Increasing drag allows a steeper approach without building excess speed.

Another way of putting it is that its got a lot to do with the drag curve; trying to get the profile drag up Vs the induced drag so that one does not sit on the 'back of the drag curve', a potentially speed-unstable place to be.
As has been said, also allows keeping a higher power setting so there is much better speed control of both engines and aircraft.

I would like to try and take a step back and explore why current sumping procedures, even when correctly applied, might be inadequate and allow significant water (whether free or entrained/dissolved) to accumulate in the 200-ER model centre tank in certain narrowly defined cold, but within certificate, conditions. By significant I mean capable of causing a restriction in a fuel line, not that it would prevent the fuel burning in the engine.

It seems important since the accumulation of significant water in the centre tank may be a contributory factor in creating the unusual restrictions in each fuel line of BA038 which triggered a double rollback.

1. Centre tank accumulation of free water

There seems to be a recognition unconnected with BA038 that significant water can build up in the centre tank and restrict fuel lines:

- the Boeing-issued 777-FTD-28-07002 (as revised 20 Feb 2008) at JetPhotos.Net Forums - The Friendly Way to Fly - View Single Post - BA 777 landing accident at LHR (and for a more summary version not mentioning 200-ER see http://www.pprune.org/3958000-post608.html) mentions that the 200-ER centre tank is reported to have accumulated water which has frozen and restricted the flow of fuel along at least the jet pump driven fuel scavenge lines.

- In the absence of any information on the water content of the fuel in the BA038 centre tank due to the damage on landing, this FTD seems to offer at least some strong circumstantial evidence of the accumulation in similar centre tanks of water and ice on long, cold flights and that this ice is capable of causing restrictions in fuel lines - this seems worth mentioning in the context of the BA038 analysis. In particular, I note from the FTD that:
• while the freezing water is causing restrictions in part due to cold temperatures in the centre tank, the freezing appears also at times to be due to the centre tank water being subjected to the even colder main tank fuel temperature.
• the centre tank is prone to the build-up of water sufficient, if not properly dispersed within the fuel, to cause enough icing in the fuel scavenge lines to restrict them in part or completely.
• free water built up is only dispersed by the operation of the water scavenge and when this is not operating the free water sinks to the bottom and is drawn into the fuel scavenge lines.
• also without mixing by the water scavenge pumps, entrained and dissolved water in the centre tank fuel is likely to separate out and end up in the OJ or fuel scavenge pick-ups.
• the pipe which takes the centre tank fuel to the outlet in the main tank on the outboard side of the wing is extended a little further outboard in the 300ER and that this extra distance in itself, due it seems to the cooling effect of the cold-soaked main tank fuel, may contribute to the accretion of ice in the fuel scavenge line sufficient to block it.
• corrective action in the centre tank on the 300-ER is being taken to move the water scavenge inlet closer to the fuel scavenge inlet to minimise ingestion of free water in the fuel scavenge line.
• 200-ER centre tanks (ie like in G-YYYM) are not covered by the FTD but ... the FTD notes that “One operator advised of this problem [of restrictions due to ice in fuel scavenge line blocking scavenge to main tanks] occurring to a lesser extent on the 777-200ER model as well.”
• the underlying cause of free water in the centre tank is not addressed, nor is the fact that the water scavenge does not operate to disperse water after the OJ pumps are turned off.
• there is a contributory factor here common to BA038 in that the FTD states that: “A review of operator QAR data to date shows that this problem primarily occurs on long distance flights from Europe, suggesting a potential operations component to the problem, but long distance flights from JFK have not been affected. Further review shows the problem to primarily occur between the months of October - April, suggesting an ambient ground temperature component to the problem.”. Is this a coincidence?

- There is no indication of the highest levels of free water which operators have found in affected 777 centre tanks (eg: 1, 10, 100 litres?) – does anyone know?

- Water will accumulate in the centre tank:
• where (a) there is free water or dissolved/entrained water in uplifted fuel; or (b) water condenses in centre tank during flight or ground turnaround, AND in each case, if it is not removed from the centre tank because it has separated out of the fuel and frozen or hidden itself in pockets inaccessible to the OJ pump or fuel scavenge or sump drain valves, then in the next sector one starts off with a base amount of water in the centre tank derived from (a) and (b).
• The next uplift of fuel may melt and/or disperse it (by entraining or dissolving it) but not necessarily or permanently.
• The process will repeat itself until the environmental conditions cease to apply or some other event intervenes as it normally would to cause the water to be consumed by the engines or drained.

