Right. First you need to know that you takeoff and land on the mains,
which are actually physically the tips. The aux tanks are in the wings
and they are NOT the mains. You're really not supposed to take off or
land on the aux but ...
The other thing you need to know is that you can't crossfeed from the
aux so you want to get them down ASAP. You're always trying to burn
the aux level down and get fuel into the mains. You can crossfeed from
the other main. You take off and land on the main. You want all the fuel
in the mains, and the aux's empty, ideally.
But, as you point out, the overflow goes back to the mains, so you have
to burn them down for a while, before you switch to the aux's, which has
the effect of refilling the mains. You can watch the gauges go back up.
If you look out and see blue mist at the tips, you're pumping fuel overboard -
this is when you are on aux, and the mains are indicating full, despite the
fact that you took off on them, and burned them down before you switched
to the aux's. Any experienced 421B pilot that says he's never seen blue mist
isn't telling you the truth :^)
The wing locker tanks also pump into the mains, selectable by their own
fuel pump switches. Again, you need to burn the mains down first, then
refill them from the wing locker tanks, just like you refill the mains with
the overflow from the return line, when you are on aux.
You always transfer fuel from the wing locker tanks
after you get the
fuel out of the aux tanks. Get the aux tanks empty, and get them empty
first.
They made it a whole lot simpler on the 421C if memory serves.
Remember with these aircraft, that any tiny angle or impatience refuelling
will result on those wide tanks being a long way down from full. For this
reason (also because I don't like jet-A in piston engines) I like to participate
in the refuelling.
I came up with this, some years back. It's not great, but it's not that bad:
www.pittspecials.com/articles/C421B.doc
