AAIB BA38 B777 Initial Report Update 23 January 2008
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Crossfeed valves....
I agree with the opinion expressed by bubbers44 about the crossfeed valves selected to OPEN, and flap retraction versus checklists!
Earlier posts have described the B-777 fuel system very well (posts 227 and 229) - this is a small addition regarding crossfeed, based on my knowledge of the B-767 system, which is very similar to the B-777 system.
It does'nt matter whether crossfeed during normal operation is selected OPEN or CLOSED - as long as there is fuel in CTR tank, both engines will get fuel from this tank - later when it is empty, automatically R wing tank will feed R engine and L wing tank will feed L engine - simple system Easy to manage
I will doubt very much, that a failure within the AIRPLANE fuel system (tanks, pumps, valves etc.) played any part in the event.
Earlier posts have described the B-777 fuel system very well (posts 227 and 229) - this is a small addition regarding crossfeed, based on my knowledge of the B-767 system, which is very similar to the B-777 system.
It does'nt matter whether crossfeed during normal operation is selected OPEN or CLOSED - as long as there is fuel in CTR tank, both engines will get fuel from this tank - later when it is empty, automatically R wing tank will feed R engine and L wing tank will feed L engine - simple system
Easy to manage I will doubt very much, that a failure within the AIRPLANE fuel system (tanks, pumps, valves etc.) played any part in the event.
Last edited by grebllaw123d; 4th Feb 2008 at 20:27. Reason: addition
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Originally Posted by grebllaw123d
It does'nt matter whether crossfeed during normal operation is selected OPEN or CLOSED - as long as there is fuel in CTR tank, both engines will get fuel from this tank - later when it is empty, automatically R wing tank will feed R engine and L wing tank will feed L engine - simple system. Easy to manage
Can you confirm this?
Sludge, water, ice, wax, whatever, could have ended up from the centre tank into the wing tanks in about equal quantities.... and caused equal problems at nearly the same time....?
I will doubt very much, that a failure within the AIRPLANE fuel system (tanks, pumps, valves etc.) played any part in the event.
But now we're back to the fuel itself.
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If you leave the center pumps on, the both engines will run on the center tank only until it is empty (900kg.). This can take a few hours or 20 mins, depending on the volume that was in the center tank at takeoff.
At some point as the mains are depleted, the 900kg remaining in the center tank will be automatically transfered to the wing tanks. (this occurs when the mains are half empty on the 767)
Last edited by Lost in Saigon; 4th Feb 2008 at 23:00.
Controversial, moi?
A scavenge system transfers fuel remaining in the centre tank into the wing tanks when wing tank quantity remaining drops below a given figure.
The quantity at which this happens varies dependent upon variant.
The centre tank pumps are switched off when centre tank quantity drops below a given figure which triggers an EICAS advisory message and corresponding low pressure light in the respective centre pump switch(s). The quantity which triggers this warning is again dependent upon variant
The quantity at which this happens varies dependent upon variant.
The centre tank pumps are switched off when centre tank quantity drops below a given figure which triggers an EICAS advisory message and corresponding low pressure light in the respective centre pump switch(s). The quantity which triggers this warning is again dependent upon variant
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Greblaw123d
.................You imply low temp fuel wax would build up at the same time in wings as in centre tank thus fuel flow problem would have occurred much earlier for that reason and was therefore not actually likely.
....What I read from posters, who I suppose know the aircraft,there is strong opinion that fuel temps dont fall below -50 , even on long hauls at -70 oat.(though I have my doubts)
...However Greb , If it were only water contamination, built up over successive flights , that could well have been mainly in the centre tank; in the last 900 kg being pumped up, although correct reserves still existed in the wings.
....Is it relevant that with repeated flights (on this route?) centre tank water contamination signals were logged , though not visible to flight crew,on at least 2 occasions very recently?
.................You imply low temp fuel wax would build up at the same time in wings as in centre tank thus fuel flow problem would have occurred much earlier for that reason and was therefore not actually likely.
