If there is one chance in a million that "BOTH" will cause fuel starvation, why use it. Use a low pressure transfer pump between tanks if you want redundancy use 2.
Georges B.
 
/-------Original Message-------/
 
/*From:*/ Rotary motors in aircraft <mailto:flyrotary@lancaironline.net>
/*Date:*/ 05/29/05 23:08:55
/*To:*/ Rotary motors in aircraft <mailto:flyrotary@lancaironline.net>
/*Subject:*/ [FlyRotary] Re: Fuel Tank Selection
 


Perry Mick wrote:
Dale:

With a BOTH function and such a low gravity head, the tanks may not feed equally, or worse, the engine quits while there is still fuel in one of the tanks, the same problem that Jim S. has described having with his Velocity sump.
If you have an EFI pump in each tank and both pumps are providing fuel to the EFI fuel rail at the same time, that would not be the same situation, there is no gravity feed from separate tanks to a common point in that design.  But if you lose a pump, you lose the fuel in that tank too.  That could get dicey when you get real low fuel in one tank, switch  on the other pump and that's when you find out it's failed.

I'm doubtful that this BOTH function was the cause of Paul's accident, agreed but I still think you want to avoid a BOTH function in a LEZ or other low-wing fuel system.  But it can get dicey if you have to use ALL the fuel in one tank or the other.

Perry

Bill,

  Thank you for that clarification. When I read Perry's
comments, I was wondering "why?" - because the system I'm
building has the functions: Left, Right, Both, None.

  Now I'm not so worried, because each high pressure fuel
pump draws from it's own tank and the only point of inter-
connection is where the lines join at the fuel rail(s).

  I borrowed the basis of my setup from Marc and Nadine
Parmalee's COZY:

http://www.marcnadine.com/fuelvalve.html

Dale R.

> From: "BillDube@killacycle.com" <billdube@killacycle.com>
> Date: 2005/05/29 Sun AM 02:03:31 EDT
> To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
> Subject: [FlyRotary] Re: Paul's Fuel System Error
>
> About a year ago I did a little "Google" research on the John
> Denver fatal crash. The bottom line appeared to be that one tank was empty,
> and the fuel selector was not fully turned to the other tank. (It was in a
> very awkward position to reach, and this may have also caused the pilot to
> auger in while attempting to reach it.) Thus, it was in the equivalent of a
> "both" position. This caused air to be drawn in to the fuel supply line
> from the dry tank. This, in turn, caused the pump to lose its prime and
> stop pumping fuel to the engine.
>
> As Perry mentions in his post, only a gravity feed fuel system can
> have a "both" type fuel selector. Low-wing aircraft that have negative
> pressure in the fuel lines from the tanks must NOT have a "both" position
> on the fuel selector, otherwise the pump (or the sump) will suck air if one
> tank runs dry (or if there is a leak in a fuel line.)
>
> This kind of makes you want to put a pump in each tank.
>
>

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