>Perry -- How is your sump supplied... gravity?

>Mark

I guess now is a good time to tell some of my fuel stories.

I've flown for 460 hours now in almost 5 years, and have yet to log any
glider time.

I've had a couple close calls though, related to fuel flow. Hopefully this
will help someone.

Yes, the selected main tank in my Long-EZ drains to the sump by gravity. But
there is only about a 1 ft. head there. I really need to replumb the fuel
line into the sump so it enters the sump at the bottom and not the top.

I have two vents to each main tank. One comes from the center of the tank
per plans, the second is near the top of the tank when the nose is parked
down. So a total of 4 vent lines for both tanks, that exit at the top of the
fuselage just ahead of the firewall behind the passenger's head. These point
forward into the oncoming air, but are only about 1/4" above the fuselage so
they are really within boundary layer air.

I have a fifth vent line from the sump tank near these four, but it had a
shallower bend in it and was higher, pointing into the airstream maybe 3/4"
above the fuselage, so it was getting more ram air.

The first sign of trouble happened a couple years ago, flying locally near
the airport and around 3500', so even if the engine had quit, it would have
been no problem. The engine hiccuped. There is not a lot of room on my
instrument panel so I have side panels, a lot of engine instrumentation and
circuit breakers on the left side and intercom and avionics circuit breakers
on the right side. So my fuel pressure gauge is on the left side. So when
the engine starts running rough, I have to think "fuel-related? look at the
fuel pressure". Well, I saw that fuel pressure was low. Throttled back and
switched tanks and it cleared up. What was happening? This only occurred on
very cold days, after I had been flying level at full throttle for an
extended period, and when the selected tank was very low, less than 5
gallons. The vent into the sump tank was getting more ram air pressure than
the vents to the main tanks, and with very little head because of the low
fuel level in the main tank, fuel flow into the sump tank stopped. I fixed
this by crimping the sump tank inlet mostly closed, leaving a small slit for
an opening.

Fast forward a year or so, and it happened again one very cold morning! So I
thought, I'll just close off the vent completely to the sump tank. I put a
rubber cap over the sump tank vent. According to Ed Anderson, this would
allow the high pressure fuel pumps to actually suck fuel from the main tank,
so I thought I would try it. I flew it this way for months with no problems.

Last February I flew to Puyallup Washington. They have a relatively short
3600' field there with one single runway. I landed there early in the
morning when it was cool, and parked it nose down as usual. Met my cousin
and went to the WA State Airfair. Early afternoon, sun is out, it has warmed
up considerably. I go out and hop in and taxi out for takeoff. Very
fortunately I had to sit at the end of the runway waiting a couple minutes
to takeoff because of two landing airplanes in the pattern. Finally, it is
clear and I take the runway, push in full throttle, the engine surges to
full power, and then dies! This has never happened before! I restart it and
it runs a little and dies. I do this a couple more times and am able to get
it off the runway at a taxiway exit. Park it and try to figure out what is
going on. I notice fuel pressure is low. This is strange, how could both
fuel pumps fail? Eventually I think of the cap on the sump tank vent, and
pop it off. I hear a "poof" sound and I hear fuel flowing into the sump
tank. That was it! Apparently parked nose down outside while the ambient
temperatures were increasing, allowed a big air bubble to form in the fuel
line into the sump tank, and because the vent to the sump tank had been
purposely blocked, fuel stopped flowing into it. I am really glad there were
some planes in the pattern and I had to wait for takeoff. Otherwise the
engine probably would have quit right after takeoff, at the worst possible
time, when there were no landing options.

When I got home, I lowered the sump tank vent inlet to the same low level as
the other vents. This way the sump tank will not get higher ram pressure
than the mains. And of course, I now maintain the sump must be vented!

My sump only holds one gallon. I really need to put a fuel level sensor
inside the sump tank, to give early warning that it is not full and possibly
draining.

Perry


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