Fuel Tank Vent - necessary???

How to keep the Cessna 170 flying and airworthy.

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Fuel Tank Vent - necessary???

Postby kidalways » Tue Mar 13, 2012 11:03 am

One of the things I noticed when I purchased my 170 last year is that the previous owner had installed a plug onto the fuel vent on top of the wing. I asked him why - he stated that with the regular standard fuel caps (not vented) - he was experiencing fuel venting all over the top of the wing. So in an effort to save the paint and avoid the stains he installed vent fuel caps on both tanks and plugged the vent tube. I have flown the airplane for awhile now and experienced no ill effects - which leads me to my question. Anything wrong with removing the plugged vent tube all together? In my mind it is doing absolutely nothing except creating drag.
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Re: Fuel Tank Vent - necessary???

Postby gahorn » Tue Mar 13, 2012 11:52 am

You should remove that PLUG! Immediately, and prior to further flight!

It is required...and altering it is a serious matter. The addition of vented fuel caps is not the primary method of venting the fuel tanks...they are alternate...and in-addition-to the tank vent above the cabin.

If you are experiencing fuel vomiting out onto the cabin roof, you are likely wheel-landing coupled with hard-braking while the tanks are topped-off with fuel. (You can operate without over-filling the tanks, or at least avoid hard-braking when so filled.)
If that is causing stains which damage polyurethane paints you are likely using auto gas and/or aftermarket additives. (Avgas only/rarely leaves residue (does not actually stain), which can be easily wiped away using acetone or mek.)

There is a long story about that "gooseneck" overhead vent. The vomiting of fuel during some manuevers/hard-braking could be smelled in the cabin and sometimes even ended up on the windshield. (I personally have never experienced any problem whatsoever with the gooseneck, so this has never been a concern which bothered me. So perhaps I'm not the most sympathetic person in this regard....but, anyway....)
Cessna tried a modification and offered it as a Service Bulletin/Letter: It drilled a 1/8" thru-hole in the goosneck to accept a thru-cotter and installed a ball, as a type of "check-valve" inside which rested on the cotter, with a plastic cap which had a reduced vent-hole on the fwd opening. The intention was that any fuel which rose up inside would lift the ball against the cap and stop the ejection of fuel. The problem was not a good one as the reduction in size of the hole was problematical to reduced venting capability in an imaginary icing encounter. So Cessna issued another letter authorizing the drilling of another 1/8" hole in the aft side of that curve in the gooseneck. This offered an alternative vent in the case of the blocked fwd-facing vent. It also allowed the fuel to spit out again. In fact, since that new hole was on the aft side, it was in a low-pressure area, and it was suspected of actually worsening the situation by siphoning fuel out the gooseneck and onto the cabin roof. It could be smelled inside.
So Cessna issued a THIRD letter, revoking previous alterations of that gooseneck and restoring it to original configuration. They then changed the design of fuel caps to include dual-venting provisions, and required at least ONE of the new vented caps to be installed (on the RIGHT wing. This also accomodated later 172 aircraft which vented both tanks thru a left wing vent by placing the vented cap at the farthest point in the system from the actual vent. Higher performance airplanes with greater fuel consumption were required to have vented caps on all tanks.)
Still later, Cessna stopped issuing non-vented caps for our models, and therefore all new caps are dual-vented without regard as to which tanks upon which they are installed.

In YOUR case.... remove that vent plug....or you are operating illegally (and you are forcing your fuel system to operate on it's secondary vent system, instead of it's primary.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight.
An originality nut (mostly) for the right reasons. ;)
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Re: Fuel Tank Vent - necessary???

Postby bagarre » Tue Mar 13, 2012 1:29 pm

I also have a question on this topic regarding vents.

Is there a simple way/trick to unblock a blocked vent hose?
My right side is blocked by unknown matter (probably dirt or bug nest). If I blow into the top center goose neck, I get air to the left tank fine but it builds pressure to the right side (blocked). I cant figure out an easy way to chase it clear.