- the volume of water contained in an uplift of properly specified fuel for a long sector is no more than a few litres so, if significant volumes of water are to be explained in the centre tank, it would seem condensation would need to be responsible. However, condensation from air drawn into the centre tank to replace fuel drawn out would, particularly at high altitude, have minimal water content (much less than a litre!). To explain higher levels of water from condensation there would seem to need to be a stream of outside air coming into the centre tank, preferably at low altitude. Question whether sufficient circulation might be provided by either:
• the continued operation of the two jet pump fuel scavenge lines after the fuel is scavenged in the last hours of flight at the end of each sector; and/or
• the descent at the end of each sector with an empty centre tank into humid air at ground level?

2. 200-ER centre tank - layout

- There are various baffles across centre tank and other dead spots in which free water may accumulate. The floor of the centre tank becomes more curved in flight and this may also hinder ensuring free water's dispersal in the fuel at certain times of flight. The layout of baffles in the central section of the centre tank can be seen in figures 6, 11 and 12 in the AAIB report produced in respect of the omission of G-YMME’s purge door at http://www.aaib.dft.gov.uk/cms_resou...ection%201.pdf but there are also significant ribs in the colder inner wing section of the centre tank.

- There are only two sump drain valves in centre tank – would anyone be able to point out (on picture of centre tank mentioned above) exactly where the two sump drain valves are located on lhs and rhs, relative to low points, baffles, OJ pump inlets and fuel scavenge inlets?

3. Weaknesses in detection of water

It seems difficult for operators to monitor easily whether free water or dissolved/entrained water is accumulating in the centre tank and thus they may allow a contributory factor to a double rollback to develop. Issues include:

- Sumping would only have a chance to remove free water if: the free water has had a chance to settle or (if the water has frozen) the ice had had a chance to melt and then settle and in each case such free water is not in an inaccessible blackspot as a result of the baffles or compromised location of the sump drain valves. Anecdotal evidence suggests that one or more of turnaround times, OATs and centre tank design (among other things) may effectively hinder these conditions occurring in long cold-soaked back-to-back winter sectors.

- Sumping will not work properly in cold conditions and is therefore potentially an unreliable procedure if required to ensure safe operation.

- Are Karl Fischer (or similar) tests to be routinely conducted several hours after uplift of warm fuel into cold-soaked centre tank at cold stopover as part of the routine Boeing/operator recommended sumping procedures on daily/transit basis I wonder? If not, then melted and dispersed ice in warm fuel uplift is arguably unlikely to be detected anyway and even then may require pumps to be run for some period before testing (though perhaps running pumps would not help sumping of free water)?

- If the water scavenge pumps operate successfully after a warm uplift to dissolve or entrain melted ice into the fuel in manner, then water sensors, which can only indicate if free water has melted and settled to the bottom of the tank, also seem imperfect as an early warning of significant levels of entrained/dissolved water. It is not even clear that excess free water MMs which are triggered are seen as significant in any event, or have immediate pre-flight or in-flight remedial actions/procedures associated with them? In particular:
• Excess water maintenance messages are not drawn to the attention of the pilot (unlike say an EICAS advisory or caution say about low pump pressure). The excess water maintenance message seems to self-clear even if correctly triggered.
• It is only designed to be triggered in centre tank if more than 627 litres (138 gallons) or so of free water is settled at bottom of particular part (any idea which?) of centre tank. This is insignificant amount in context of burning 80 tons of fuel but even a part of such amount might represent more than enough water, suitably frozen, to create a significant restriction in each of the fuel manifolds.
• Perhaps, if free water in the centre tank is seen to be a contributory factor of the double rollback of BA038, more prominence needs to be considered for this message?

- perhaps in winter after each couple of long very cold sectors out of LHR test all of the centre tank contents by draining the centre tank in a warm hangar and then examining and measuring whatever else was inaccessible in the centre tank by opening it up. Also perhaps to repeat process but on final sector to disable fuel scavenge completely (though this might reduce centre tank condensation (if any) too) in order to enable examination of contents of 800kg of remaining fuel for any build-up of free, dissolved or entrained water. The key seems to be to test the correct categories of flight legs and not simply to rely upon a sump test.


4. Centre tank scavenge

It may be worth considering, for more risky types of long, cold flights:

- scavenging the centre tank fuel into one main tank only in order to break the centre tank fuel being a common cause of a failure condition and to preserve the independence of the main tank fuel supplies to each engine? Downside would be an imbalance between the main tanks.

