Didn't you get FUEL IN CENTER message on the EICAS? That should remind you that you center pumps are off.

I believe he did say that he got that message but did not realize what it meant. 

I believe he did say that he got that message but did not realize what it meant. 

Ah. I must have missed it. Always keep that FCOM handy.  B)

willywonka, on 10 Jun 2014 - 4:39 PM, said:

Ah. I must have missed it. Always keep that FCOM handy.

All we need to remember is that if there are ANY yellow EICAS messages at all, something needs investigating. Also, keep an eye on the STAT page as some non-critical failures pop up here as an advisory, such as the PRSOV valve failures. But yeah if there is a message in yellow (or red!) you need to see why.

 

EDIT: The non-critical failures on STAT page are more for your interest. There is nothing that needs doing as there is nothing you can do and it is not flight critical.

I believe he did say that he got that message but did not realize what it meant. 

 

Yes correct.

 

 

All we need to remember is that if there are ANY yellow EICAS messages at all, something needs investigating.

Yep. I certainly wouldn't have gone into ETOPS with an unresolved EICAS message, much less close the doors and push back from the gate :lol:

Mike,
I see now that you said the exact same thing; I guess that’s what I get for skimming instead of reading.   :smile:


To try and answer your question the simple answer would be because with the current design there would be no occasion where fuel would only be drawn from center tank bypass valves.

 

 

To understand this we need to review the basics of the fuel delivery system. Fuel is delivered to the main fuel pump (the gearbox driven one on each engine) via a common engine feed manifold, the boost pumps are responsible for creating a positive pressure in this manifold. The two override pumps in the center tank operate at a pressure three times greater than the pumps in the main tanks making that source preferred while the center pumps are on. When they’re turned off the main tank boost pumps are responsible for creating this positive pressure. There are check valves between the center pumps and the manifold to prevent the main tank boost pumps from pushing fuel back into the center tank. The main tanks bypass valves work the same way in that they are only check valves and not something that is actuated. This means that for the bypass valve to work all positive pressure in the manifold must be removed, and a negative pressure from the main fuel pump demand needs to exist.

 

So assuming you had the same bypass valves in the center tank, the only situation where they would draw fuel would be one where all boost pumps were off on that side of the manifold and the crossfeed valves were closed. In that case the best you could hope for (removing any gravity differences as Wes pointed out) is fuel to be drawn from both the center and main tanks at the same time. With the situation in this post where the main tanks were run dry, the bypass valves would be sucking air into the manifold making any center tank bypass valve useless anyway.

 

I hope this makes sense, I know I’m not nearly as good at explaining things as some of the other folks on this forum.

Brian

 

 

I hope this makes sense, I know I’m not nearly as good at explaining things as some of the other folks on this forum.

Makes sense. I had suspected that the way the centre tank pumps "overpower" the wing pumps had something to do with it. So if BOTH centre tank pumps were to fail, your total USABLE fuel would immediately reduce to the sum of the two wing tanks?

Correct, in fact the FAA MMEL even allows for dispatch with one pump inoperative provided it’s accounted for in weight & balance, and a suitable airport is reachable with the remaining fuel in the wing tanks.

 

Brian

Correct, in fact the FAA MMEL even allows for dispatch with one pump inoperative provided it’s accounted for in weight & balance, and a suitable airport is reachable with the remaining fuel in the wing tanks.

 

Brian

I suppose this would also put an absolute limit on ETOPS - if you were that far out to sea that if you lost both centre pumps, the remaining fuel in wings would not be enough to get back to an ALTN. Not sure if there is any point on earth that would cause this situation though.

And now for something completely different....

 

Um to the third poster in this subject on ATC Vectors I hate to burst your bubble but vectors are not that efficient. As a real world ATC Controller myself, you get far fewer A/C in by vectoring. Now a days if you want to maximize your traffic flows in an out we use the STARs and SIDs as much as possible (Preferably RNAV based), and only vector when needed. Especially with arrivals, most of the SIDs (RNAVs) have continuous decent profiles built into them. These RNAV SIDs along with the ever increasing precision of the nav systems on the aircraft allow for a much tighter patter (we can literally run everyone usually at 3 miles sep minus the A380s and Heavys), while the A/C descend on the SID.  Vectoring was the old school way of increasing traffic flow, but now a-days we try and avoid it, if possible and just run as many planes as we can on the RNAV SIDs.

 

Oh and people have literally done PHds on this increased efficiency of RNAV SIDs vs Vectors and vectors are far less efficient then they are. one of the studies I personally read on this subject was centered on KATL. The research team even mapped out the routes and time flown by hundreds of A/C when on ATC Vectors vs when they were placed on RNAV routes. The A/C on vectors routinely flew about 4-8 min longer then on RNAV routes. Don't get me wrong, vectors have there place and I do use them almost every day, they just are not usually the most efficient when dealing with high traffic flow periods.

