fuel supply is normal in twin engined jet aircraft. The B737 B757 and B767 are basically identical. The A320 in nearly the same.

WR,

He might not have wanted to recommend anything lower to line pilots. I'm sure the laws of physics change depending on who's sat in the seat. ;)

I don't know what the figures are - I would be interested to know. What I am saying is that with all the LE & TE flap deployed (plus flaperon deflection), the wing profile becomes much more like the high-lift sections of old, where best performance occurs much closer to Vs. Given the amount of (deliberate) drag from the flap and the unavoidable bits from the gear, you may find that max. L/D occurs at a speed slower than 1.3/1.4Vs (or not... This is just a suggestion based on my experience with other aircraft.)

Also there was some thrust being produced by the engines: "above flight idle but below the commanded thrust" which complicates the situation somewhat. If you can get rid of a bit of drag by flying more slowly, that residual thrust will have more of an effect on the outcome. Food for thought...

Not a pilot - just an interested pax,

Could cavitation in pumps be caused by another source, say an ultrasonic presence or vaporised fuel due to abnormally fast pump speed rotation? Would this initially encourage spool up before starvation (witness on the ground did comment on excessive engine noise)

Go easy on me - very limited knowledge

Rgds

Conservative FBW Architecture: If it ain't broke, don't fix it
 

Quote from ve3id [Feb24/03:30]:
Chris Scott said:
"PS: Isn't it remarkable that, 20 years on, the SECs may still be employing an Intel 80186 chip, now ancient history in the home-PC world? We were all buying PCs with 80286/80386 chips, even as the A320s were first going into service."
I couldn't let this go! NO, It is not remarkable, it is just good engineering. If a chip performs the designed task within spec it should stay in the design forever. The engineering world is not motivated by specsmanship and marketing like the consumer electronics world is. If you put a more modern chip in there, what benefit is it going to give you? Absolutely none, but what about the risk of mask errors introducing bugs that the original programmers did not test for because the new chip has circuits that were not even known back then? Very probable.
If you re-design with a new chip, you have to re-test all system components, and that costs a lot of money.

I'm just an ex-driver, so thank you for confirming my sentiment from an engineering perspective. "Remarkable" was not written in the pejorative sense of the word; more in admiration.

In the A320, the mother of civil digital FBW, the designers took a deliberate decision to go for well-proven COMMERCIAL chips, which were already being used in large numbers in a wide range of applications. For the SECs (spoiler-elevator computers), they chose the 80186. This, I am now told, was a close derivative of the 8086, which had brought PCs to offices and homes across the world, and whose bugs were well understood. Airbus and Sextant Avionique had the option of developing what they called "mathematically correct" micro-processors. Without wishing to sound complacent, their decision not to do so has so far stood the test of 20 years' service and (presumably) well over a million flight cycles.

If the missions of the SECs and ELACs (elevator-aileron computers) are not much more demanding in 2008 than in 1988, and replacement chips are still available, why indeed go for a more complex chip? As you put it, ...... It's not a matter of keeping up with the Joneses!

Despite all their differences, I expect the "Joneses" themselves would be in broad agreement.

Fuel System Design Question.
 

Thanks Sweedish Steve.
So the answer is b). In one sense I hoped as much.

I don't have time to explain my full proposed theory at the moment but I will ask another question if I may.

What prevents the CWT pumps from restarting due to aircraft attitude change once they have initially shut down due to the 9000kg limit being reached. Presumably the answer is nothing.
In which case a "slug" of "bad" fuel could be sent down the lines impacting both engines at a critical time the slight time difference being due to small differences in the LH to RH installation of pickups etc.
The key difference to dxzh post is that the fuel in the CWT is common to both engines. All we need to show is that this fuel was contaminated by high water content. Since the water scavenge pumps presumably shut down with the boost pumps water would have separated out and may well have been liquid due to the heat from the ambient as the aircraft descends, the A/C packs + of course reduced volume left in the tank.
Evidence of such an event may have been removed by a further attitude change causing the CWT pumps to shut down again and purge "good" fuel through the lines from the wing tanks...but too late to save the day. I believe somebody has already posted something similar a long way back in this thread.

Evidence of water in the CWT cannot be validated due to accident damage and contamination.

I'll try to write up the full sequence later today unless somebody shoots me down at this point.

Such a mechanism brings issues like the "spurious" maintenance water warnings, the extra cold temps, low ground temps etc into play and maybe explains why only this aircraft suffered such an event.

I wonder how long an engine takes (at idle) to use the fuel present in the lines from tank to engine? 5 secs, 10 secs, 30 secs, min before starting to spool down?

I'd still love to know if other 777's have shown evidence of cavitation. I suspect they have and that this is a red herring from the point of view of the accident.

I suppose it's a question of alternatives. With the autopilot engaged, it will carry on down the LOC and GS, slowly losing speed; this is not necessarily a bad thing (as a temporary measure) and it frees up a bit of mental processing power to be used on something else. No need to try to explain what you're doing with the controls to the other pilots as they know how the AP will function in these circumstances.

If the first reaction to the problem is taking out the AP, what does the HP do? Lower the nose to point at the houses? Fly at what speed? At that point I think mental models would start diverging and the sight of an aiming point that is obviously a crash into buildings/roads would introduce unneeded fear/consternation into the scenario.

Another factor is that these guys had been woken up in another country some 15hrs before. I know from experience that I'm not the freshest flower in the bunch at the end of a long duty; all your skills/reaction times/thought processes are degraded and there is precious little you can do about it.

To contaminate both engines with center tank fuel you will need to have either one pump running and an opened X-feed, or the 2 CT pumps running.
In case the crew didn't switch off those pumps, they are supposed to switch off automatically in low press situation. Would it be possible for those pumps to restart in case liquid fuel would become available ?

Apart from the fact that they would have been physically turned off on the overhead panel about 1/3 the way into the flight. The scavenge pumps would have run later on to leave the CWT empty some time before TOD. Nice theory otherwise! ;)

snanceki
What prevents the CWT pumps from restarting due to aircraft attitude change once they have initially shut down due to the 9000kg limit being reached. Presumably the answer is nothing.

When the 900kg message is shown to the pilots, they turn off the CWT pumps by operating the switches on the overhead. They will not restart with these switched off.

I wonder how long an engine takes (at idle) to use the fuel present in the lines from tank to engine? 5 secs, 10 secs, 30 secs, min before starting to spool down?

A lot longer. When you shut an engine down using the spar valves it runs for 30-50secs on the fuel in the pylon, so from the boost pimps it will probably run for about 2 minutes.

These are optional, according to Boeing, and are currently fitted by some Far-East airlines. ;)

To assist those who are still asking about fuel in the centre tank and how/when the autopilot disconnected ... I quote from the:
AAIB Bulletin S1/2008 SPECIAL
Published February 2008
I know that everyone participating in this thread has already read it but here are two items.

At 1,000 ft the aircraft was fully configured for the landing, with the landing gear down and flap 30 selected. The total fuel on board was indicating 10,500 kg, which was distributed almost equally between the left and right main fuel tanks, with a minor imbalance of about 300 kg. The fuel crossfeed valves indicated that they were closed and they had not been operated during the flight.
[edit]

The airspeed reduced as the autopilot attempted to maintain the ILS glide slope and by 200 ft the airspeed had reduced to about 108 kt. The autopilot disconnected at approximately 175 ft ...
[edit]

The recorded data indicates that there were no anomalies in the major aircraft systems. The autopilot and the autothrottle systems behaved correctly and the engine control systems were providing the correct commands prior to, during, and after, the reduction in thrust.