- not running the fuel scavenge pump when the centre tank is empty, by removing the motive power to the fuel scavenge after 30 or 40 minutes of operation (or no later than centre tank indicating empty)? Otherwise is there a risk that continued operation of the fuel scavenge while the centre tank is empty:
• could draw into the centre tank towards the latter stages of the flight a significant stream of (potentially humid) cold outside air from the vents (causing unnecessary condensation build-up)?
• could result in a significant volume of air being entrained into the main tank fuel (causing issues by its likely release at high altitude elsewhere in the fuel supply system)?

- requiring a water scavenge at all times while the centre tank has fuel in it, rather than just when the OJ pumps are working, in order to minimise the settlement and accumulation of free water in 800kg of fuel remaining after the OJ pumps are turned off?


5. Centre tank OJ/jettison check valves

Is there any possibility of undiluted centre tank water being injected directly into the fuel manifolds without passing through the main tanks first?

- The AAIB’s comment in a different context about the possibility of the OJ/jettison valves opening to admit air from the centre tank into each fuel manifold if pressure difference was sufficient presents an intriguing prospect if it might also allow the possibility of dregs or slugs of free water (maybe a bucket or two) being swept into the cold-soaked main tank fuel in each fuel manifold towards the end of the flight.

- Pressure differences as a plane rapidly descends are to be expected – the fuel manifold might just be at least on a localised basis be at lower pressure temporarily, perhaps as a result of having pockets of undissolved air in it (if the scavenge pump’s operation while dry could provide a mechanism temporarily to introduce air into otherwise unsaturated fuel) which might allow the centre tank pressure to overcome each OJ/jettison check valve.

- However even then would water be able to pass through each switched-off OJ pump with the same ease as air say?

- Clearly more than one contributory factor is required for undiluted centre tank water to end up in each fuel manifold at the end of the flight, but it might offer an alternative explanation as to how significant water might unusually move into each fuel manifold, near simultaneously, towards end of flight, sufficient to cause a restriction to each engine and without increasing the concentration of water in the main tanks.

- Should there be a cut-off valve (in addition to a pressure dependent check valve) which could be closed after OJ pumps turned off to ensure that this could not happen? Is there a safety reason why the operation of the OJ/jettison check valve must be allowed after OJ pumps turned off?

F25
 

NigelOnDraft:

Now that you put it so simply it would indeed explain the actions of the crew. Your inputs regarding the prevalence of F25 landings, it also makes it more difficult to understand the critics. This certainly was a rapid descent with (Very) low power.

Notwithstanding the fact that fuellers are unwilling to resell fuel that has been offloaded, at carriers' home bases, I could envisage centre tanks being emptied entirely prior to refuel as a result of this, though greater fuel line heating throughout flight may be the answer.

However, as the AAIB point out, the flight profiles were extremely rarely encountered - I would like to know how rarely they have been encountered on back to back sectors as in this case in their next report.


Regarding flap - from day 1 of flying training in single-engine piston aircraft, people are taught to get rid of drag flap upon engine failure to stretch the glide.

Anyone commenting here who questions the elimination of drag flap should, I feel, return to the basic books and relearn what they clearly have forgotten. It was neither a typical glider whose aerofoils are distinctly differently designed, nor a procedure for which they had trained - the instinctive response is to be applauded.

Drag flap permits a combination of more rapid speed decay without power, and a more suitable deck angle at final approach. Perhaps if someone has a suitable 777 graph, this pointless diversion to the thread can be put to rest?

NoD puts it best:

Thanks for the replies against F25 vs F30.

From what you guys have said looks like F30 vs F25 is more of a brake and engine control feature rather than low speed lift assist otherwise procedures wouldn't have been changed to use F25 ilo F30 if I understand the replies correctly.

Maybe its unwise to raise the issue but I warm to the explanation of "intuitive action".

Seems logical from my limited understanding perspective.

snanceki:

I think your question has been answered above. No further comment required from me.........

CONF iture, your post said it all. Luck of both sorts contributed hugely to this accident and the amazing lack of casualties associated with it.

As SLF, I may be missing something, but having a fair bit of experience with Diesel-powered vehicles, I am struck by the cold operations similarities.

It seems odd that the pumping-system, with huge overcapacity/redundancy, could be used more effectively, in conjunction with the oil-cooler.

Again, It seems strange, from my simplistic viewpoint, that oil is first air-cooled,THEN dumps surplus heat into the fuel.

