REFLECTOR:Low fuel light - NO!

Brett Ferrell reflector@tvbf.org
Wed, 23 Jul 2003 18:46:22 -0700


Please, please, please do not do this (install a vent shutoff valve)!  Air must enter the system as the fuel drains out or 1 of 2 really bad things will happen.
1)  Pop-Bottle plug effect - where the gas glug-glug-glugs to your engine, or stops altogther as air cannot replace the lost fluid-or-
2)  The tanks will crush as the fuel pulls vacuum on the tank and the outside air pressure pops the strake open

Having seen a 30,000 industrial tank get crushed like a soda can from a plugged vent & overflow line, I'm horrified by the idea of this happening in flight.  Allowing air into the tanks is the primary purpose of the vent, and why you should install the check valve to allow cabin air into the tanks (but no gas-vapor into the cabin) should the external vent get blocked by ice.

If there's an adequate fuel level in the stakes, an air bubble should not be able to form in the sump, as the head pressure from the strakes should fill it to the level (in the vent line) of the strakes (fluids seek their own level and all).  If a bubble were to form it would indicate that the fuel pump is pulling the level faster than the tanks can replenish it.

Brett

Quoting Bob Kuc <bkuc1@tampabay.rr.com>:

> Re: REFLECTOR:Low fuel light 
> I was think about that also.  However, if you are on 
> the low side of > fuel and then make a turn, the fuel on the inboard
> stake would rush to > the end.  Would that not introduce air into the sump
> tank that you would > have no way of relieving because you have the vent
> blocked off?
> 
> Bob
>   I plan to install a manual vent shut-off valve
> between the sump tank > and the vent manifold.  The result will be that the
> vent system can > allow the main strake tanks to drain, but can't
> introduce air into the > sump tank.  I have a theory that the vent line to the
> sump tank causes > more problems than it solves, once the initial "burp"
> of air is allowed > to escape from the sump tank (before engine start).
>