Am I doomed to pulling down the head liner (again)?
David Ross
N2481D
1952 170B
Freeway Airport (W00)
My Tail Number at gmail.com

One can not appreciate Density Altitude without a tree line.
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Re: Fuel Tank Vent - necessary???

Postby jrenwick » Tue Mar 13, 2012 3:52 pm

gahorn wrote:If you are experiencing fuel vomiting out onto the cabin roof, you are likely wheel-landing coupled with hard-braking while the tanks are topped-off with fuel. (You can operate without over-filling the tanks, or at least avoid hard-braking when so filled.)
If that is causing stains which damage polyurethane paints you are likely using auto gas and/or aftermarket additives. (Avgas only/rarely leaves residue (does not actually stain), which can be easily wiped away using acetone or mek.)

I've seen fuel spewing out the vent on a 1953 170B when power was applied for the takeoff roll. (The tanks were full.) That airplane always seems to do it, but I don't think my 1955 170B does it.

100LL avgas stains my white Imron paint blue if it drips on it long enough (e.g. a drip from a tank sump drain and the side window left open to catch the drip). I haven't found anything that removes these blue stains, but they do fade slowly over time.

This is just my own experience, YMMV.
John Renwick
Minneapolis, MN
Former owner, '55 C-170B, N4401B
'42 J-3 Cub, N62088
'50 Swift GC-1B, N2431B, Oshkosh 2009 Outstanding Swift Award
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Re: Fuel Tank Vent - necessary???

Postby n2582d » Tue Mar 13, 2012 5:16 pm

If fuel "vomiting" through the gooseneck is a real problem, it seems to me the simple solution would be to add two check valves in the overhead vent line. Put one on each side of the gooseneck to allow air in but prevent fuel from flowing out the gooseneck. Get the approval through Blueldr! :wink:
Andair Check Valve.jpg
Andair Check Valve.jpg (11.29 KiB) Viewed 1013 times
Disclaimer: The valve pictured is not an FAA-approved part.
Last edited by n2582d on Tue Mar 13, 2012 7:04 pm, edited 1 time in total.
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Re: Fuel Tank Vent - necessary???

Postby minton » Tue Mar 13, 2012 6:02 pm

n2582d wrote:If fuel "vomiting" through the gooseneck is a real problem, it seems to me the simple solution would be to add two check valves in the overhead vent line. Put one on each side of the gooseneck to allow air in but prevent fuel from flowing out the gooseneck. Get the approval through Blueldr! :wink:
Andair Check Valve.jpg


Be sure to reference A/C 20-27G :lol: :lol:
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Re: Fuel Tank Vent - necessary???

Postby kidalways » Wed Mar 14, 2012 4:57 pm

Okay - so I believe I have touched on a nerve with some 170 owners. My next question - why hasn't someone done the paperwork to remove the vent? Honestly it provides nothing but drag, stains all over the wing and possible water in the fuel. The only place this belongs is either somewhere where it is protected from the elements getting in or in the trash can. My PA-12 has just vented fuel caps - and that was the way the plane was certified. My intention with this plane is to make a nice plane better - keeping the plane original for the sake of it all is not my intention. I like the suggestion of the check valves but still not convinced that flying in a rain situation wouldn't lead to some type of fuel contamination. I mean really guys if there wasn't a problem then why are people spending the money to put two vented fuel caps on their planes????
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Re: Fuel Tank Vent - necessary???

Postby blueldr » Wed Mar 14, 2012 5:23 pm

minton,
Right on, Sport. Send me the paperwork. I'll approve almost anything.

kidalways,
You're worried about water contamination from the gooseneck in the rain? What the hell are you going to fly in, a typhoon?
If the drag of the gooseneck worries you, see Harry Delicker. He'll sell you a STC and a bigger engint to overcome the airspeed loss. Of course, you could always sell the airplane and buy a couple of more PA 12s that have no gooseneck drag problem.
BL
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Re: Fuel Tank Vent - necessary???

