Fuel venting on your averge GA spamcan
 

Am I correct in thinking that your run of the mill cessna 172 fuel system is vented by both a check valve on a dedicated breather line and thru the tank gas caps? Where the check valve will let air into the tank but not let gas out?

Had the experience once of flying in the uprising air in front of a cold front in a 172 following refuelling to "full". Level flight, throttle at idle and at Vne, with fuel continually venting from the vent behind the left strut. Can't comment on the 172 system, but see from my book shelf that other high wing Cessnas vent through both the fuel cap and a check valve on the vent line.

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 :eek:. 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 :(.

My 'after landing' check in a 172 is to turn the fuel to 'R' tank and leave it that way until starting again, even if the a/c is going to sit in an allegedly level hangar.
With a 150, which only has a 'both' or 'off' option, if you fill to full then leave it in the hangar, best put a dustbin under the behind-the-strut vent. Personally, I leave the tanks on a 150 no more than 2/3 full, then top off immediately before flying if I really need full tanks.

TOO

My RV10 has two fuel vent lines, one from each tank. No check valves. Fuel cap is sealed. Fuel likes to piss out if its overfilled.

The reason I'm asking is I still have the tank pressurisation issue I raised here a while back but have still not got to the bottom if it. I talked with the factory and they suggest the issue could be the check valves are installed vertically and need to be re positioned horizontally (or vice versa, I cant remember what they said till I check) as they work better in one position as compared to the other. Another thing that has got me bothered is my previous Maule an M7 had the airspace between both mains connected, my M4 doesnt, this plane was 1 of 12 build in 2007 as a 50 year celebration of the original M4 . Maule has decided to put the this M4 back into production for 2017 but the FAA have mandated a connector pipe between the mains that my "limited edition" doesn't have. I wonder why the pipe was mandated. My old fuel system mains and aux (with xfer pumps to move fuel from the aux to mains) on the M7 was very predictable, alternating between left and right every 30 minutes with expected associated drops in quantity indicators, with the M4 it bizarre even when Im running on the left it seems to pull from both. Something aint right

There are two things to do to reduce fuel loss, first, as already mentioned here use left/right tank selection to prevent movement from the right tank to the left (the one with the vent).
Secondly, it has always puzzled me as to why at many airfields there seems to be a habit of filling aircraft until they are full when the plan is to do a local trip or circuits. A standard 172 has endurance of 4+ hours, put full fuel in and three big adults and you may well be overweight. I digress slightly.........if you fill anywhere near the top (particularly on a warm day) the cold fuel will then warm, expand and pour out of the vent.
Personally if I need lots of fuel I fill the right tank and "nearly" fill the left then use the left tank first until the trend indicator (fuel guage) suggests a drop before using the right tank.

We always try and fuel plan for a day's flying, including the first trip the following morning. What we DON'T want to have to do is to taxy the a/c in the morning from the hangar to put fuel in for the first trip of the day. It wastes time and wear and tear, so the last trip of the day is a 'hangar' flight (as do glider clubs) and the first trip ideally is with the student helping getting the a/c out of the hangar and doing the Check 'A'.

TOO

I have fitted several of the kits below which stops the venting problem, its available for most high wing Cessna models. My current Aerobat has one on and even totally full to the brim does not lose a drop!

Pays for itself rather quickly

McFarlane Part Number MC0400311-119 FUEL VENT LINE, LH-Std ( replaces 0400311-119 )

They should all have 2 vented Caps as per an AD that applies to pretty much every Light Single Cessna.


Wrinkles In Cessna bladder tanks are also a big issue because they will trap water that will not be removed by draining fuel from the quick drain.

Yes, BPF is correct. My recollection was foggy. It's Canadian AD 79-10-14 R1, I did not find an FAA equivalent. I recall the AD coming out, and my assisting in changing out caps at the flying club. Our instruction was to replace the unvented cap on the right tank of the 150's and 172's (I suppose the rationale was that the left tank was already vented, and they wanted to save the cost of second caps per plane). When I bought my 150, I changed both caps, just because I inherited two off a wreck, so I did not give it more thought after that.

The fuel cap AD 79-10-14 R1 was issued by the FAA

It is also referenced on the EASA AD site.