New Luscome

[Big Pistons Forever]

There are eleven fuel pumps, 2 mechanical and 9 electric

[anofly]

3. if you takeoff with all tanks full, and switch to the aux tanks , the excess fuel, the fuel injection system does not need or use,goes up the common return line to the mains, (tip tanks but mains!) and because it is already full, it goes overboard.
there are a bunch of fuel pumps working on this plane with no switch, and some with switches. some just push fuel around inside the (tip) main tanks I think ... they might not be on diagram...?

you sort of takeoff on mains(tips) burn an hour or so out of them, then burn the aux, and some of that goes back into tips (mains)?

[Colonel]

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

[Colonel]

Not sure anyone operates twin Cessnas any more, but I liked them.

The geared ones were nice and quiet, cabins were roomy, lots of useful load,
and the pressurization was really convenient for flying in the teens. Great
airplanes for 15/16/17k feet. Over a lot of wx, and nobody else there except
climbing or descending kerosense burners near big airports.

You can really cover some distance in these airplanes. I flew them to Bahamas,
Key West, New Orleans, you name it. They had long enough legs that you could
often to a trip without a fuel stop, which is really important. You could do day trips
that the airlines couldn't, and be home by 4pm.

[Squaretail]

Not sure anyone operates twin Cessnas any more,

Mostly because Cessna is trying its best to make it as prohibitive as possible to keep operating their legacy aircraft. Didn't they get into a scrum with the FAA about issuing AWDs?

While I like a good fuel system diagram as much as the next guy. It must be said that as with any increase in the complexity of a system, there is a proportional increase in the chance of failure in said system. Few things worse than finding out one of those little pumps ain't working and having to recalculate how much fuel you can still access.

[Colonel]

one of those little pumps ain't working

Yeah. I remember my friend Joe had a tuna tank 310, the boost pump
stopped working on one side, so he crossfed the prime to start. Not
sure that's on the checklist?

If the SIDs don't scare you, the spar AD's on twin Cessnas should!