It is well known that the standard Europa sight gauge is less than
satisfactory for several reasons. It is very dependent on aircraft attitude
and is difficult to see both due to position and rapid discolouration of the
tube material. Its only advantage is cheapness and it can be calibrated
directly, at least in one attitude.
The following solution appears to have all the advantages and only one
disadvantage.
It uses the "pipette" principle. A clear rigid tube approx. 10mm dia. is
placed vertically in the tank reaching from near the bottom to around 2
inches above the top, entering via a grommet or O-ring which enables the tube
to be withdrawn vertically into the cabin. The bottom of the tube is open and
the top of the tube carries a cap (of the type seen on liquid soap bottles)
which serves two purposes. 1) it keeps the cabin free of fuel vapour
without the need for vent pipe, 2) when momentarily opened, it equalises the
level in the tube to that in the tank. The maximum withdrawal ( 15", tank
empty) is easily available in the 22" above the tank.
Thus when the cap is reclosed and the tube withdrawn, the fuel column will
appear, trapped in the tube, where it can be read against a calibrated scale
on the tube. It is immaterial whether the bottom of the tube leaves the fuel
or not. You just have to see the meniscus.
Perhaps the neatest most visible and durable scale is to attach a common
measuring tape,* fixed to the top of the cockpit module, with the free end
attached to the top of the tube. The rear of such rules is usually unmarked,
so this surface can be calibrated directly in litres or gallons or both. (*
Stanley "Eurolock" seems appropriate, though you don't need the lock).
Positioning.
The deepest part of the tank is either side of the tunnel, and the only
reason to have the tube reach lower than the level of unusable fuel, is to
avoid you pulling it right out when measuring nearly empty levels. So only
the extreme outside is barred by curvature of the fuselage (and doors).
Laterally, just inboard of the headrests is best. Longitudinally, zero
sensitivity to attitude ( for the uppermost (largest) part of the tank) is
obtained 6.5" from the front of the headrest. i.e. just forward of the rear
of the lowest part of the tank. By arranging for "Main" to be passenger side
operating on the other side would enable reserve side to be checked before
take-off. This could be positioned in the same place but angled for least
attitude sensitivity (in the lower part of the tank, or alternatively placed
behind the pilot headret.. This would be difficult to activate and read in
the air if solo, but a pilot using reserve should be preoccupied with
immediate landing.
Materials.
The tube should be clear and rigid - glass is clearly too frangible so
perspex is the ideal. Nylon is also possible with thin enough walls. An
4mm. internal diameter is optimum but the system has been tested up to 6mm
internal dia. Above this it is difficult to keep short columns from running
out of the tube. If you can't find a suitable grommet then punch a ring from
that black stuff you have removed from your trailer ! and superglue it into
the tank top.
Oh yes, the disadvantage....... you have to DO something, rather than just
look at it, but it takes about the same skill as picking up a pencil, so I'm
sure you'll cope.
Yes, you could make it remote reading/electronic/servo-automatic etc. but why
bother ? As described it costs peanuts, component count is 4 ! (and that's
not factorial 4), has a giant scale length and is dead reliable (at least
while your squeazy cap doesn't leak (:-))) - if so your finger would have to
be pressed into service.
Approval applied for, dxf available if required..
Graham Clarke G-EMIN
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