Also mentioned, this overheats in cases of low fuel-flows, to the point where fuel is burnt purely to circulate and reduce temps.
It's self-evident that Airliners are designed to operate mainly in high,cold conditions....they only pass through "warmer" environments on their way to and from their main environment.

SO, why is the "waste" heat, from the oil ,not fully utilised to warm a full-flow RECIRCULATING fuel system? no doubt there are also other sources of "waste" heat,within the engine -system...but full-flow would , with appropriate by-passes / pressure spill-off etc. keep the lube oil cooled and warm the fuel.

Yes, I appreciate that warm fuel is considerably less dense and therefore contains less energy per unit-volume, but I don't see big elastic bands at the airport, launch -assisting on hot summer's day :}

just about all road-vehicles (petrol and diesel) are fuel-injected,nowadays (emissions control! ) Invariably the pumps have surplus capacity over peak demand and all surplus spills off to a return to the tank....so I'm not inventing anything, just suggesting that maybe aircraft designers should think laterally.

(maybe they have and i'm talking bollocks! :\ )

At a given configuration, the only thing that matter to achieve the longest gliding distance is the AOA. The best gliding distance is obtained at L/Dmax. Beside following the GS is useless as it leads far beyond the threshold, this leads to increase the AOA, building up drag thus highly reducing L/D and gliding distance. Energy is a factor of force by distance. The more you spoil in drag trying to follow the glide, the less distance you can fly. Once no more kinetic energy is available (stick shaker), the only one available is the potential energy, which is also based on distance... but only the vertical one. The potential energy will anyway be used till the plane reaches the ground, what matters is to improve the L/D in order to spread the use of the kinetic energy. The drag reduction between 25 to 30 is probably meaningless compared to the generated drag at a 16 degrees nose up attitude.

http://www.aerostarwind.com/images/3...Drag_Graph.jpg

dxzh thanks for the info - 2 items that stand out from your post from my point of view are :-

1 - • Excess water maintenance messages are not drawn to the attention of the pilot (unlike say an EICAS advisory or caution say about low pump pressure). The excess water maintenance message seems to self-clear even if correctly triggered.

2.-• It is only designed to be triggered in centre tank if more than 627 litres (138 gallons) or so of free water is settled at bottom of particular part (any idea which?) of centre tank. This is insignificant amount in context of burning 80 tons of fuel but even a part of such amount might represent more than enough water, suitably frozen, to create a significant restriction in each of the fuel manifolds.

Can somebody confirm to me that the two ponts above are absoloutley correct. That is to say that the excess water message will self clear and that approx 138 gallons is the amount of water that could be in the tank prior to any sensor being activated.

NigelonDraft wrote:

"suninmyeyes - excellent post, especially the second para about us not saying half of what really goes on, since we just then get attacked by all the MS FSim experts"


While I can understand your frustration with how this is interpreted, the little snippets that emerge from those in the know are fascinating in their own right and I prize them for that fascination.

There may well be people that don't understand the realities of flying and moan and carp about procedures they can't comprehend, but to those of us that know enough to not do that but can never be flight crew because our bodies let us down at the first hurdle these chances to experience that "Oh I _see_!" moment are really valued.

Please keep letting the odd one slip here and there....

We need to be a little careful when we say that the conditions for this flight were unique. The AAIB only say the flight is "unusual within the 13,000 flights analysed" and only IF you look at two factors together:

a) the fuel temperatures
and
b) the fuel flow rate during step climbs and on approach.

If it turns out that one or other of the above wern't a factor in the accident then all of a sudden the flight isn't that unusual. They found 280 flights with colder fuel temperatures for example.

The AAIB do use the word unique, but in the context of this being a unique accident.

It's what keeps bothering me too.
So far, nothing yet points to something 'uniquely unique' in what? 6.5M flights?
Yet we do not have 777s scattered just short of the threshold all along similar 'cold' routes.
So where are the other Gruyère holes that lined up?

CJ

It was a little unique in the airframe operating back to back "cold" flights. So the fuel never had a chance to warm up. Relatively speaking.

Was the water in the fuel, ever water long enough to be drained?

For the first time ever in 6.5M flights? I don't believe that.
That on the other hand sound like a more plausible question.
I find myself staring at a figure for water (138 gallons in the centre tank alone!) more than two orders of magnitude larger than the the 5 litres (40 ppm) quoted in the AAIB report, yet "acceptable" before an EICAS advisory message is triggered?

