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AAIB BA38 B777 Initial Report Update 23 January 2008

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AAIB BA38 B777 Initial Report Update 23 January 2008

Old 25th Jan 2008, 16:44
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Pedant,

No, if you look at my original post, the AAIB, and if I may say so yourself, are trying to hide behind "bad grammar".

The full stop does not protect the AAIB from it's attempt to squirm.
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Old 25th Jan 2008, 16:46
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I'm not convinced that the extent of damage to the engines is relevant. It seems clear from all reports that they were both running (i.e. burning and turning) before the incident started. The aircraft continued to move forward at 100+kts until it finally reached the ground. Even if the engines had both flamed out when the thrust loss occurred, they would still have been windmilling fast enough to wreck the fans on impact.

The damage says they were both turning. It says nothing about whether they were burning, and if not, when the flame went out.
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Old 25th Jan 2008, 16:51
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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.
I interpret this statement to mean the autothrottle and EEC output were correct, or as "expected".

This work includes a detailed analysis and examination of the complete fuel flow path from the aircraft tanks to the engine fuel nozzles.
I interpret this statement to mean that AAIB thinks the problem is a fuel flow problem of some kind.
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Old 25th Jan 2008, 17:08
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The engines did not shut down and both engines continued to produce thrust at an engine speed above flight idle, but less than the commanded thrust.

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
Originally Posted by Flight Safety
I interpret this statement to mean the autothrottle and EEC output were correct, or as "expected".
I interpret this statement to mean that the autothrottle and engine control commands were performing as expected when responding to false or erroneous information or parameters.

This work includes a detailed analysis and examination of the complete fuel flow path from the aircraft tanks to the engine fuel nozzles.
Originally Posted by Flight Safety
I interpret this statement to mean that AAIB thinks the problem is a fuel flow problem of some kind.
I interpret this statement to mean that AAIB simply wants to eliminate any doubt as to the integrity of the fuel system.

I guess we can all read between the lines while we wait for the official results of the investigation.
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Old 25th Jan 2008, 17:10
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Question

How much extra data logging takes place in the B777? I know there's the official stuff at the back in the orange-coated titanium, but do the engines have some short-term logging available as well as part of their control system? I ask because I have vague recollection of such data being available and a useful input to an investigation once where data from the FDR wasn't available. I assume it's there in part to allow the engine manufacturers to obtain performance data in normal use rather than any real expectation of its availability for post-crash analysis.

Last edited by llondel; 25th Jan 2008 at 17:10. Reason: spurious apostrophe
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Old 25th Jan 2008, 17:51
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AAIB wording

Hi Phil,

Re. your #54, I must admit that, strictly speaking, you are right: their assertion "as previously reported" does not stand up to scrutiny of the rest of the paragraph.

We now know that an autothrottle demand for more thrust at about 600 ft achieved a response from both engines, but this was short-lived. The auto throttle would continue to send a series of demands (presumably, many times per second) until both engines achieved the desired thrust setting. We are not told if that was ever achieved, and in any case the 'spool-up' was short-lived. The right engine decelerated after 3 seconds (T+3 secs)

During the following 8 seconds, as the airspeed decayed, the autothrottle would have been demanding more and more thrust. This would be reflected, on the B777, by both throttle levers moving progressively forward, and the handling pilot would have been aware of this (as he endeavoured to keep the A/C straight). At some stage, the crew augmented the autothrottle demands by pushing the throttle levers further forward. Nevertheless, at T+11 seconds, the left engine decelerated to join the left one at something above flight-idle, and the airspeed would have been decaying faster.

What we do not yet know is: what happened earlier during the descent and approach? Could the demand for more thrust at T+0 have been the first since top-of-descent? On a daytime arrival into LHR, that would be (sadly) a very rare event. So, assuming thrust above idle was needed earlier (e.g., on initial approach, or to stabilise at 160 kts till 4 miles for ATC spacing) how did the engines respond to the autothrottle demands?

If the engines had gradually been contaminated with dirty or waxed fuel, would this not have been revealed earlier? Why did the two engines suffer no apparent problems until 600 ft, and then - despite feeding from separate tanks - only 8 seconds apart?


Re. the engines' differing fan damage, as seen in the photos, there are several things we don't know:
When did the left gear collapse?
When did each pod settle firmly enough on the ground to deform its fan intake?
When did the crew close the engine master switches (to "CUTOFF")?

Chris
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Old 25th Jan 2008, 19:34
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If we're picking holes in the AAIB report, then I can't help commenting that

established on late finals
is very poorly composed. Any runway only has one final approach, as any fule who's been properly taught knows, and late is neither the same as short, nor defined anywhere, other than in its inference that the aircraft was behind schedule.

Perhaps not surprising that it was a little slap-dash, given the circumstances, but no more forgiveable for that.

