Good point Mark. This sounds like another good argument for the arrangement that I use. Always draw & return to the same tank. Refill from the other tank when necessary using a low pressure facet fuel pump. This eliminates the need to ever run the tank in use low and risk running dry (unless you are really OUT of gas). No fuel selector needed. Simple plumbing. Only requires a single return port. Eliminates that nervous moment when you wonder "Should I switch tanks now or use that last gallon of fuel in the tank & risk running dry". Tracy ----- Original Message ----- From: Mark Steitle Sent: Tuesday, May 25, 2004 4:12 PM To: Rotary motors in aircraft Subject: [FlyRotary] Fuel Pump Design 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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