For the moment, most of our assumptions and guesswork are all wrong somewhere, or BA038 would have happened far earlier, and maybe not with the same outcome....

But since it happened on BA038, it's lurking somewhere to pounce again.... so I'm among those that hope a plausible "probable cause" will be found, and if I can contribute my minute bit to that... so much the better.

CJ

That is why I wrote "a little".

Regarding uniqueness, it is not the 6.5million flights, but how many of those 6.5 million were a 777. And of that number how many were that variant, and that configuration, with those low temperatures mixed into the cooking pot.

6.5 million is a red herring.

They will not resell fuel because they don't know what it is. It may come out of an airliner tank but it will be a mix of Jet A and Jet A1 and even Chinese fuel. So they give it back to the airline it came from.

Centre tanks are always empty before refuelling starts. Do you envisage someone removing the dregs?

I believe the B777 was the first aircraft to have a built in water detection system. The reason for the 138gallons is that the bottom of the centre tank is flat. This is why the boost pumps are switched off with 800kgs remaining, and the rest sucked out by the jet pumps,
In hindsight perhaps Boeing should have made the EICAS message a Status message which the pilots could see.

333333333333333333

FAR 121, Appendix M FDR parameters
 

FAR 121, App. M, Item #43. Additional engine parameters:
There are several parameters such as vibration level, N2, EGT, Fuel flow, N3 and:

Fuel cut-off lever position. Is this the only parameter that is recorded to determine spar valve position? FAR 121, App. M does not mention any (required) parameters for recording spar valve position other than fuel cut-off lever position.

If the premise is that spar valve control has been tested and could not be demonstrated to fail under any condition, would the tests (investigating BA38) have been a duplication of tests performed during certifiation of the control system? If so, would there have been a reason to add any FDR parameters other than the required fuel cut-off lever position to determine spar valve position if the system was tested as fool-proof during certification?

The AAIB state: "Any uncommanded movement would have been recorded on the FDR and warnings would have been enunciated on the flight deck."

FAR 121, App. M only mentions the fuel cut-off lever position as a required recorded parameter, does this mean that any additional parameter must have been added by the manufacturer or as a JAR-OPS requirement? After checking JAR-OPS 1.715 table A1, table B and table C, applicable to the 777, I could not find any such requirement.

Referring to warning enunciation on the flight deck regarding spar valve position. The warning I am aware of is presented only when there is a disagree between the spar valve control relay position and spar valve actuator position as a result of a jammed actuator. According to the AMM/SDS, OPAS monitors control relay position and valve (actuator) position. As long as the spar valve actuator follows control relay position, there is no warning (no status message on EICAS). Even if for whatever (very remote) condition the control relay would move from open to closed position uncommanded with fuel cut-off lever in the run position. As long as the valve actuator follows control relay command there will be no warning presented on the flight deck because OPAS does not sense a disagree between these two components, hence, sends no fault message to AIMS. OPAS does not monitor a disagree between fuel cut-off lever position and spar valve actuator position.

Spar valve position can be displayed to the crew, but only if the crew selects the fuel synoptic or maintenance pages. Neither are likely to be selected during an approach phase unless there is a specific reason to do so.

It would have been interesting if the FDR parameters (and which parameters) for the spar valve positions were included in figure 2 of the interim report. Perhaps they will be included in the final report.


Green-dot

Last 720 ft
 

Just tripped over this in a US flight safety article:

`Or, as legend R.A. "Bob" Hoover has been quoted, "If you’re faced with a forced landing, fly the thing as far into the crash as possible.".

Probably not that relevant to these chaps as so little time to assess and act, but I will certainly listen to any thing said by Bob Hoover, and a nice refreshing reminder from an Ancient Aviator (with apologies for the `Ancient').

:ok:

Modern DFDRs can record hundreds (if not 1000+) of parameters - way more than the minimum required set, plus they have the QAR (which may record even more) for almost all of the flight (including through the onset of the problem).

I think it is highly likely that the AAIB have spar valve position data, their statement is that the data shows valve open, not indicated open or commanded open. They also state that movement (even uncommanded) "would have been recorded", which implies to me that there are FDR parameter(s) recording actual position as well as commanded.

They don't actually state that they couldn't make it fail, they state that they couldn't reproduce "uncommanded and unrecorded" movement (my emphasis) - which is actually quite a different premise. They then infer that there was no failure because none was recorded, rather than because it was failure proof.

