Fuel Vent Question

[Gary Snell]

I'm doing the Harer STC upgrades for an 85-8 conversion and guess I need to comply with these fuel vent mods for whatever they are worth. The STC drawings show the vent lines running to the tank filler necks, but that would suggest that the lines run across the top of the tank that is flush with the fabric. Seems like a hose nipple in the side of the tank just below the top would be a more appropriate place. Also, are folks typically running a new connection in the top of main tank or connecting to the filler at that end. Any comments from those with the experience would be appreciated.

Thanks
Gary Snell

[barnstmr]

Re: Fuel Vent Question

I have not seen the Harer STC drawings. But I am familiar with fuel tank vent design practice. One thing to bear in mind is that the position of the vent could affect the fuel tank capacity. If you position it too low, it will allow fuel to drain out after fill. Typically the filler neck is positioned so that it will extend slightly into the tank to ensure the tank cannot be filled beyond 98% of the tank volume. This ensures the FAR requirement for allowing for 2% expansion space. Any fill beyond the vent will drain out.

[Gary Snell]

Re: Fuel Vent Question

Terry, thanks for the info. After closer inspection, it appears that the tank is low enough to just get a 1/4" vent line on top of the tank. Only problem is my filler neck is too short to get the cover on if there is a line attached to the side of it. Guess I need to get a longer neck for the tank.

[Ragwing nut]

Re: Fuel Vent Question

I think it is pointless to vent the wing tanks. They are aux fuel only and gravity feed to the main tank. The reason for the main tank is to maintain positive pressure at all times so the head pressure requirement is met for fuel flow. However any changes to the STC will need to be documented on your 337 and you don't want to argue fuel system requirements with the FAA. Some tanks have a vent port on the back side of the tank which is what I used since there is little to no room where the drawing calls for it.

Mike

[Gary Snell]

Re: Fuel Vent Question

I agree that the wing tank vents are pointless but as you say Mike it's probably an argument not worth trying to make. With regard to the fuel filler neck, I need to put a longer neck in to accommodate the vent line but I'm not clear on how it's attached. I thought these were normally secured with a threaded weld flange to accommodate the steel neck. Mine seems to be press fit with a sealant or solder around it. Is this normal?? If I weld in a new flange it looks like it will interfere with the vent connection where the neck threads in.

[N96337]

Re: Fuel Vent Question

The last 2 that I did didn't need any longer neck. We just drilled the hole and soldered it in...worked fine. Personally, I don't see why everyone makes such a big deal out of following directions.... it's part of the STC, and has to be there to be in compliance, so why get all hung up on it??? It's not like it slows the aircraft down or does anything negative to the aircraft.
John H.

[Gary Snell]

Re: Fuel Vent Question

Although I (and others) question the technical merits of having the vent line, I'm not "hung up" on the requirement to do it. My problem is the filler necks on my tanks are too short to get the lines in and still be able to put the cap on. Maybe a different cap will work with shorter rim will work. My question is how are the necks secured in the tank since appears I may need to remove them and install longer ones.

[Ragwing nut]

Re: Fuel Vent Question

Steel tanks they were soldered on and aluminum tanks were welded if not mistaken. I agree it is a PITA if you put it where the drawing shows.

Mike

[Dano"T"]

Re: Fuel Vent Question

Like THIS From Grant Baily's Model 19 album.

[Robert Lees]

Re: Fuel Vent Question

Even on the ali tanks, the steel filler neck was pressed/soldered/whatever. But not properly affixed.

My own opinion is that steel filler necks should be screwed into a welded fitting...but this means tank removal to achieve. Photo below.



Still, it means the line boy won't rip the neck out!

Rob

[Gary Snell]

Re: Fuel Vent Question

Thanks to all that responded. Rob, I agree that is the right way to do it. Guess I need to contemplate my options.

Gary

[NY86]

Re: Fuel Vent Question

I don't know about these tanks specifically, but a common practice was to form a flanged hole in the aluminum, fit a threaded steel neck in and run a bead of aluminum weld around it. It wasn't really welded, but the aluminum got pretty friendly with the steel threads.