Svein, That is all sound advice, except there is nothing
to stop you switching back to the main after having run
the reserve dry, to tap into what you left there and what
will have spilled over from the reserve. Some of that will
of course then go via the return line to the reserve side
again and when your main runs dry you can yet again switch
to reserve and expect to find a little bit there. I am not
suggesting this should be your normal practice - clearly
it is preferable always to plan to land with your reserve
fuel unused, but if one of these days you find yourself
having to do a massive diversion for say unexpected
weather problems, it is worth remembering that either tank
running dry gives enough notice (with engine coughing) to
allow you to switch to the other tank and that you can get
a few extra miles by repeating the process.
Regards, David Joyce, G-XSDJ
"Sidsel & Svein Johnsen" <sidsel.svein@oslo.online.no>
wrote:
>Johnsen" <sidsel.svein@oslo.online.no>
>
> Graeme,
>
> What we need to keep in mind is the following:
>
> When we run off the main tank, about 28-30 liters of
>fuel goes out per hour
> at cruise power (912ULS). Abt 10 of these liters return
>to the reserve tank
> (i.e. abt 1/3 of the total flow out of the tank), and
>flow back over the
> saddle into the main tank. This ensures that the
>reserve tank is always
> full, even if we should fly a little uncoordinated once
>in while or
> experience some bumpy turbulence, and the net drain from
>the main tank is
> 18-20 liters (or whatever the cruise consumption may
>be).
>
> What we should NOT do is switching to the reserve tank
>before we have
> decided that "OK, this is it for the main tank, from now
>on it's only the
> reserve tank that will take me home." Take the
>following case: 9 liters
> in the reserve, 11 liters in the main. If this were the
>case during the
> last part of a flight, we have 20 liters total, i.e. one
>hour's flying time.
> If due to uncertainty about actual volume remaining in
>the main tank, we
> switch to the reserve tank (9 liters), then we have a
>little under half an
> hour before the engine tells you something. If we now
>switch back to the
> main tank, we suddenly do not have 11 liters consumable
>fuel there anymore!
> Why not? Because about 1/3 of that fuel is returned to
>the reserve tank,
> but the returned volume does not any more overflow into
>the main tank
> because the reserve tank first needs to be filled to its
>top. 10-15
> minutes' flying time is thus lost, which could be
>critical (unless we are
> conscious about this, and switches back to reserve
>again).
>
> What it all boils down to is knowing how much you have
>left in the main
> tank, so we can confidently run this tank almost empty
>and switch to reserve
> just in time before the engine tells you to, and then
>stay on the reserve
> tank.
>
> I completely trust my fuel totalizer (feed and return
>flow senders) and the
> sight tube. By the way, I have connected the two vent
>tubes (the one from
> the top of the sight tube and the one to the top of the
>filler neck) by a
> crossover tube at their highest point. If one inlet
>should be blocked by an
> insect or whatever, the other will provide equal
>pressure on top of the tank
> and on top of the fuel in the sight tube. It is amazing
>how erroneous the
> sight tube reading gets even with a very small pressure
>difference in the
> standard tubing arrangement - just try it by blowing
>gently into one of the
> outlets.
>
> /////////
>
> About return flow: I mentioned early this spring that I
>would re-wire the
> fuel flow system so that I could get temporary direct
>reading of the return
> flow only. I have the relay deck and the push button,
>but just have not had
> the time to wire it in ........ Will do - will report!
>
> Regards,
> Svein
> LN-SKJ
>
>
>
>
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