I don't know enough about 777 instrumentation to be sure, but I believe that on older boeing types there was(is) a spar valve light that indicates closed and transit. It would be interesting if that has been lost (or relegated to a maintenance page) going to glass cockpit. However, I also think that the AAIB must have some basis for their statement that warnings would have occurred.
I agree - but on the other hand they may regard a flat line graph as superfluous if they believe they have already stated the same in words.

Don't get me wrong, I quite liked your spar valve theory, and the AAIB clearly also thought it was a line worth investigating - but I do think it is looking a little dead now.

[I have to admit I don't like the ice conclusion, I feel I need to re-read and digest the report a bit more, but it does seem to be what's left when they've eliminated other things, rather than driven by positive evidence. Feels very unsatisfying as a conclusion (even interim).]

6.5 million is hours not flights (1.4M flights) - but it is all 777s (in fact all RR powered 777). That does cover several variants however, and fuel tank configuration is different between the variants, which may well be significant.

Of those flights, they've only looked at (only got?) data on 13,000 RR powered 777 flights, and the conditions are unusual within that set of data.

Because they don't makem with 6
 

My sentiments entirely. And, therefore, in the meantime it COULD happen again. Even if the chances are only 1 in 13,000, why take it if alternatives are available. Most of my circle of frequent flying business travellers now selects flight carefully on longhaul NE Asian/Europe flights to choose the 744. Ice or otherwise. Maybe irrational, and was ridiculed earlier, but the fact is there is a PERCEPTION amongst frequent flyers (Who DO know one end of 744 from the other end of a 772/3ER) that this airplane has unidentified problems and should be avoided until otherwise.

With all respect,
Can someone confirm that the FARs would allow this condition? From what I've quickly gleaned (Gimli Glider) the RAT should always provide enough hydraulic pressure for the primary control surfaces.

Hours not flights. I stand corrected. Thanks.

PS. No RAT on a 747-400.

snowfalcon2;

It is true that the RAT will stall out if the a/c is permitted to fall below a certified airspeed which is about 140kts if I recall - I'll have to check the manual. The RAT on early 320s was subject to interference from the nosewheel slipstream - can't remember what was done to rectify the situation - perhaps the starboard offset, not sure.

A few more thoughts related to centre tank pump inlets:

- Each OJ pump must be able to pump, while in climb and then cruise, 99% (as in (80t- 0.8t)/80t) of the fuel out of the centre tank before the inlets are free from the remaining fuel. The OJ pumps free of fuel, lose pressure and are subsequently switched off - the main tank boost pumps, which are permanently on, maintaining flow to engines in the meanwhile. In this case, the centre tank reduced to a little under 750kg indicated at its lowest at the time of the engines switch to main tank supply, before apparently increasing for some reason by 125kg or so to 800 kg indicated a few minutes later.

- Each OJ pump inlet is raised from the floor slightly and is in fwd part of the centre tank. What exact depth of liquid on top of floor (or liquid on top of ice on top of floor) immediately below the fuel pump inlet is required to submerge OJ pump inlet? How localised could that depth of water be given the geometry of the tank, position of ribs and the tank's shape during flight? Can anyone produce a diagram of exactly where everything is at the bottom of the tank near the OJ pump inlets?

- I assume that the fuel scavenge worked given drop from 800kg indicated to 0kg but ... given the knowledge that on cold flights like these that it is not uncommon for some of the fuel scavenge pipes to become blocked by ice, what would happen if the fuel scavenge lines local to the fwd positions on the left hand side and right hand side of the tank where the OJ pump inlets are were iced up, but the scavenge lines in the centre of the centre tank were working?

- I note from the rate of scavenge in Figure 1 the rate increased as the level of fuel indicated declined to 0kg. Could that suggest that as the level lowered: the rate of scavenge appeared to increase as less fuel/free water was accessible than expected as it was trapped where the scavenge lines were not working locally; and/or an expected increase given the vagaries and the geometry of tank?

Is it a red herring even to contemplate the boost pumps or centre tank pumps allowing the passage of fuel/water when they are selected OFF? I guess the premise of the suction feed test on the ground (as a pure test of the suction feed) would be flawed if it was a possibility that an engine-driven LP could draw fuel through any of the unpowered pumps from centre tank - and thus I would like to discount the idea completely. Yet as a Mad Hatter is drawn to a tea party, I am curious about the possibility given the suggestion in the interim report that air at least might possibly pass through the OJ/jettison check valve (and presumably therefore through the OJ pump when selected OFF too) ...

Kind of like a computer with a hundred USB ports for data inputs. But do you have a hundred devices to hook up to these ports?