Still, doubtless some mod will come along and delete this post as they did my last (pertinent and worthwhile) one about human factors. Why bother?
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Old 25th Jan 2008, 20:18
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EICAS would have brought up the ENG FUEL FILTER L + R and then possibly gone into the by-pass stage.
You would have to ask how fast an impending fuel bypass event progresses...

The impending bypass alerts are Status Messages. Cautions, Advisories and Status messages are inhibited below 800' (someone reminded me in an earlier message thread about inhibits)

If fuel flow was being restricted/slowed, is it conceivable that the engines could sustain idle power (for a considerable time), but not above-idle thrust? The 8 seconds between both engines not responding to increased thrust lever input might not be such an important factor here.

Rgds.
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Old 25th Jan 2008, 21:24
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I expect the AAIB initial reports to be purposely vague at this early stage in the investigation, as they are just beginning to sort this one out. I don't expect statements to be any more detailed or specific at this point, than they are now. Therefore it's tough to "read between the lines" of these initial statements or to speculate, because we have so little information to go on right now.

Last edited by Flight Safety; 26th Jan 2008 at 09:22.
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Old 25th Jan 2008, 23:08
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FADEC Logging

I believe that the FADECs log certain information to their (separate) internal non-volatile memories. I asked for confirmation of this in an earlier thread, but without any response.
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Old 25th Jan 2008, 23:16
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FADEC logging

There is non-volatile 'FADEC memory', mentioned re. SR111: http://bst-tsb.gc.ca/en/reports/air/...rep1_12_09.asp
and in a mechanics' forum, re. BA038, at: http://www.airmech.co.uk/forums/show...t=8005&page=13

According to Goodrich, the EECs chosen for the Trent provide 'a wide range of diagnostics data to the engine and aircraft maintenance systems...80% is for maintenance, 20% is for engine control.'
http://www.enginecontrols.goodrich.com/large/ecu.shtml
I don't know how much of this 80% would end up in non-volatile memory.

Last edited by John Marsh; 25th Jan 2008 at 23:19. Reason: Goodrich quote accuracy
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Old 25th Jan 2008, 23:58
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I can confirm that the FADECs have a fault code store - but that's the operative thing, it will only see faults. If the system is operating normally within its parameters there will be nothing to see.

As far as I know the FADEC FCS is being analysed as part of this investigation and the resuls will be included in the final report.

VnV
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Old 26th Jan 2008, 08:45
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If we're picking holes in the AAIB report, then I can't help commenting that
I really think this is going a bit far, and the AAIB are between the devil and the deep blue here...

They are making fairly simple statements about fairly basic things so far... they don't have to so any nitty picky comments seem pedantic in themselves!
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Old 26th Jan 2008, 09:22
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I totally disagree with those who suggest that the AAIB is being disingenuous in the initial or revised reports.
We are being kept informed by very early progress reports and have some way to go in the diagnosis of the cause of the accident.

Will we ever know the true cause? Well, if it was due to a cellphone jamming transmitter, then you youngsters should put a note in your diary for 2038 when the government confidential files for 2008 MAY be released.
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Old 26th Jan 2008, 09:57
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To "bug smasher"

You are evidently not qualified to decide what is satisfactory or not.
If either main fuel tank in the 777 goes below 2 tonnes the crew go into a low fuel checklist. I have done this on the 777 a few times. As long as you land with reserve fuel which is typically 3 tonnes there is no problem.
The 038 did not go into the low fuel procedure. The fuel on board was actually a little more than the average 777 lands with according to a ground engineer at LHR. The fuel quantity was therefore adequate.

I've ben reading posts on PPRUNE for years but this is the first time I have been sufficiently provoked into making a post. Many of the posts on the previous 038 thread were complete drivel and the suggestions of pilot error at such an early stage of the investigation were mudslinging and should be libellous. If you throw enough mud it can stick. As for those critical of the AAIB it is amazing arrogance for some of you to think you know better about how they should be handling it.
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Old 26th Jan 2008, 12:44
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'Adequate Fuel'

Quote from bugg smasher:
Top of descent fuel has been unsatisfactorily described as ‘adequate’, does anyone know the actual figure?
The AAIB didn't say it was 'top of descent' fuel that was adequate, they didn't qualify it at all, so one must assume they meant on board at the time of the accident, nothing else would make much sense. This should be enough to put an end to the theorising about BA's fuel policy.

I would love to know what else, if anything, happened on this flight, warnings, status messages etc, not to mention any MEL items, last maintenance and so on, but I guess I will have to try to be patient. My current view is that the crew played an absolute blinder, retracting one stage of flap on instinct may well have saved the day (or at least the boundary fence...), I hope this will still be the correct position after the final report is published.

GBNF
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Old 26th Jan 2008, 12:54
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As for those critical of the AAIB it is amazing arrogance for some of you to think you know better about how they should be handling it.
Last I knew they were a public entity and therfore subject to public comment in order to better serve the public

Like most government functions they have a very thick skin (accepted procedures). However there is always room to better communications to satisfy the public.

