The fuel vent system on my Mark I is built per (I believe) the
latest info and includes: vent pipe from fitting to front of
tank, immediate rise of 10 inches through the baggage area, and
two 1/16 holes at the back of the vent just above the fuselage.
The vent line runs downward from the front of the tank to the
vent outlet from the tank, then upward to the vent fitting on the
fuselage. This allows a slug of fuel to collect in the low part
of the vent line during trailering, probably as a result of
sloshing on the road.
I trailer to the airfield and fill the tank from cans on return.
During the filling process there is a gurgling/rattling sound
---From the tank vent which ceases when the fuel level covers the
vent end inside the tank. Very helpful, sort of like the fill
whistle used on home heating oil tanks -- no need to watch the
gauge, just listen to the vent.
However, there seems to be a problem with my current setup. I
use a Mr Funnel conductive plastic funnel for filling to prevent
water in the fuel. I noted fuel around the filler opening and
initially assumed it was my poor technique causing slopping out
of the Mr Funnel, which constrains the flow rate slightly. More
care in filling didn't solve the problem but careful observation
provided the answer: the gurgling of fuel in the vent line pushes
a mist of fuel out of the two vent holes at the rear of the vent
inlet just forward of the filler opening on the Mark I. Thus,
the filling process causes a small quantity of fuel to be sprayed
and vaporized near the filler area. In addition to being untidy
this could be a safety issue, i.e. there has been a report of a
fire in a Europa possibly due to a static spark at the filler
opening -- this fuel spray could cause a flammable mixture in
exactly that area.
To minimize fuel spray from the vent during filling I am
considering adding an expansion volume in the vent line, the
concept being that air being vented pushes droplets of fuel along
through the vent line due to the air velocity. By adding a
section of larger diameter to the vent line the velocity in that
section will drop, reducing its ability to carry droplets which
should then collect and run back toward the tank. Flammable
fumes will continue to the vent outlet but droplets and spray
should be reduced or eliminated.
Two alternatives for the expansion volume come to mind. Simplest
is adding a fuel filter in the vent line since filters have a
much larger diameter than the fuel line. Alternatively, cutting
a 4 inch long section from the vent line and replacing it with
larger diameter plastic tubing which slips over the existing
tubing, securing it with hose clamps should also work. This
could optionally incorporate a one way valve by ensuring the new
section is approximately vertical in flight, making short radial
slots in the cut end of the small vent tubing which leads to the
tank, and installing a suitable size ball bearing inside the
large tubing. In use, vapor flow around the bearing would occur
via the slots (where the material between the slots would support
the bearing); in the event of an incident in which the aircraft
overturns the ball bearing would fall to the top and block the
tubing to the outlet (no slots here), minimizing fuel flow from
the vent in the inverted position.
Is my vent setup the latest? Has anyone else experienced fuel
spray from the vent line while filling? Solutions?
John N44EU Newtown, CT
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