....What I read from posters, who I suppose know the aircraft,there is strong opinion that fuel temps dont fall below -50 , even on long hauls at -70 oat.(though I have my doubts)
...However Greb , If it were only water contamination, built up over successive flights , that could well have been mainly in the centre tank; in the last 900 kg being pumped up, although correct reserves still existed in the wings.
....Is it relevant that with repeated flights (on this route?) centre tank water contamination signals were logged , though not visible to flight crew,on at least 2 occasions very recently?
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There are 2 pumps in each tank, identical, but the pumps is the CTR tank are in series, whereas the pumps in wing tanks are in parallel, which means that you get higher pressure from the CTR tank system with the result that fuel are fed from this tank first - in the all pumps ON scenario.
The Centre Tank O/J pumps put out 36psi/31,750kg/hr each
The Wing Tank Boost Pumps put out 12psi/19,320kg/hr each
With the crossfeed valves (normally) closed, the fuel system is split into 2 halves.
Looking at the left half.... (composed of 1 CWT pump and 2 wing tank pumps)...
Initially, with fuel in all tanks, the CWT left hand O/J pump (1 off) feeds the left engine. The 36psi from this pump forces the check valves on the wing tank Boost Pumps to close (because they are putting out much less psi than the Centre Tank O/J pump). This stops fuel coming from the wings. If the centre wing pump fails or is switched off or there is no fuel in the tank, the check valves on the wing tank pumps open automatically (simply because the pressure in the manifold is less than it was before). No electronics involved.
All the pumps are in "parallel". If two pumps were in series, a blockage in one would mean that both would fail.
Rgds.
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On average, how long would it take for fuel level in the Centre Tank to show zero after switching off the Centre Tank pumps? 3 hours?
On average, how long would it take for the fuel level in the Center Tank to show zero after switching off the Center Tank pumps at the appropriate time on the BA 777 (type which crashed) using a typical BA Cost Index.
The 777 manuals I currently have at my disposal don't tell me
1) fuel quantity trigger point for the BA 777 CWT pump low pressure warning
2) the level in the main tanks at which scavenging begins on the BA 777
3) the scavenge rate
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Snanchecki:
"How did you conclude the "....right tank interfered with..." bit?"
The left engine ran up longer.
I concur about the fine airmanship.
Someone suggested that fuel would be sucked up by the burners without any pumping. In a fan jet, isn't the air pressure at the burners higher than that in the tanks?
Of course I don;t know any more about it than what I read in PPruNe!
"How did you conclude the "....right tank interfered with..." bit?"
The left engine ran up longer.
I concur about the fine airmanship.
Someone suggested that fuel would be sucked up by the burners without any pumping. In a fan jet, isn't the air pressure at the burners higher than that in the tanks?
Of course I don;t know any more about it than what I read in PPruNe!
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NSEU
1) fuel quantity trigger point for the BA 777 CWT pump low pressure warning
2) the level in the main tanks at which scavenging begins on the BA 777
3) the scavenge rate
1) The low pressure warning is a pressure indication from a pressure switch in the CWT output line. Fuel qty ind is not involved.
2) At 10400kg Wing tank level the inlet check float valves open.
At 17000kg Centre tank qty the outlet float check valves open.
The fuel scavenge system starts working, transferring fuel from centre to wing tanks.
3)Scavenge rate is according to the AMM at least 200kg/hr. No max rate is quoted.
As CWT pumps are typically turned off at about 900kg remaining, and as there are two fuel scavenge jet pumps, it should take just over two hours to empty the centre tank. As the wing tanks have fuel for about eight hours flying, the centre tank should be empty six hours before landing.
1) fuel quantity trigger point for the BA 777 CWT pump low pressure warning
2) the level in the main tanks at which scavenging begins on the BA 777
3) the scavenge rate
1) The low pressure warning is a pressure indication from a pressure switch in the CWT output line. Fuel qty ind is not involved.
2) At 10400kg Wing tank level the inlet check float valves open.
