An update, fuel tank vent restriction.
We trailered our Europa home from Colorado and commenced with the annual
condition
inspection. As part of that, we sent out the carbs for overhaul - no problems
noted. We replaced all the 4mm turbo control hoses per the 5 year requirement
- no defective hoses were found. The carb sockets were updated to the latest
specification 2 years ago and inspection showed no cracks. A few hours
prior to this trip we had inspected all fuel filters including the ones in the
electric pumps. No debris was found - we haven't had debris in the filters in
several years.
What we did find were restrictions in the fuel tank venting system. The result
of these restrictions is that the fuel flow was limited to the amount of air
that could get by the restrictions which was way less than 5 to 6 gallons per
hour that we normally use at cruise!
We have a modified fuel vent system, similar to that described on Custom Flight
Creations website. The tank vent (Tygon SE 200 fuel line tubing) leads to the
top of the cobra where it is connected to the cobra. From the top of the cobra,
a vent line (more Tygon SE200) goes over the head of the cobra to the belly
of the fuselage. The sight gauge vent and long range tank vent join together
and go over the top of the cobra independent of the fuel tank vent. All three
vents join together a foot below the top of the cobra and then a single tube
goes to the belly vent. The vent on the belly is fabricated similar to the build
manual instructions for the original factory vent on the top of the fuselage
- it is a piece of 1/8" tubing bent to face forward into the airstream and
has two 1/8" bypass holes in the rear in case the main opening gets clogged.
In Colorado we discovered the belly vent was packed full of debris. There was
even debris sticking out of one the bypass holes. We managed to clear and clean
that vent tube using carb cleaner in an aerosol can. We did this before the
test flight. It was really plugged because it is difficult to access on
preflight
and difficult to tell if it has debris. In our case we hadn't blown it
out in several years and between a little oil from the engine and operating
mostly
off grass strips a oily mess accumulated. We have added cleaning the belly
vent with aerosol cleaner to our condition inspection checklist!
In Colorado we also noticed that the Tygon tubing going over the cobra looked a
little kinked, but figured it hadn't changed or gotten more kinked during this
trip. But now we think that it did. On this trip we filled the tank all the
way up into the neck of the cobra to have the maximum amount of fuel on board.
We also added a quart of oil about 10 hours before all these problems - but
that actually overfilled the oil tank and caused it to blow by and out the
oil tank vent onto the belly of the airplane helping plug the fuel tank vent.
With the belly vent blocked or nearly so, there was very little air available
to replace the fuel being drawn from the tank, so the air space in the top of
the tank and in the vent line was being put under vacuum which caused the tygon
tubing to kink more.
We did one test flight in Colorado and the engine ran normally. However that
was after using a few gallons of fuel doing all kinds of ground tests, so there
was more air space in the fuel tank and the tank walls could collapse a little.
But we topped up into the cobra again before our last departure and subsequent
precautionary landing.
So we have replaced the tygon tubing with aluminum tubing over the cobra head to
prevent kinking in the future. We also moved the return line from the fuel
regulator from the bottom of the fuel tank to the top of the tank because our
tech adviser advised that certificated aircraft do not return fuel to the bottom
of the tank.
We've only made a couple test flights since (runway is snow covered now) but
they
were normal.
The big learning experiences: be sure the fuel tank vent on the belly is clear,
be sure there are no kinks or restrictions in the fuel tank vent system and
clean the belly tank vent thoroughly every condition inspection.
Funny how we always thought about the fuel tank vent system in terms of space
for
fuel to expand while sitting in the sun and in terms of letting air out of
the tank during refueling, but we never thought about it's most important job
of letting air into the tank to replace fuel consumed!
Jim & Heather
N241BW
XS Mono, 914, Airmaster
425 hours
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