The International Cessna^® 170 Association

Hi Flyguy,

Your 170 probably looks like mine with all the fuel stains from the right wing filler down across the flap, onto the fuselage, and all over the right horizontal stabilizer. All this with NEW caps Of course the right one is vented as the "we are here to help you" folks have mandated At least I can show the FAA guy that the cap really does vent....gas all over my plane Then I will let the FAA guy justify the vented cap to the EPA guy who is ranting about all the gas on the exterior of my plane and we will see who wins I wonder if anyone has really experienced a problem with the factory venting system? I do not have enough experience with it to know if it is REALLY a problem. Perhaps that little vent hole in my new fuel cap needs to plugged up with a little bug...but not before I get up there and blow into the vent standpipe and listen for the bubbles in the tanks

Well I just finished climbing up on the nose and blowing into the fuel vent stand pipe Guess what? It worked just like it is supposed to I heard airflow out of the tank filler necks and when I put the vented cap on the right tank, I heard my bad breath hissing out of the vent So then it really hits me, the original venting system is functioning normally and is placing a slight positive pressure on the fuel system at flight speed. Air is therefore always flowing out of the right vented cap and every time a bit of fuel sloshes up against the cap....there it goes outside with the vent air and all over my 170

So I am back to square one. I know there was an AD issued on the fuel system and mandates a vented cap on the right tank. WHY? Does anyone know why this AD was issued? Is there a component in the original venting system that is prone to failure? The only possible weakness I can see is if a big bug smaks the vent stand pipe and completely blocks the intake of ambient air. I have a screen over my stand pipe, and the drilling of a small "reliever" hole on the underside of the stand pipe just aft of the intake would ensure that the vent stays open even if a big bug managed to seal off the intake.

I would appreciate some ideas here. This particular AD smells like a rat to me and I am ready to plug up the vent holes in the right cap. Keeping the fuel in my tanks by letting the factory vent system do what it was designed to do....unless someone can show me that the original system is in some way very prone to failure...and even if this is so, there must be a fix for the venting system. Just slapping on a vented cap and watching one's fuel vent overboard from the positive tank pressure is NOT the way to go

The original fuel vent was found to be susceptible to blockage from insects and ice, so a vented fuel cap was mandated by the FAA. (If you alter your vented fuel cap you'll be in violation, for what it's worth.)
According to Cessna test pilot Wm. Thompson who wrote (from notes in his diary) about this issue, the original vent was found not only susceptible to such blockage, but the fix was also not much better. The first fix was to drill a small hole in the aft side of the gooseneck to guarantee that the vent system still worked in case of icing. But that was in a low-pressure area of the vent-tube and all that did was suck fuel out and all over the roof. Then another problem was discovered. When heavy braking was applied during full-tank landings, fuel would splash out all over the windshield. So in another mis-guided effort Cessna developed a ball check-valve arrangment (which has been discussed elsewhere in this forum). That solution only aggravated the accumulation of impact ice and bugs and the loss of fuel through the bleed hole.
If the vent tube (gooseneck) does indeed become blocked, then fuel starvation or collapsed fuel tanks are possible. So the only workable fix, however messy, was the inclusion of a vented fuel cap.
The long-term fix was when Cessna moved the vent to a location behind the upper wing lift-strut on later airplanes. If you've ever flown those airplanes full of fuel you've seen fuel splash out of them as well when performing a turning takeoff or when in rough air. But the fuel just goes away and not all over the airplane.

I don't think I've ever seen a bug (other than one of ol gar's mudbugs) big enough to plug the fuel vent on my 170. A mud-dauber nest might do it,though. I think I'd probably reinstall original non-vented caps,& save the fancy vented one(s) for annual time,"violation" be damned.

Anonymous

George you are a walking cyclopedia! Where do you come up with all this stuff? "Hole here" then a "hole there" then "another hole" somewhere else! Heck by the time they got done drilling holes it would seem there wouldn't be much left! Why not just a little screen pasted on the opening end of the little pipe? Too darm cheap that's why! Need for someone to sell a bunch of faa - pma - tso- do nuthin gas tank caps for $49.95 a pop (at that time)!

Now Geo. don't fret too much about my lack of seriousnes in my writing here. Safety is of major concern for each of us when dealing with most aspects of flying. I understand that very well and I will never demean serious posts. My off the wall comments are usually sent before I have time to generate serious responses, but I will never in any way belittle questions adressing genuine safety concerns!

ICE! I never thought of that one George...not that I am going to go flying in it So, there are several ways I can go here if the reliever hole trick has already been used. The screen over my stand pipe will keep the bugs and other "large" debris out of my precious check valve and vent system. Now I have to design for ice. I am thinking about a little "hat" if you will, something like I used to see on the natural gas vent stack of old houses. A small cone shaped hat to be supported on three legs and mounted to the front of the stand pipe with a wormgear clamp. Could be made from scrap aluminium sheet. This hat would stream the bugs and other stuff around and past the opening of the stand pipe. Only negative would be the loss of the positive tank pressure generated by the impact of the relative wind on the stand pipe in flight. Hummmmmm The whole gizmo could come off in a snap and the vented cap go back on for annual time

On the vented cap note, the only way I can see the vented cap being a good solution to the "problem" of venting our tanks is for the cap vent to consist of a one-way valve. A vent valve that would let ambient pressure inside the tank, but would close off to movement of air or fluids from inside the tank. Is there a cap like this at reasonable cost

Trojan has one

There has been plenty of experimentation with solutions for this problem and there's no reason to endanger yourself or your airplane over a little fuel stain.
The vented caps have a one-way vent or check valve built into them. If yours is damaged, then fix/replace it. Look under your cap for the little flat reddish-colored silicone flap under the vent. If it's missing, then that's your problem.
Screens on the vent tube are not a new idea. The only problem is: they are unapproved (yet clearly visible) and they become the next problem when they are clogged with bugs/ice. And yes, a positive air pressure must exist in the fuel tanks, which is why the vent is 1) there and 2) facing forward.

