REFLECTOR:Low fuel light

[Dave Black]

John,

> On my plane, each wing has an overflow line exiting the bottom 
> of the wing with a 90 degree bend to face forward, exactly like 
> one would see on a Cessna.  I know it's an overflow line because 
> fuel pours out if I try to fill my tanks to the very top and the 
> wings are not completely level. 

This is the original factory design of the Velocity fuel vent system, before
the manifold was introduced. I call it the 'black magic' vent system, because
it relies on odd twists and perforations in the vent line to somehow keep fuel
onboard while allowing vent air in. In fact, it works best as an overflow
drain -- exactly as you have observed. 

Since each tank had its own separate vent line, it's easy to see how fuel
imbalance could occur. In fact, the only way an imbalance would not occur is
if you somehow were able to pressurize both tanks equally. A tricky job when
you have separate lines. To eliminate the fuel overflow issue, it was
necessary to bring the vent lines UP from the fuel tank -- which meant into
the rear cabin area. Hence the concept of tying all the vent lines together at
the manifold was born. 

This was SUPPOSED to fix the all-too-common fuel imbalance problem. But in a
number of cases, it did not. Most common causes are leaking fuel caps (can
easily overpower the vent lines), vent lines which have a low spot (trapping
fuel and blocking the line), and flying in a skid. 

As to the low fuel sensor, it has often been mentioned that the float sensor
supplied by Velocity can, over time, absorb fuel and thus quit floating. This,
of course, will give a low fuel warning. The other likely cause is mounting
the float sensor too high in the sump tank. Remember that under ideal
conditions, fuel in both main tanks and the sump tank will be at the same
level. In most installations, I believe the top of the sump tank is several
inches above the bottom of the main tanks. Thus, when you are using your last
couple inches of fuel, you should expect to see the level of the sump tank
drop as the tank level drops. Again, if the float sensor is mounted high in
the sump, it'll trigger. 

I would think it best if the top of the sump tank were below the bottom of the
strake tanks. Then the sump would stay full right up until the mains were dry. 

Best,
Dave Black
Shortwing RG