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The attached
drawing is my idea of a simple fuel system for the 9a that I am building, with
Aux fuel tanks akin to Berne Kerr’s Project. I would like some feed back as to what
problems this arrangement might pose, and what the remedy would be for such
problems. The plan is to use large
diameter pipe between the main fuel tanks so as to allow the fuel to
equalize
Bob Perkinson.
Fuel system design hasn't come up in quite a
while. But since a large portion of accidents are fuel related, I
thought this would be worthy of discussion.
I have experienced a
failure mode during ground runs on my Lancair ES with 20B that seems to be a
problem with other types of installations as well. I recently read a
first flight report by a professional test pilot, David Allen, that almost had
to set a Lancair ES, N711RG, down on I-70 due to the inability to restart the
engine after running a tank dry. (See www.geocities.com/daveandjj for the full
story.) This was a certified fuel-injected engine installation.
Also, the Subaru guys (I have been lurking on the Eggenfelder Subie site
today) had a similar situation, resulting in some glider time and a dead-stick
landing.
The problem is that after exhausting the fuel in one
tank an airlock forms between the fuel pump(s) and the fuel pressure regulator
that the efi pump cannot overcome due to lack of fuel in the supply
line. This is what I have experienced with my installation on ground
runs. I can't get the efi pumps to re-prime unless I momentarily break a
line loose between the pump outlet and the pressure regulator. As soon
as I do that the pump will re-prime and all is well.
For the
record, I have two of Tracy's efi pumps, with two efi filters, hooked in
parallel. They are mounted low on the fire wall. Upstream is a
gascolator and an Andair 6-port selector valve. Downstream of the
pumps/filters is a map sensing pressure regulator. There is a -6AN going
to the fuel rail, and a -4 return line back to the Andair valve/tank.
I
was curious if anyone else has had this happen and what they did to resolve
the issue. I have come up with two possible solutions.
1. Tee off the pump output and put an electric solenoid valve in
the line and tee the outlet of the solenoid to the fuel return line, past the
regulator. Push-button switch on panel would activate the
solenoid. If needed, a second or two of activation should relieve the
air-lock and allow the pumps to re-prime.
2. Install a bleed line
around the pressure regulator with a small metering orifice (.020-.030") that
would bleed off any air that might get trapped. (This appears to be the
solution the Subaru group is focusing on). This would be a full time
bypass. This seems to be the simpler solution.
3. Install a
low pressure, high volume fuel pump in one, or both, of the wing tanks.
Procedure would be to always keep reserve fuel in this tank.
Any
comments from seasoned flyers would be welcomed.
Mark S.
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