On 06/24/2013 12:10 PM, Sidsel & Svein Johnsen wrote:
> I maintain that the best procedure is to run the main tank almost empty (or
> until coughing engine, if one dares to), and only then switch to the reserve
> tank. If that tank should run dry, there is nothing left in the main tank
> (no return flow has spilled over to the main tank after consumption from
> reserve tank started) - you have used all fuel there was.
I have two tank sensors, one in the main and one in the reserve side. My
tank sensors are very sensitive near the tank bottom and they display
the amount of fuel with a resolution of 1 liter! So I can offer some
insight in how the tank empties.
What I observed when I got home with little fuel left, is that once the
main tank is zero, quite a lot of fuel sloshes from the reserve to the
main side. While draining the main side, all return fuel (for the 914 a
multiple of what the engine actually uses) returns to the reserve side,
so although fuel is sloshed over constantly, it is also refilled
constantly. So, the main tank slowly decreases until it reaches zero and
the reserve side remains filled to the brim all the time.
When switching to the reserve side, the situation changes dramatically.
Fuel being sloshed over from reserve to main is now no longer pumped
back automatically. And a lot of fuel sloshes over because the tank is
wide near the top of the saddle and of course the reserve is filled just
to the edge of it. It is like walking with a bucket completely filled
with water and you don't need much movement to loose quite a lot of the
contents. In a short while I had collected 4 liters in main again (4
liters isn't much but it is half the contents of the reserve side!). It
was just moderately turbulent, the usual thing on a hot summer day with
some cumulus above the airplane, and being aware of the low fuel I tried
to keep things as calm as possible.
So, if you don't switch back to main, you can't count on having 9 liters
in reserve, but only half of it.
When I switched back to main, the main tank got empty again in two
minutes, because the 914 pumps displace 120 liters per hour and all the
excess fuel is put in the reserve side again. I kept my hand on the fuel
valve, it was just a matter of counting down and seeing the fuel quickly
move from the main to the reserve side again. Now I had 8 liters again
in the reserve side.
I had to repeat this once more, and after that the fuel level was low
enough to stay in the reserve side. And of course about that time I was
landing, with 4 liters left plus a silent amount of 2 liters of
"unusable" fuel.
The lessons here:
1) If you don't switch back to main, you only have half the amount of
the reserve tank available.
2) Avoid to empty the reserve side while there is still (or again) fuel
in main left, because once you switch back to main the fuel will be
pumped over again in the reserve side in just a minute!
3) Be prepared to switch a few times between main and reserve to keep
the fuel as much as possible in the reserve side.
4) Be aware, especially with a 914, that the main tank will drain very
quickly once the reserve tank is no longer full.
5) If you don't have separate fuel sensors, fuel management will become
very difficult while flying on reserve: The reserve will empty more
quickly than anticipated, and once it is empty, you will find fuel again
in main, but as it is pumped over, it will quickly disappear and then
the reserve side has fuel again!
Frans
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