 

Again sorry dude to burst your bubble.

And now for something completely different....

 

Um to the third poster in this subject on ATC Vectors I hate to burst your bubble but vectors are not that efficient. As a real world ATC Controller myself, you get far fewer A/C in by vectoring. Now a days if you want to maximize your traffic flows in an out we use the STARs and SIDs as much as possible (Preferably RNAV based), and only vector when needed. Especially with arrivals, most of the SIDs (RNAVs) have continuous decent profiles built into them. These RNAV SIDs along with the ever increasing precision of the nav systems on the aircraft allow for a much tighter patter (we can literally run everyone usually at 3 miles sep minus the A380s and Heavys), while the A/C descend on the SID.  Vectoring was the old school way of increasing traffic flow, but now a-days we try and avoid it, if possible and just run as many planes as we can on the RNAV SIDs.

 

Oh and people have literally done PHds on this increased efficiency of RNAV SIDs vs Vectors and vectors are far less efficient then they are. one of the studies I personally read on this subject was centered on KATL. The research team even mapped out the routes and time flown by hundreds of A/C when on ATC Vectors vs when they were placed on RNAV routes. The A/C on vectors routinely flew about 4-8 min longer then on RNAV routes. Don't get me wrong, vectors have there place and I do use them almost every day, they just are not usually the most efficient when dealing with high traffic flow periods.

 

Again sorry dude to burst your bubble.

I am confused you say you do real world ATC but keep referring to SID when referring to departures "These RNAV SIDs along with the ever increasing precision of the nav systems on the aircraft allow for a much tighter patter (we can literally run everyone usually at 3 miles sep minus the A380s and Heavys), while the A/C descend on the SID." - How does an aircraft DESCEND on a Standard Instrument Departure? 

To try and answer your question the simple answer would be because with the current design there would be no occasion where fuel would only be drawn from center tank bypass valves.

 

To understand this we need to review the basics of the fuel delivery system...

 

Brian,

 

Thanks for your very detailed explanation of the fuel pump and valve systems.  I think I followed it pretty well after a few readings (which is fine for a complex subject).  If I understand correctly, it would be possible for a suction bypass system in the center tanks to work, in the situation Tony faced (main/wing tanks empty) if there were a way to lock closed the suction bypass valves in the main tanks.  Then the engine mounted pumps would not suck in air, but fuel from the center tanks.  This would not eliminate the problem Wes and you raised, that gravity would be a greater obstacle to suction from the center tanks than from the main ones.  I am not sure why this would ever be necessary RW though -- if all center tank pumps failed it would be too risky to proceed on a flight that required the center tank fuel, as suction-action is not a reliable means of delivering fuel (the FCOM notes that air from the wing tanks can enter the wing/main suction-bypass system causing a flameout, under some circumstances).

 

I have another question -- why not turn OFF the main/wing tank pumps until the center tank gets low?  This would save wear on them, and possibly on the center pumps as well, since they wouldn't have to "fight" the pressure of the wing/main pumps?  The main pumps could possibly operate at a lower pressure.  The wing/main pumps could come on automatically when the center tank starts to get low (in case the pilots neglected to do so).

 

Mike

Your right 77west. I mis-typed a bunch referring to STARs while calling them SIDs. I was rushing and should have re-read my note before posting. Good catch and thanks for the heads up.

I have another question -- why not turn OFF the main/wing tank pumps until the center tank gets low?  This would save wear on them, and possibly on the center pumps as well, since they wouldn't have to "fight" the pressure of the wing/main pumps?  The main pumps could possibly operate at a lower pressure.  The wing/main pumps could come on automatically when the center tank starts to get low (in case the pilots neglected to do so).

 

 

I don’t know the official design decision on this one, but I would guess one of the reasons would be that in the event of an override pump failure it would insure uninterrupted positive fuel pressure.   That said, the pumps have a pretty high MTBF, so the chances of losing a fuel pump on a flight are pretty remote, and two even more so.  Pump automation could be done but it’s another thing that could fail, and would add complexity just to save the flightcrew a couple of button pushes during a phase of flight where the workload is usually low anyway.

 

Brian

Thanks, Brian, for explaining the fuel system in such detail.  It strikes me that the 777 is not just automated, but is designed on a principle of simplicity -- the suction/bypass valves and the fuel pumps work properly without either human intervention or complex automation.  No valves have to be shifted, either manually or automatically, to make sure the center tanks empty before the main ones.  It's just inherent in the system: the center pumps overpower the main/wing pumps.  If a main/wing tank pump fails, if I understand it correctly, the change in pressure in the manifold opens the suction/bypass valves from the main/wing tanks.  Neither the pilot nor an automated detection system has to intervene.

 

Tony, thanks for sharing your experience and provoking a very interesting discussion.

 

Mike