At 17000kg Centre tank qty the outlet float check valves open.
The fuel scavenge system starts working, transferring fuel from centre to wing tanks.
3)Scavenge rate is according to the AMM at least 200kg/hr. No max rate is quoted.
As CWT pumps are typically turned off at about 900kg remaining, and as there are two fuel scavenge jet pumps, it should take just over two hours to empty the centre tank. As the wing tanks have fuel for about eight hours flying, the centre tank should be empty six hours before landing.
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Thanks Swedish Steve
Just a thought, however....
The wing tanks being 29,320Kg (full), the centre tank scavenging process wouldn't begin until almost 2/3rd's of the fuel in the wing tanks had been used.
This would appear to me that if the fuel in the wing tanks lasts 8 hours, burning off 2/3rd's of this would take 8 x 2/3 = 5 hours (approx)... so, adding over 2 hours scavenge time to this... the scavenge process would finish with less than 1 hour to go, rather than 6.
Unfortunately, I forgot where I was going with this train of thought....
but thanks again (to all)
Rgds
NSEU
Just a thought, however....
2) At 10,400kg Wing tank level the inlet check float valves open.
This would appear to me that if the fuel in the wing tanks lasts 8 hours, burning off 2/3rd's of this would take 8 x 2/3 = 5 hours (approx)... so, adding over 2 hours scavenge time to this... the scavenge process would finish with less than 1 hour to go, rather than 6.
Unfortunately, I forgot where I was going with this train of thought....
but thanks again (to all)Rgds
NSEU
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Must admit I haven't read the entire 14 pages but have gathered that the crossfeed was open and the flaps were at twenty. Maybe it's just coincidence and "out of the box" thinking but these are also 2 actions called for by the Fuel Qty Low checklist.
Controversial, moi?
2) At 10400kg Wing tank level the inlet check float valves open.
At 17000kg Centre tank qty the outlet float check valves open.
The fuel scavenge system starts working, transferring fuel from centre to wing tanks.
At 17000kg Centre tank qty the outlet float check valves open.
The fuel scavenge system starts working, transferring fuel from centre to wing tanks.
Fuel transfer begins when EITHER main tank is below 13,100Kgs.
Must admit I haven't read the entire 14 pages but have gathered that the crossfeed was open and the flaps were at twenty. Maybe it's just coincidence and "out of the box" thinking but these are also 2 actions called for by the Fuel Qty Low checklist.
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of course it's not a coincidence, but clever thinking of the crew, who remembered these items and just did it as they ran out of all options, ground rapidly approaching and still no engine spool up...
Last edited by Dani; 5th Feb 2008 at 22:14.
Controversial, moi?
It was a coincidence that those two items were carried out but also happened to be part of an unrelated 'Fuel Qty Low' checklist.
I happen to know what the thinking was behind the flap re-selection and it was nothing to do with low fuel quantity. Also they had greater than 10 tonnes of fuel on board at the time.
I happen to know what the thinking was behind the flap re-selection and it was nothing to do with low fuel quantity. Also they had greater than 10 tonnes of fuel on board at the time.
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"Also they had greater than 10 tonnes of fuel on board at the time."
Thanks for the clarification Mickey. I hadn't come across that piece of information so far - only that they had what was described as "adequate" which left some question.
Thanks for the clarification Mickey. I hadn't come across that piece of information so far - only that they had what was described as "adequate" which left some question.
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We dont KNOW exactly when and if FUEL LOW drills were performed .The Captain did not mention them in his first debrief.In his second debrief there was a feeling of uncertainty as to when and how they were done.
...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?)
...I continue to be puzzled by the crews initial claim of an electrical shut down , but the investigation says there was not.
.....Whatever did go wrong, the Crew, Boeing ,and R.R. got an awful lot right
...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?)
...I continue to be puzzled by the crews initial claim of an electrical shut down , but the investigation says there was not.
.....Whatever did go wrong, the Crew, Boeing ,and R.R. got an awful lot right