Since engines are run only when the fuel pumps are on and vice versa , why are there separate switches for this? Why can't it be automated ?
You're asking two questions:
Why can't they be automated?
McDonnell Douglas has analysed the eleven million hours of experience with DC-10 systems, which are retained on the MD-11 for the most part. The cockpit has now literally automated the flight engineers' job. Drills and checklists have been converted into algorithms for operating system controls. Checks can now be made on each system simultaneously, rather than sequentially. (Flight, 1987)
They certainly can be, and are on the McDonnell Douglas MD-11. Manual control is possible, but in normal and most abnormal operations the fuel panel is fully automated.
(flightforum.ch) MD-11 fuel panel: the dark 'system' button indicates automatic operation mode, when pressed the word 'manual' is illuminated and then the rest of the panel (buttons) become selectable by the user.
Why are they separate?
In short, they're not engine pumps (engine-driven fuel pumps are run directly by the engines via each engine's accessory gearbox). They are tank pumps, and selective usage of tanks is important. Whether automated or not (automation can fail), you still need separate controls for various situations, of which:
- What could the fuel pump failure have been that my flight experienced? – selective use of pumps to keep the left/right wing tanks balanced if the cross-feed valves failed open
- Why are the center tank pumps deactivated when slats are extended on the A320? – deselecting the center tank on various planes during takeoff
Since engines are run only when the fuel pumps are on and vice versa<<
Not true. Engines can be run without fuel pumps, to allow for fuel pump failure.
The need for fuel pumps depends on the location of the fuel tanks relative to the engines, and the relative thirstiness of the engines - there are some airplanes where the fuel pumps are only used at some times (i.e. near the ground).
So it all depends.