REFLECTOR: Re: Reflector Digest, Vol 9,Issue 68-Fuel Feed-blinking low fuel warning

[Alex Balic]

would it be a good idea to install a check valve just below the fitting that
connects the sump vent to the tank vents ( mine has a cross fitting that
continues up to the roof, then down to a hole in the floor- if the check
valve is located on the line from the sump, it would allow air to escape
from the sump, but not allow it to drain down due to high demand from the
engine- just a thought...... in a rollover, the line running to the roof and
down to the floor would still prevent fuel from siphoning out.
  -----Original Message-----
  From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org]On
Behalf Of Jim Sower
  Sent: Thursday, February 17, 2005 9:02 PM
  To: Velocity Aircraft Owners and Builders list
  Subject: Re: REFLECTOR: Re: Reflector Digest, Vol 9,Issue 68-Fuel
Feed-blinking low fuel warning


  <... The only way I could see that happening is if the flow from the mains
is somehow restricted due to inadequate venting, and that the fuel pump is
pulling fuel out the sump tank and the sump vent line is letting in air into
the sump lowering the fuel level. ...>

  Bingo!!
  And once your sump fuel level falls low enough to light the low fuel
light, you never know how much usable fuel you have until that light goes
out.  The mains might be providing 95% of demand in which you'll make it
home with a gallon or two in the sump.  Then again, they might be only
providing 50% of demand in which case you flame out in under an hour.  If
the strakes are only providing 25% of demand, you've got about half an hour
of fuel.
  And you have NO WAY of knowing which situation you're in until either you
get where you're going and LOOK at the sump, or the fan goes quiet.

  Go ahead and ask me how I know this ... Jim S.


  Chuck Harbert wrote:

    To John Dibble:

    This is very strange that it only occurs intermittingly at high
altitudes, unless it is just a intermittent bad electrical connection or
switch. Do you have the same type of low fuel switch that I have ( mine is
green and floats in the sump tank)? I installed mine on the pilot side of
the sump near the top. I didn't quite understand your description and thot
maybe it was a different type switch. Are you sure the warning light itself
isn't just loose in it's socket?

    I'm trying to understand what changes at higher altitude that would
affect the low fuel switch. The only thing I could think of was that
atmospheric pressure drops, so the sump tank could possibly expand a little
I guess. Do you have a sump tank vent line and the main tank vent lines per
plans?

    I'm assuming that you have pretty much ruled out the possibility that
the sump tank is partially emptying and that the low fuel switch is working
properly? The only way I could see that happenning is if the flow from the
mains is somehow restricted due to inadequate venting, and that the fuel
pump is pulling fuel out the sump tank and the sump vent line is letting in
air into the sump lowering the fuel level.

    Chuck H



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