As I always understood it you should before starting the APU switch at least one fuel pump ON. Today I found this in the documentation If extended APU operation is needed on the ground, position an AC operated fuel pump ON. If fuel is loaded in the center tank, position the left center tank fuel pump switch ON to prevent a fuel imbalance before takeoff. Sorry for being stupid but what exactly does this mean, how many and which fuel pumps should be switched ON before starting the APU for a normal flight? TIA,

As I always understood it you should before starting the APU switch at least one fuel pump ON. Today I found this in the documentation If extended APU operation is needed on the ground, position an AC operated fuel pump ON. If fuel is loaded in the center tank, position the left center tank fuel pump switch ON to prevent a fuel imbalance before takeoff. Sorry for being stupid but what exactly does this mean, how many and which fuel pumps should be switched ON before starting the APU for a normal flight? TIA,

That is correct ! Fuel for the APU flows using just gravity and it takes it from the wing tank by default.By turning on left centre pump you will avoid an imbalance in the wing tanks. You can see this for yourself on the Engine Indication screen.There wil be less fuel in the left wing.

Thanks for your quick reply Frederic! Just to make sure I understand this correctly, what you say is that I should only switch the left centre pump ON and no other pumps? And this would mainly be when I expect extended APU operation where I could otherwise switch any fuel pump ON since the amount of fuel wouldn't be that much for normal operation of the APU? Sorry for being priggish but I'm about to make my own checklist and I just want to make sure I get everything right and according to how things are done IRL.

That is correct ! Fuel for the APU flows using just gravity and it takes it from the wing tank by default.By turning on left centre pump you will avoid an imbalance in the wing tanks. You can see this for yourself on the Engine Indication screen.There wil be less fuel in the left wing.

Apu doesn't take much fuel but actually it takes fuel from LEFT AFT tank by default. So you only have to turn on the LEFT AFT fuel pump when APU is on the Bus !

Thanks for your quick reply Frederic! Just to make sure I understand this correctly, what you say is that I should only switch the left centre pump ON and no other pumps? And this would mainly be when I expect extended APU operation where I could otherwise switch any fuel pump ON since the amount of fuel wouldn't be that much for normal operation of the APU? Sorry for being priggish but I'm about to make my own checklist and I just want to make sure I get everything right and according to how things are done IRL.

When I flew for Ryanair, if we had fuel in the centre tank then we would always turn on left centre pump for the APU.If we did not have fuel in the centre tank then we just used the default left aft tank but no pump turned on. (al pumps off)

Thank you so much for sharing your knowledge and experience Frederic, now I got it :(

If you've got center tank fuel, use the L Ctr pump. If you don't, use the Aft #1 pump.One pump only. If you start to get an imbalance, use the Aft #2 pump with x-feed open to even things up.

Yep, got it - thanks! As for using a pump (when you don't have fuel in the center tank) I guess that might be different from airline to airline. As Frederic said above, at Ryanair they normally didn't switch any fuel pumps ON when they didn't have fuel in the center tank but I also read in the doc that it's good practise to still use a fuel boost pump whenever the a/c has AC power since that will extend the service life of the APU fuel control unit.