Question about 747 fuel crossfeed valves

[Guest Peter Bowcut]

Hello Gang,On the B747, the number 2 and 3 crossfeed valves are normally left in the guarded AUTO position. When the aircraft is in takeoff configuration, both of these valves automatically close, then will automatically re-open when the aircraft is airborne. When the fuel pumps are manually configured to tank to engine mode of operation, the fuel synoptic display shows x-feed valves 2 and 3 remain in the open position. X-feed valves 1 and 4 are of course manually closed by the aircrew according to SOP. In tank to engine configuration, how do engines 2 and 3 not crossfeed from the left and right main fuel tanks? Why doesn't fuel system logic automatically close valves 2 and 3 in tank to engine configuration?The fuel system in the PMDG 747 simulation appears to be very accurate indeed, amazingly so in fact. I am posing this question to understand why the real 747 fuel system logic is designed this way.Thank you, Thomas M. Bowcut

[Guest Zass]

Hi. The 747 has three "modes" of fuel operation: all main tanks full and with CWT (center wing teank) fuel (and, if installed, fuel in the horizontal stabilizer). Then the mode with no CWT fuel and main 2&3 > main 1&4. And last but not least the tank to engine configuration. In the first one, main 2&3 feed their respective engines, and the CWT feeds engines 1&4. The crossfeed valves are controlled automatically (so crossfeed 2&3 close automatic, so all crossfeed valves ON/IN-LINE and the CWT pumps ON). This occurs before T/O with the trailing edge flaps extended. After flap retraction, all angines are fed from the CWT. After the EICAS alert >FUEL LO CTR L/R, the center tank pumps are switched off, since fuel quantity in the CWT is low. The second mode operates the OVRD pumps (which have a larger output power then the main pumps) and feeds all engines, and all crossfeed valves are open ("in-line"). This continues until main 1&4 are approx the same value as main 2&3 (so EICAS >FUEL TANK/ENG is displayed).Then the third mode kicks in. The OVRD pumps are switched OFF, and crossfeed 1&4 closed. This means that every main tank is feeding its respective engine. If the horizontal stabilizer also contains fuel at take-off, this fuel will be pumped to the CWT in level flight, when indicated on EICAS (eg >FUEL PUMP STAB L/R). Only then these pumps are switched ON, until they are again indicated on EICAS, you will switch them of because they are emptied.

[Headley 26]

As my "mother"language is not English it is sometimes difficult to understand the technical explanations explained in different tutorials and manuals.After reading this topic and the advanced tutorial written by Holger Seilz i begin slowly understand the "fuel logic". Thanks guys.

[Guest Peter Bowcut]

Hi Zass,When in tank to engine configuration though, the fuel synoptic display shows x-feed valves 2 and 3 open, along with the common crossfeed manifold still being in the 'fuel circuit' (i.e.) the x-feed manifold being green between engines 2 and 3. This is the part I'm confused about - how do the number 2 and 3 engines not crossfeed from one another's fuel supply in this situation? While in tank to engine configuration in the PMDG sim, shutting off the main AC fuel pumps in one of the main wing tanks (either #2 or #3 tanks) on one side will immediately result in crossfeeding taking place from the opposite side main tank (with it's AC pumps still active). The number 2 and 3 x-feed valves and the x-feed manifold seem to stay 'online' for x-feeding on demand during tank to engine configuration, and I'm sure this is by design. I just don't fully understand the whys and wherefores..... :)Thanks a lot for your reply.-TM Bowcut

[Guest Zass]

Yes, you're completely correct, crossfeed 2&3 remain in-line. Still it results in a correct fuel distribution, which is very important. Only with flaps in take-off position, the crossfeeds 2&3 are closed automatically, which is then the ultimate tank/engine config.

[Qavion 19]

"Only with flaps in take-off position, the crossfeeds 2&3 are closed automatically, which is then the ultimate tank/engine config."Depends on which tanks are being used at the time. If the CWT is in operation, then you still don't get one tank feeding only one engine.No one has been able to explain to me why this is so.Cheers.Q>