It ain't the DFDR recording limitation that just meets or slightly exceeds the regulated requirement, it's the availability of aircraft system sensors and wire routings that sets the actual limit.

With that said I really don't know what parameters were recorded for BA038

The report
 

I certainly believe the AAIB are very trustworthy and honest people simply trying to do thier best in what seems an unfathomable case.
However there are naturally those conspiracists who will always look for a cover up, bribery, corruption in any organisation and lets face it it can happen.
It should therefore be incumbent upon the board to be as transparent and straightforward as possible in all their reports and findings to the public.I have read all the 5 reports and all the posts on this thread. The reports are comprehensive and well written but that doesnt mean we cant criticise them!
My criticsim is this. Not once before did the AAIB mention the actions of the crew and I agree with Nod that the AAIB have never shown much interest in those actions, clearly focusing all their attention on fuel interuption and its causes.
So why do they now suddenly tell us that it was the first officer who disconected the Autopilot and selected flap 30? and why is this more relevant now that they have established that ice is the cause?
They must have known those facts from the very beginning but declined to tell us about it. I am sure it is not the case that they are trying to hide something but nevertheless it gives ammunition to those who think they are!
Why cant they just lay all the facts on the table from the start without worrying about whether it is relevant or not or who it might hurt or what the consequencies might be or the costs to the industry etc etc

1. Main tank water scavenge

If centre tank fuel scavenge pipes can ice up, perhaps the main tank water scavenge ones could in certain conditions too? Maybe a scenario with cold-soaked fuel in each main tank at critical icing temperature at end of flight with a relatively high concentration of water after centre tank fuel scavenge and then, during cold stopover, a very cold water/ice mass accumulated in and around water scavenge inlets (near bottom of main tank), blocking them? As long as no more than 7 gallons of free water was in main tank at any one time, no excess water message would be recorded about this issue.

If each main tank's water scavenge lines became restricted by ice before the return flight, would that also explain how there was a very low level of suspended/entrained water to be found in fuel in main tank as it would not continue to be mixed into the fuel by the water scavenge and tend to separate out?


2. Icing

It almost seems easier to explain icing if there is some form of cascade effect where:

- first, relatively minor icing over time restricts narrower and colder scavenge pipes.
- then, unforeseen major condition such as lack of water scavenge and/or fuel scavenge develops.
- then, unlikely accumulation of free water/ice where it is not expected.
- then, accumulation goes on to cause bigger and worse things downstream.

And has to be readily duplicated on either side. And lack of warning messages and tell-tales such as water ppm accounted for.

Sorry Lads. Trawling through all these posts and your honest attempts to vindicate the AAIB's preliminary findings, seriously, what are the odds of ice causing both engines to fail at the same time? One I could understand, but not two.

If there was low fuel pressure to the engines caused by a build up of ice, why was this not noticed? Why didn't the PEK/LHR 777 that day also exposed to low temps did not reciprocate the problem. A mate of mine flying a BA 747 at the same time from HKG said fuel temps were in the amber for some time, but no engines stopped.

The fact that this debate continues proves the ice theory is not convincing.

It is something else.

You are seriously trolling.

That is the whole point of the investigation

As high fuel flow was not demanded or required until final approach.

They don't know, but are probably considering the consequences of two back-to-back sectors in some of the coldest temps recorded for long durations on 777 missions to date.

Different plane, different route, different design. What is your point?

Well Re-Heat, many years ago flying back from ANC in very cold weather (-70f+) with US avgas, the main concern then was the fuel waxing, not any water freezing in it.

You would think a newer design aircraft would have better protection against deep cold for long periods. This is not a modern phenomenon.

To lose both engines at such a critical time is unbelievable. I landed 30 minutes before the BA038 but on a GE90 777 from the Gulf and saw it skid to a halt, as we were parked on a bussing stand at the Hatton Cross end of T4 on the opposite side of the runway to where YMMM came to rest.

On our approach quite high amounts of power were required as it was a gusty wind and we were shaken around a bit in our shoulder straps.

New Ad Note For 777 Cold Weather Ops
 

See following link:

This is a final rule request for comments.

Justia Regulation Tracker Airworthiness Directives: Boeing Model 777-200 and -300 Series Airplanes Equipped with Rolls-Royce Model RB211-TRENT 800 Series Engines, - Federal Aviation Administration - 52909?52911 [E8?21138]

I know, and I don't disagree, but the more modern types have moved away from point-to-point wiring to bus-based systems. That allows you to simplify the scarily complex wiring looms (or to do more with the same level of scarily complex). The 777 is ARINC 629 bus, which is multi-transmitter (and more advanced than anything other big jet until you get to 664 / AFDX on the A380 / 787), which should provide a lot of help with this issue.

