cwatters

Increasing drag allows a steeper approach without building excess speed.

HarryMann

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.

dxzh

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?

philipat

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.

Re-Heat

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:

snanceki

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.

777fly

snanceki:

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

windytoo

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.

cockney steve

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! )

sispanys ria

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.

scrivenger

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.

Feathers McGraw

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....

cwatters

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.

ChristiaanJ

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 Gruyθre holes that lined up?

CJ

L337

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?

ChristiaanJ

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

L337

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.

Swedish Steve

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.

Rainboe

333333333333333333

Green-dot

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