jmk wrote:


If it is a simple fuel shutoff issue, then just mount the appropriate
valve for which ever you consider (or your FAA guy considers) less
critical. That is, normally open or normally closed. Loss of
electrical system reverts it to the "normal" position.

I doubt that they'll approve it. Too dependednt on the system
one way or another. If you had a fuel leak in flight that resulted in
an engine compartment fire, you'd need to shut everything down, fuel
and electrical, in order to have any decent chance of making a
survivable forced landing, and you wouldn't be able to shut the fuel
off. If you left the elecrical on, the fire would shut it off for you.

The only other real failure mode is that it sticks... and hey, it stays
in its last position.

Depends on what you consider most critical... if this is a "fuel tank
selector" thing you are trying to create, then two of them - mounted
NO, and held closed by the electrical system. Mount a mechanical 90
degree cutoff valve somewhere for those times when you want to work on
things on the ground. In flight you have a left-selected, or
right-selected, or (lose your electrical system) both.

If it's a low-wing airplane, you don't want a BOTH position. If
one tank runs dry before the other, the pump will happily suck air from
the dry tank instead of fuel from the one with fuel. FAR 23.951
addresses this issue in certified airplanes.

http://www.airweb.faa.gov/Regulatory...2?OpenDocument

There are a bunch of FARS that are worth paying attention to when
designing a fuel system. I've been around homebuilding since '73 and
have read too many accident reports involving homebuilt airplanes that
fell down because their builders didn't have the information to alert
them to possible problems.
Fuel tank venting is one of the really big killers. In a
gravity-feed system using two or more tanks fed through a valve that
has a BOTH position or though two valves that may be ON at the same
time, the tanks must be vented together to keep pressures equal lest
uneven fuel flow results. I was involved in a Glastar project, and that
airplane had two wing tanks with separate vents. Uneven fuel flow was a
problem, and if the pressures had been far enough apart the fuel from
the tnak with the lower pressure would have had trouble reaching the
engine once the other tank ran dry. The pressure in the dry line will
keep fuel from dropping through the other line.
The Glastar's vents were also at the wingtips, and if the tanks
were full or nearly full, the fuel would run out of the low vent if the
airplane was the least bit off level. Fuel would crossflow through the
system from the higher tank into the low tank and a lot of fuel ended
up on the floor.
The lesson: copy a certified system.

Dan