So, tell us Mouse...where does all this inside info come from? Are you part of the investigation team, a crew member, a BA flight dept pilot, or are you having us on? We dont KNOW exactly when and if FUEL LOW drills were performed. It will become apparent that the flight crew did what would seem like a good idea at the time. We will have the comfort of our armchairs to decide what they should or should not have done. On the evidence so far I have no criticism and a sense of relief that my abilities were not the ones that were tested that day nor subject to detailed scrutiny, officially and unofficially, in the days past nor weeks to come. Failure of one Xfeed to open suggests it was either done very late , after engine "rundowns" and not not fully performed by the system, ...OR , it was selected before , and,(with one Xfeed failed at CLOSED) actually contributed to the "rundowns". ...Or ,is it possible that in fact one Xfeed was failed OPEN throughout the flight?(then what?) Neither had a crossfeed valve failed open throughout the flight. |
FUEL QTY LOW 777 flight manual
CROSSFEED SWITCH (Either) ON
(Ensures fuel is available to both engines if the low tank empties) FUEL PUMP SWITCHS (AII) ON (Ensures all fuel available for use) Plan to land at nearest suitable airport GROUND PROXIMITY FLAP OVERRIDE SWITCH OVRD Use flaps 20 and VREF 20 for landing (Increased speed at flaps 20 provides improved elevator control for landing flare in the event of a dual engine flameout) Avoid high nose up attitude and excessive acceleration or deceleration (prevents forward pumps from uncovering) |
Would be interested to know if it took more than 45 seconds to look that lot up :-)
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It's great to witness a much more balanced, calm and knowledgeable discussion of these matters here than was the case a short while ago...
With this seeming more like a one in a million 'act of God' occurrence, whatever it might have been, we can only give thanks that the outcome was so benign and that hopefully it will provide pointers to making air-travel even safer in the future... |
Suction feed
Ref post 275
" The burners sucking fuel without pumping" ...... Fuel is injected into the combustion chamber of any jet engine, in the form of a very fine spray, so it will ignite easily and combust as completely as technically possible. Such a fine spray can only be achieved with fuel under very high pressure so you definitely need a high pressure fuel pump (which is mechanically driven by the high pressure rotor, N2 or N3). Any pump that delivers pressure on one side, will suck on the other side. Normally, an aircraft manufacturer will make the plumbing of fuel pipes of such dimensions, that in principle, the suction of the high pressure pump, possibly assisted by gravity, will be enough to get fuel from the tanks to the engines. However, simply "enough" is never sufficient in aviation, so every tank will have a redundant number of low pressure supply pumps in the plumbing system to guarantee ample delivery of bubble free fuel to the inlet of the high pressure pump under all conceivable conditions. Those supply pumps are normally electrically driven, and their electrical power source of course is never just the same source for all pumps. I hope that post 275 is thus clarified for interested readers. |
Fuel transfer begins when EITHER main tank is below 13,100Kgs. So scavenging would probably begin 3 1/2 hours after the Center Tanks pumps had been switched off. Add another 2 and a bit hours to this to get almost 6 hours. So 2 or more hours before arrival, the fuel level in the Center tank should be zero. Rgds. NSEU |
From my numbers G-YMMM held 29,100kgs of fuel in each wing tank when full (usable, level attitude and using a density of 0.8029 kgs per litre).