What better way of listening to their public for feedback than through a discussion board like this, after all they can take it or leave it.
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Old 26th Jan 2008, 14:40
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The 'Beeb' is not a reliable source of info!
For online news, most organisations have an exceptionally easy to use contact/feedback mechanism. If you use this, you can help them both correct/update existing pieces and give them the opportunity to see where they can improve the quality of their journalism.

If you don't tell them directly, they're likely never to know...
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Old 26th Jan 2008, 15:34
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The Mechanics of Waxing in Super-Cold Fuel

The logical explanation as to why a fuel "blockage" occurred (when it did - and no earlier) probably goes like this:

a. Wax globules forming in fuel during cold soak on a lengthy cruise segment would probably adhere to each other, sink to the bottom and gather (courtesy of the fuel-flow's directionality) at a tank low-point (most credibly in the collector box at a pump inlet or filter). Because of mutual adherence making such a clump sizeable (and its icy nature) the wax would not proceed any further (during cruise or descent) into the Lp (then the HP) fuel system.

b. I'm guessing that there are in-tank bypasses and gravity feed ports to cover such a situation and avoid fuel starvation due to supply-side pump inlet and fuel filter blockages.

c. During the low altitude hold that BA038 was subjected to prior to its approach to 27L, the fuel would have warmed slightly and the mutually attractive coalescence of the icily waxed fuel globules would have broken down and permitted a subdivision of that singular icy clumped wax mass into smaller (and more fluid) waxy globules (but most still with a frozen core). In envisaging this, think about what happens to those gimmicky fluid oil globules as an oil-filled electric lamp heats up and then cools off (i.e. the size of the oil clumps rising and then falling varies with the duration the lamp's been operating).

d. Those smaller detached icy wax globules would have migrated into the HP fuel system as the fuel slowly warmed.

e. Ultimately some of those globules would've been progressively ported into the FADEC's reference ports (i.e. the small diameter lines of those bootstrap labyrinths that, together with various air pressure pick-offs, establish the FADEC's reference datums). The fact that the sticky congealed masses arrived at each FADEC at slightly different times after an autothrottle call for more thrust isn't surprising (but perhaps best explained by the different numbers of oil-fuel heat exchangers in the port and starboard wing-tanks of the 777).

f. In this theory there's no suggestion of any blockage of the main HP or LP fuel supply lines. The blockages would have been in the FADEC's reference port lines (only). As the GE engine's AD at this link says: "...simultaneous Loss of thrust control events on both engines installed on the Boeing 777 series aircraft due to common mode threats, such as certain atmospheric conditions that may result in ice in the Ps3 or P3B pressure sensing system and causing corrupted signals to the FADEC in both engines."

g. It's not just probable, but obvious therefore from the FAA's concerns, that a "corrupted signal" will cause a FADEC lockup. Why "wax" and not just ice? Water in suspension in fuel tends to stay that way, however once a fuel "waxes" it takes on an icy constituency, and a component of the mass will be water (in ice form).

h. Questions that remain would seem to include whether FADEC reference port errors actually need a blockage to occur or simply the much increased viscosity of wax causing a "loss of fluidity", a constrained throughput and an illogical scenario for the FADEC's programmed logic. It's my understanding that waxy globules can pass through micron mesh fuel filters. However undeniably also, an icy mass could block both a fine-mesh filter and the alternate path of a small diameter bypass line. It's doubtful whether fuel filters are designed to cope with anything more than the particulate contamination of (say) bad fuel, displaced fuel-tank sealants or a displaced or deteriorating o-ring.

i. The fix most probably will be to warm the fuel picked off to "prime" the FADEC's innards need for data... and to correct for that in the software.

j. Whether a TOGA PRESS would have added thrust because of fuel bypass geometry? That's the multi-million dollar question. Without such a readily available assured solution, the situation remains unacceptably perilous.
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Old 26th Jan 2008, 16:16
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Re The Mechanics of Waxing in Super-Cold Fuel

Unctuous - Sounds plausible to me as a non fuel chemist, knowing nothing of these globules. Can you just enlighten me further:

1. Would this imply something wrong, or out of spec, with the uplifted fuel?
2. If not, how common is this, or how close have we been before?

While the fuel at TOD will be cold (not necessarily the coldest in flight, depends where these very low temperatures were observed), the reduced thermal capacity of the remaining fuel means it does warm significantly by the start of approach (not only in the case of low level holding, though that will increase the time spent at less cold levels, and hence the amount the fuel warms). Previous posts, not yours, seem to have confused fuel well below freezing point of water (which is normal and common) with fuel so cold it starts to wax etc. In my time in BA, we were all aware of the limitations on 'freeze point' and observed them.

I am not aware that asking for GA power in any way alters the fuel system, by bringing in some bypass, other than just demanding a lot of it (your point j).
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