Best public reference I've got for parameters recorded on a recent jet is for N651AA (flight 965, crashed in columbia). Quoted in various places as recording approx 300 parameters (see eg. Hall's Testimony on March 7, 1996). That is some way over the minimum (which is still 88 I think).

That flight was a 757 with previous generation (ARNIC 429) bus - so I would expect the wiring on a 777 to be able to route at least that many parameters to the FDR & QAR.

Nor do I, and I doubt we'll be told.

The signal is slightly noisy - there are other blips - and that could account for this. Also, consider that when the OJ pumps switch off, does fuel already in the pipes / pump but upstream of the check valve fall back into the tank ? That might be enough to account for this signal.


I think there have been drawings posted way back in the various threads on this incident - but you'll have to trawl through a lot of posts to find them.

Could be, but I suspect it is more likely a result of the way it's controlled from float valve in main tank - I think the final scavenge rate works out higher than the fuel burn rate when scavenge started.

I think that was only if all the main tank inlets were blocked and suction feed woudl pull in either nothing or air from the CT. Nothing vs. air is not a great choice of fuels :uhoh:. Also, if this happens they've found different effects to those recorded on this flight, so it didn't happen in this case.

Overall, I would have thought that the important thing to do with the fuel system FDR data (ie. fig 1 in the report) would be to compare with a similar but incident-free flight. We don't have the data to do that but the AAIB will - I would be very very suprised if this hadn't been done already and anthing unusual in the fuel system data would have been looked into.

Whenever I read these posts I get the impression that contributors are talking about free ice floating around in the fuel.
I see this ice, whether it be from water in the fuel or the result of condensation being a sudden release, either due to temperature, flexing of the wing or vibration, of ice which has frozen and adhered to internal wing surfaces during the flight.
i.e. all the scavenge systems, fuel mixers and heaters could never touch this ice as it was attached firmly to a surface and released itself in quantity at roughly the same time, thereby clogging momentarily, the filters to the pumps.

Thanks for the link, to quote the AD:

Hmmm. Is it me, or does that imply a far greater degree of certainty as to the mechanism than the latest AAIB report did ? I read the AAIB report as saying that this was one icing scenario (of two), that they couldn't reproduce it yet, but it was likely to have happened as it is the only hypothesis they haven't disproved. :confused:

AD
 

1. AD

AD pretty much as predicted last week.

As a stop gap measure to keep 777s flying (given regulatory imperative to avoid certain levels of risk for significant failure conditions hazardous to flight), it is definitely to be welcomed though it (almost inevitably due to its immediate nature) misses the opportunity to address some of the likely underlying contributory factors to "hazardous amounts" of ice accumulating in the "main tank fuel feed system".

I would highlight the AD does not reflect:
- the potential build-up of free water and ice in the 200-ER centre tank over two cold-soaked stages.
- the fuel in each main tank remaining cold soaked (together with its scavenge lines) on cold stopover.
- the timing of delivery of any centre tank water into each main tank fuel supply.
- the accretion of ice might not be of the "releasing" type in which case until it fully develops, full flow may not remove it or be affected by it.

In particular, while the in-flight run-up can happen 3 hours before TOD, any ongoing build-up of free water in the centre tank (not addressed by AD) may only be scavenged into the main tanks one hour or two before TOD as on BA038 - if run-up is done before fuel scavenge completed then arguably chain may not be broken unless require another run-up, say, 20 minutes after centre tank indicated as zero?

2. Location of "95%" restriction

If the restriction in each manifold is elsewhere than the "engine fuel" system, as could be suggested from the test rig results which showed significance of a time lag after the accelerations started but before the flow reduced, and operates as an effective restriction on 95% of the cross-section at a point in the "main tank fuel feed" system upstream of the engine, then this AD seems to miss the obvious risk that the restriction was never located in the "engine fuel" system! I assume though that (if I may try and be diplomatic) it is currently expedient to ignore that suggestion.

Ironically, pity the non-RR engined 777 crew and passengers - at least those flying in 777s equipped with RR should now face a reduced risk of icing-induced double rollbacks. Perhaps more risk-averse non-RR 777 operators though might see some advantage in being seen to implement the proposed AD on a voluntary basis, at least pending the next report?