FINALLY, A SIGHT GAUGE THAT WORKS!
This sight gauge really works, the vent tube
keeps the top turtle deck clean,
and it doesn't burp the gas back at you during refueling.
|Click to enlarge.
The Europa vent system puts the vents on the top of the aircraft just aft of the lowest area of pressure. A full tank
with a roll to the right and you have fuel streaming aft through the vents. The low pressure area on the top of the aircraft
pulls the sight gauge from its true value at low speeds, and at high speeds makes it read low. So, why not move the vent to
the bottom, keep the aircraft clean, and any over fill will drain the fuel to the ground. Make the vent as the plans call
for and just put it on the bottom. I put it on the forward side of the rear bulkhead for a tri-gear and on one of the baggage
bay floor supports for a monowheel. We make one vent tube normally, but you can use two. Just don't hook it directly to
the sight gauge. You must use an FS09 for your vent fitting. Vent the sight gauge to the aluminum 5/16 outlet and the bottom
of the site tube to the Tee off the bottom main drain line. Never the feed line. Attach the vent line to the brass 5/16 nipple
of the FS09 (the 5/16 nipple is attached to a 1/4 inch tube copper pipe as per the extended range tank setup) then to a Tee
located near the top of the filler neck, then over the top of the neck and down through the floor of the baggage bay or into
the tri-gear support to where you put the vent. Finally, consider putting the sight gauge on the seat back bulkhead, between
the pilot and passenger. That way acceleration forces are minimized.
As for the placement of the
sight gauge, it is imperative that the vent side of the sight gauge be completely free of fuel. Any fuel trapped
in the sight gauge will affect the reading. This is a common problem if during fuelling, the cobra neck is filled
to the top. As you can see in the drawing fuel will vent overboard, but not in the vent line, if it is looped and twisted
through the cockpit module.
As shown in the drawing, it does not give a desired vent
line path. I prefer my sight gauge to be on the inboard side of the passenger seat. The vent side travels
up to the top of the module then into the headrest and up to the cobra neck. In tri-gear installations, it can
be run on top of the fuel tank between the tank and the fiberglass if room allows. In mono wheel aircraft,
it is better to run the vent side up into the passenger headrest, on an uphill all the way to the cobra neck. If clear
polyurethane tubing is used in the vent line, it is easy to determine if fuel is trapped. In our installations, we put
the vent fitting in the cobra neck for the sight gauge line in a position where one can put a small piece of tubing on the
vent line with the fuel cap removed and blow air by mouth into the vent to purge any fuel that the refueller may have forced
into the vent line of our sight gauge. If the builder is careful to bring the vent line down on a consistent downward
slope with no loops in the vent line, the vent line will self clear.