Has anyone ever gone the route of "moving" the vent from the top of the cabin to under the port side wing like the newer Cessnas? I still get alot of spillage from the goose neck; particularly in turbulence or steep turns. When using Mogas, that not only stains real bad, but it stinks, big time! There has to be a way to install an in-line check valve or something to get rid of this problem once and for all. I'd be really surprised if after some 50 years, someone has not come up with a real fix for this (besides a nose clip and stain remover). JD

wa4jr wrote:Well I just finished climbing up on the nose and blowing into the fuel vent stand pipe Guess what? It worked just like it is supposed to I heard airflow out of the tank filler necks and when I put the vented cap on the right tank, I heard my bad breath hissing out of the vent So then it really hits me, the original venting system is functioning normally and is placing a slight positive pressure on the fuel system at flight speed. Air is therefore always flowing out of the right vented cap and every time a bit of fuel sloshes up against the cap....there it goes outside with the vent air and all over my 170

So I am back to square one. I know there was an AD issued on the fuel system and mandates a vented cap on the right tank. WHY? Does anyone know why this AD was issued? Is there a component in the original venting system that is prone to failure? The only possible weakness I can see is if a big bug smaks the vent stand pipe and completely blocks the intake of ambient air. I have a screen over my stand pipe, and the drilling of a small "reliever" hole on the underside of the stand pipe just aft of the intake would ensure that the vent stays open even if a big bug managed to seal off the intake.

I would appreciate some ideas here. This particular AD smells like a rat to me and I am ready to plug up the vent holes in the right cap. Keeping the fuel in my tanks by letting the factory vent system do what it was designed to do....unless someone can show me that the original system is in some way very prone to failure...and even if this is so, there must be a fix for the venting system. Just slapping on a vented cap and watching one's fuel vent overboard from the positive tank pressure is NOT the way to go

BTW, it's best not to blow into the vents. Any trash, spiders, etc will be blown into the tank. It's better to hook up a siphon-blow-gun and apply suction to the vent and listen at each fuel filler for the sound of air.

For those who've now looked under their vented caps (that have been venting fuel onto their wings) and discovered the little red silicone check valves have their flaps missing,...here's a cheap fix.
Several years ago an AD note came out on 182's thru 210's requiring their fuel filler caps and man-hole covers be replaced with new ones. The old caps were usually made of a red plastic mechanism under the metal tops.
Anyway, lots of shops have a junk-pile in the back-room and lots of those old useless items were tossed back there or into a 55-gal trash drum that's stored at the back of the hanger in the corner. Go find it.
Look at the vents in those caps and you'll discover they are identical to the ones that are in your current vented 170 caps. Do NOT try to pry the red silicone flap-seal out of the vent, you'll only tear it.
Instead, use a screwdriver (or allen wrench in some cases) to remove the vent entirely. Then install it into your cap. This should allow your cap to still vent correctly, but prevent/reduce fuel from blowing out of your tank onto your wing.

Yes I suppose I should stop fiddling with the original fuel vent. The cap I proposed would have probably produced a low pressure area around the stand pipe and sucked fuel out I do, however, wonder how I could redirect the vent line down into one of the wings and install a small NACA vent on the bottom of the wing. THIS would indeed keep bugs, FOD and ice away from the vent opening and also provide a nice positive pressure to the tanks I'll put this project on the back burner for when I don't have much else to do.

My new vented cap from Spruce does not have any valve at all. All it has is a 1/16" hole drilled so as to vent to ambient air just under the outer shell of the cap. When gas comes up against this cap with positive pressure from the original vent system, the fuel goes spurting out. I'm not dealing with a "little" stain here...I'm dealing with a wet airplane that will go off like a roman candle if some yahoo comes a smokin around my machine on the ramp. The fuel is all over the right horizontal stabilizer so when my 2 year old wants to help daddy push the airplane back into the tiedown, her hands burn with fuel This is why I am a bit...no...a lot pissed with an AD I see as a kneejerk reaction so some isolated problem. The AD has created a much larger problem for me in the form of a severe explosion hazard I'll be glad to let the nice FAA man tell my little girl why her hands are burning with fuel.

Someone had said that Trojan carries the vented caps that do not leak. Can someone forward contact information from Trojan and I will try the vented cap rountine one more time before going back to unvented caps....damned the kneejerk AD

John, if you have the caps I think you do from Spruce, they are neither PMA'd nor FAA approved caps. They were intended for homebuilts. The red plastic caps you removed were the correct caps, just probably had the vent problem I mention above.
An alternative is the FAA/PMA'd caps from Monarch. http://www.monarch-air.com/

Thanks for the URL George, I will try it when I get back from my next trip. When I saw the post regarding the silicone flapper on the red caps...I wished I had not thrown mine away The caps from Spruce indeed do not appear to be FAA/PMA approved. Normally I would not worry too much about the approval stamp, but this time it looks like it really means something

Monarch will not sell just one cap. They insist that they are certified only in pairs, and offered me a pair for $95 And they are not approved for the 170....must have a field approval. All I need is ONE good vented cap to keep my gas in the tank. Does anyone have an address for Trojan???? Otherwise can anyone advise if NAPA has a good vented cap that will work...to let air in and keep gas at the same time? I'm gettin real close to puttin that nonvented cap back on