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At 13:06 2004-05-25, you wrote:
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.
Hi gang, I'll toss in my answers as a Control Systems Engineer.
I like things in this order: Simple and effective, complex and
effective.
Simple and effective:
Item 2 above fits the bill. Since the fuel pressure regulator is
always bleeding off fuel, adding an extra bleed simply makes the
regulator valve operate in a different position. This only
complicates things by adding a couple of extra fuel line
connections. Conversely you could make a TEE fitting out of steel
or Al stock that had a bleed built in and have no extra fittings.
Now, if you have an EFI pump soft-failure (slowly looses pressure) you
will end up with a low fuel pressure condition sooner. Most of the
E-Subaru guys have a low-fuel-pressure switch for an auto pump fail-over
circuit. This starts the backup pump and throws up an alarm.
System is tested on every startup.
More complex and effective:
Take item 1 and loose the push button switch for a low pressure switch
and an alarm light. No need to add pilot work load to the
situation. The solenoid does add the extra complexity and potential
failure mode. The alarm circuit always gets tested on
startup.
Not effective:
Item 3 won't work because the low pressure pump will not make enough
pressure to open the pressure relief on the fuel rail so you're still
blocked. It MIGHT push enough into the high pressure pump to allow
it to catch but not guaranteed.
I'm going with the bypass bleed and low pressure fail-over/alarm
circuit.
YMMV
Mike, PE
Mike McGee, RV-4 N996RV, O320-E2G, Hillsboro, OR
13B in gestation mode, RD-1C, EC-2
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