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?
Last edited by RandomPerson8008; 31st Jul 2020 at 02:09.