asymmetric fuel feed was Re: REFLECTOR: Fuel Shutoffs

[Jim Sower]

That was my impression too.  It's the intuitive answer.  It's not been 
my experience.  I did all the stuff you mention FIRST, and then went 
further.  After all roads led to dead ends, I did the math.  That's when 
I discovered how small the numbers are.  Like the head pressure of 5" or 
6" of fuel is measured in very small fractions of one psi - as a 
practical matter, you can't measure it.  Any number of very small 
factors (like an undetectable leak in a filler cap) could make a major 
impact.  I had a list of stuff I did a mile long, to no avail.  I always 
ended up with asymmetric transfer (not always from the same side) on a 
regular basis.  I like simplicity, so I was loathe to go to all the fuss 
and bother of installing [de]selector valves, but I'm glad I did.  I'm 
still baffled by the whole thing.  I had a hideous problem.  I'm aware 
of a number of (particularly Velocity) people who also do.  But there 
are lots who don't.  No really satisfactory explanation has come to my 
attention yet.

I've concluded it's largely pure dumb luck ... Jim S.


Scott Derrick wrote:

> As I understand it, if you use the standard vent system of all tanks 
> and sump tied into one vent line and your lines are unobstructed there 
> are only two reasons for asymmetric fuel flow.
>
> 1.  leaking fuel cap. the low pressure over the cap pulling air out of 
> the tank.  Get a new O ring or use fuel valve lube on the O ring to cure.
>
> 2.) Your airplane is skidding/slipping when in straight flight, ie not 
> trimmed with the ball in the center.  This is the most common 
> reason.   Fix the skid by adjusting the correct rudder outboard and 
> the tanks will drain evenly within a couple gallons.
>
> stepping on the fuller tank with the same rudder for a short period 
> will equalize the tanks,
> adding more valves and lights to fail is solving the symptom, not the 
> problem......
>
> Scott
>
> At 11:06 PM 9/27/2004, you wrote:
>
>> If you don't vent the sump, the [marginal] gravity feed will be 
>> enhanced by the fuel pump sucking the fuel out of the strakes into 
>> the sump.  That said, if you have assymmetric feeding of the strakes 
>> (a common occurrence in these airplanes) once one tank gets empty, 
>> the sump will vent through that strake vent and the strake that still 
>> has fuel will have only gravity feed (which it has demonstrated is 
>> not working for it very well) available. Isolation valves would solve 
>> that problem.
>>
>> <... a turn the wrong direction on take-off (away from the fuel 
>> outlet) could all contribute to less than adequate fuel flow ...>
>>  Of course if you're in balanced flight (ball centered) the fuel has 
>> no way of knowing you're turning so fuel flow would not be affected.
>>
>> I had assymmetric fuel transfer from the strakes to the sump for the 
>> LONGEST time.  I tried everything (except 1/2" lines) to no avail.  
>> Finally solved the problem with electric "de-selection" valves that 
>> light a LED on the panel when they are closed.
>>
>> If you vent your sump, all bets are off .... Jim S.
>>
>>
>>
>> Chuck Jensen wrote:
>>
>>> Regarding a dual fuel shutoff, I understand there was some 
>>> work/testing done on this years ago that raises an issue that is 
>>> worth thinking about.  It seems Velo (or someone) did flow tests 
>>> from a single tank (with a low level of fuel in it) through the 3/8" 
>>> fuel line and found the flow rate was marginally greater than the 
>>> expected fuel consumption of an IO-540 engine at max power.  Given 
>>> that some installations are non-standard and less than optimal, 
>>> there was thought that with low fuel, a turn the wrong direction on 
>>> take-off (away from the fuel outlet) could all contribute to less 
>>> than adequate fuel flow unless both tanks were feeding.  Since then, 
>>> most have gone to 1/2" fuel lines, which should resolve the flow 
>>> capacity from each tank--but not the exposed outlet from a flight turn.
>>>
>>> So, 1) I don't know if there is anything to it!, 2) on the side of 
>>> caution, item #86 on the checklist would be to confirm both tanks 
>>> are on line.  With all that said, I wish I had dual isolation valves!
>>>
>>> Chuck
>>