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Old 16th May 2017, 02:24
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9 lives
Join Date: Jan 2008
Location: Canada
Posts: 601
Most 100 series Cessnas have either one or two venting caps. They will allow air in (as fuel is burned) but prevent fuel escaping. I addition (but not the 177 Cardinal) will have one vent projecting down behind the left strut. The position of this is very important - too high or low, and it will suck fuel out of the tank. During a walkaround, have a look at the tube, and observe if the painted portion of the tube lines up nicely with the rubber grommet. If it is wrong, consult the maintenance manual. The C 177 has two vents, which are on the trailing edge, between the aileron and the wingtip. They're squashed out of round tube.

The vent tube(s) are connected to the tank(s) with a one way check valve, which allows air in, but prevents most of the fuel which might escape. However, these one way valves have a little hole drilled, so a small amount of fuel could escape. This is why a 100 series Cessna parked with the left wing low, and full tanks will dribble out the vent. It'll keep dribbling, until the fuel level goes below the tank vent (many liters) so best not leave the plane that way.

The air spaces in the left and right tanks must be interconnected, if the fuel selector is to have a "both" position. This is why Cherokees do not have a "both" fuel selector position, as it is not possible to interconnect the tank air spaces with the dihedral.

It is wise to assure that the vent tubes are not obstructed (some insects really like to make mud nests in them. Failure to vent causes bug problems. I was transferring fuel by pumping from my wingtip float tanks [into the main tanks]. The wingtip float fuel tanks do not have vented caps, as those tanks do not provide fuel directly to the engine, they only top off the mains. I notice that the whole side of the wingtip float had buckled in . I landed, and opened the cap - whoosh pop, and the float restored itself (thank goodness! 'Could have been an expensive repair! Bug mud nest in the vent tube. I drilled four tiny holes so I could string fine lockwire to form an "X" across the opening of the tube - problem solved.

For Cessnas with bladder tanks (some 180 series), venting is extra important. If the tank does not vent well, it'll unsnap itself from inside the wing, and collapse. Then wrinkles in the tank will prevent using all of the fuel, and it will not be possible to fill the full volume next time. Resnapping them into position is an unpleasant job .
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