Therefore once the centre wing tank pumps had been selected off 16,000kgs of fuel would need to be used before scavenge transfer started. Using an average fuel burn per engine of say 3,000kgs per hour that is roughly 5 hours of flight. I cannot recall ever monitoring the scavenge transfer that closely but although I do not doubt the MINIMUM transfer rate of 200kgs per hour quoted earlier I think the normal rate is a good deal greater. For example at 200kgs per hour just 800kgs in the centre tank would take 4 hrs to transfer. I may well be wrong and will endeavour to time the transfer when next I have the opportunity but I am puzzled where this somewhat esoteric discussion is leading! |
For example at 200kgs per hour just 800kgs in the centre tank would take 4 hrs to transfer. but I am puzzled where this somewhat esoteric discussion is leading! As the dregs were being sucked from the centre tank, I was wondering at what time this process would be completed. Of course, the contaminants have to find their way down from the outlets of the scavenge pumps to the bottom of the wing tanks where the wing tank pump inlets are. I don't know how long this would take. |
HEre is some more info on the fuel system. Note the last paragraph
http://online.wsj.com/article/SB1201...googlenews_wsj Pilots and safety experts said both engines of the British Airways plane normally would have been draining fuel directly out of the same center tank during final descent to the field. Jetliners rely on additives to prevent ice formation inside tanks and fuel lines, but they don't typically have heaters to warm fuel. In the past few days, investigators have found internal damage to at least one fuel pump suggesting that the pumps weren't receiving adequate fuel flows, according to one person familiar with the details. |
As SwedishSteve said earlier, there are two scavenge pumps, so the minimum rate would be closer to 2 hours. |
Hallo boys and girls :cool:
Look what I found: http://meriweather.com/777/over-777.html Please, don't tutch anything. It may be connected to a real trippel seven! |
Pilots and safety experts said both engines of the British Airways plane normally would have been draining fuel directly out of the same center tank during final descent to the field. Jetliners rely on additives to prevent ice formation inside tanks and fuel lines, but they don't typically have heaters to warm fuel. In the past few days, investigators have found internal damage to at least one fuel pump suggesting that the pumps weren't receiving adequate fuel flows, according to one person familiar with the details. |
I certainly hope these 'experts' aren't involved in the real investigation! |
As the dregs were being sucked from the centre tank, I was wondering at what time this process would be completed. Of course, the contaminants have to find their way down from the outlets of the scavenge pumps to the bottom of the wing tanks where the wing tank pump inlets are
Yes there are two transfer jet pumps. One on each side. But there are water scavenge jet pumps as well in all three tanks. These operate all the time and scavenge water from all the odd cavities it may lodge and direct it to the booster pump inlets. So when you get down to the dregs of the centre tank, it will not be a chunk of water, if the water scavenge jet pumps were working. In my last incarnation as a Tristar engineer, we used to regularly go in the tanks to clean the jet pumps. the jet used to get blocked by debris, usually thiokol from the tank sealing. There is no indication that these pumps are not working. |
I certainly hope these 'experts' aren't involved in the real investigation! Why jetliners do have fuel heaters. They use heat exchangers on the engines (using hot oil to warm cold fuel). To a lesser extent, there are also heat exchangers in one of the wing tanks to cool hydraulic fluid. Even if the damage to the pumps was prior to the crash, all 4 pumps would have to be damaged to cause both engines to not respond... and the gravity feeds would have to be blocked, too. From where are they getting these so-called experts??? :{ |
Descent fuel tanks
My experiance is with 737's but I can't imagine this is different on the 777. Fuel is normally used first from the center tank so descents would normally be using wing tank fuel. This is because weight in the wings is more easily supported from a structural point of view (this is why there is a concept of maximum zero fuel weight on transport aircraft). In the 737, the center tank fuel pumps operate at a slightly higher pressure so that the center tank will empty first even if all tanks are online.
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Is it just me.....
A 777 of which there are many hundreds flying around (over the pole, over London etc) loses power on both engines on short finals and crashes and almost no information advice etc from any source... Boeing, CAA, FAA, AAIB. Not so much as a, just in case, check the wing nut on the outer flange on the right wing thingymbob. The FDRs are so sophisitcated these days they can tell how many times you scratched your a*s during the flight (thousands of parameters monitored) but, after three weeks, almost nothing............ but BA seemed to know enough to parade the pilots |
A sophisticated FDR can tell you a lot about what happened and when. But it can't necessarily tell you WHY, and if you don't know that you can't tell people to check for a cause.
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The fuel temperature at the outlet of the fuel cooled oil cooler would be lowest at the moment the crew/AT increased the thrust at 600'. This is when the engines lost thrust.
I've spent far too much time thinking about this and my conclusion is that a prolonged cold soak followed by a long low/idle thrust descent cooled the whole system (engine oil and fuel) such that when thrust was requested the increase in fuel flow through the fcoc resulted in the fuel temperature and therefore viscosity being low enough to disrupt the proper flow through the fuel metering unit or other downstream fuel components such that the requested fuel did not reach the fuel injectors. If I'm right do i win a prize? |
"It is a capital mistake to theorise before one has data. Insensibly, one begins to twist facts to suit theories, instead of theories to suit facts"
Observation by Sherlock Holmes in Arthur Conan Doyle novel. Still so true today............. |
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