B-747-400/-8 Fuel System Design Question
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B-747-400/-8 Fuel System Design Question
What is the primary reason for the fuel system management cards closing the #2 and #3 fuel crossfeed valves during flap extension for takeoff (and then opening them after takeoff)?
I understand that each individual CWT or Main #2/3 Override Jettison Fuel pump is only sufficient to supply 2 engines with takeoff power, so I would speculate that the reason is fuel pump failure mode redundancy (e.g. if the 2 CWT O/J pumps were supplying fuel to all 4 engines during takeoff, if one CWT O/J pump failed, the remaining pump would be insufficient for all 4 engines, even though the lack of CWT O/J pump pressure should cause the armed #2/3 O/J pumps to turn on).
However, for most flights at our airline, there is no fuel in in the CWT, in which case the benefit of dedicating 2 boost pumps AND 2 O/J pumps to the #1&4 engines while dedicating just two boost pumps to each the #2 and #3 engines for takeoff isn't readily apparent. Maybe the outboards being "critical engines" is a partial reason for the architecture?
Are there other reasons?
I understand that each individual CWT or Main #2/3 Override Jettison Fuel pump is only sufficient to supply 2 engines with takeoff power, so I would speculate that the reason is fuel pump failure mode redundancy (e.g. if the 2 CWT O/J pumps were supplying fuel to all 4 engines during takeoff, if one CWT O/J pump failed, the remaining pump would be insufficient for all 4 engines, even though the lack of CWT O/J pump pressure should cause the armed #2/3 O/J pumps to turn on).
However, for most flights at our airline, there is no fuel in in the CWT, in which case the benefit of dedicating 2 boost pumps AND 2 O/J pumps to the #1&4 engines while dedicating just two boost pumps to each the #2 and #3 engines for takeoff isn't readily apparent. Maybe the outboards being "critical engines" is a partial reason for the architecture?
Are there other reasons?
Last edited by RandomPerson8008; 31st Jul 2020 at 02:09.
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From memory on my B747 Classic and -400 courses a VERY long time ago (no longer have course notes) - think it had something to do with rotor (compressor/turbine) catastrophic failure and potential FOD damage to inner tanks. The underwing areas on the inboard engines is more exposed due to the geometry of the wing and engine mounts. Can't remember the actual logic and I could be wrong.
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Ok, thank you Tom Sawyer, I was also going to speculate that compartmentalizing the fuel manifold by closing x-feed 2&3 might offer some measure of protection from fuel contamination if it were limited to one tank or a fuel leak within the system, as you stated.
SLF, I was under the impression that for the take off fuel feed must be tank to engine so that each engine was totally independent from the others ie a fuel feed fault will only affect one engine, unless I'm misunderstanding the question.
https://www.slideshare.net/theoryce/fc744-fuel
https://www.slideshare.net/theoryce/fc744-fuel
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From memory, also a long time ago, I think Megan is right on the money. Take off being a critical phase of flight fuel feed had to be tank to engine.
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Thanks for the answers!
The jet would become tank to engine with no crew action if all of the override jettison pumps failed during takeoff, I guess. Perhaps that is sufficient to satisfy the certification requirements.
The 744/-8 is not normally "tank to engine" for takeoff unless the total fuel quantity at blockout is less than ~119,000 lb (relatively uncommon, would be only for flights shorter than 4 hours or so). Otherwise, some of the fuel tanks are providing fuel to multiple engines during takeoff, albeit with multiple fuel pumps.
The jet would become tank to engine with no crew action if all of the override jettison pumps failed during takeoff, I guess. Perhaps that is sufficient to satisfy the certification requirements.
The 744/-8 is not normally "tank to engine" for takeoff unless the total fuel quantity at blockout is less than ~119,000 lb (relatively uncommon, would be only for flights shorter than 4 hours or so). Otherwise, some of the fuel tanks are providing fuel to multiple engines during takeoff, albeit with multiple fuel pumps.
Last edited by RandomPerson8008; 31st Jul 2020 at 07:41.
From memory on the classic the take-off fuel configuration was "H" ie. #1 and #4 crossfeed valves open and #2 and #3 valves closed. With no centre tank fuel then this was "tank to engine" however with centre tank fuel it would feed the outboard engines thereby helping wing bending relief since the outboard tanks would stay full since the CWT pumps had a higher pressure to overcome the #1 and #4 tank pumps which acted as backup in the event of CWT failure.
After the aircraft was clean then the centre wing tank pumps would feed all engines.
After the aircraft was clean then the centre wing tank pumps would feed all engines.