Postby minton » Wed Mar 14, 2012 7:11 pm

kidalways wrote:Okay - so I believe I have touched on a nerve with some 170 owners. My next question - why hasn't someone done the paperwork to remove the vent? Honestly it provides nothing but drag, stains all over the wing and possible water in the fuel. The only place this belongs is either somewhere where it is protected from the elements getting in or in the trash can. My PA-12 has just vented fuel caps - and that was the way the plane was certified. My intention with this plane is to make a nice plane better - keeping the plane original for the sake of it all is not my intention. I like the suggestion of the check valves but still not convinced that flying in a rain situation wouldn't lead to some type of fuel contamination. I mean really guys if there wasn't a problem then why are people spending the money to put two vented fuel caps on their planes????


The answer to your question is: FAA OVERKILL :lol: :lol:
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Re: Fuel Tank Vent - necessary???

Postby kidalways » Wed Mar 14, 2012 7:39 pm

BL,
Grow up - there was no disrespect meant in my post. I just find it hard to believe that in this day in age no one has pursued changing the venting system given it's problems. Cessna certainly changed it when they manufactured the 172. Oh well - I will pursue this more - maybe even get an STC for it. Hey BL - you could be the first one to purchase the STC and my custom made blank off plate. :wink: Lighten up - switch to decaff!!!
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Re: Fuel Tank Vent - necessary???

Postby bagarre » Wed Mar 14, 2012 10:26 pm

Well, worrying about the drag produced by the fuel vent is a little silly; considering all the other drag these airplanes have :roll:

I imagine an STC to re-engineer the fuel vent system would be pretty darned difficult to get approved in this day and age. It might be possible to get approval to remove the center vent in lieu of the under strut vent and dual vented caps since later 172's are set up that way. But, I think the cost to get it approved would be prohibitive.

If you're looking to improve the 170 in some way, there may be more effective battles to take up with your FSDO.
David Ross
N2481D
1952 170B
Freeway Airport (W00)
My Tail Number at gmail.com

One can not appreciate Density Altitude without a tree line.
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Re: Fuel Tank Vent - necessary???

Postby blueldr » Wed Mar 14, 2012 11:07 pm

The best way, and a legal way, to get rid of the goosneck vent on the C-170 is to somehow get the STC to install the Continental IO-360 engine in the airplane. That STC calls for the installation of a vent behind the wing strut on BOTH sides of the airplane. Unfortunately, that STC is not presently available. In addition to modifying the vent system, the STC and the
IO-360 engine give the airplanes performance a real kick in the ass.
BL
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Re: Fuel Tank Vent - necessary???

Postby 170C » Wed Mar 14, 2012 11:24 pm

Early 172's had a goose neck fuel vent on top of the left wing. On rare occasions one might capture a bug, but it is rare. The good thing is that if the tanks are full and the left wing is lower that the right wing it doesn't siphon out fuel like the later model ones with the vent on the bottom of the wing behind the strut. I am sure if the left wing were enough lower it probably would vent fuel. As far a creating drag, there are so many other things on our planes that create drag that the goose neck vent is hardly worth mentioning. If you did remove the vent tube you likely don't have the super critical measuring equipment to determine what, if any, additional speed you would have gained. IMHO
OLE POKEY
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Re: Fuel Tank Vent - necessary???

Postby n2582d » Thu Mar 15, 2012 12:07 am

kidalways wrote:I just find it hard to believe that in this day in age no one has pursued changing the venting system given it's problems. Cessna certainly changed it when they manufactured the 172. Oh well - I will pursue this more - maybe even get an STC for it.
You might want to talk with Steve or his son Brian at Steve's Aircraft. He's "been there, done that" and found that there is not enough interest to pursue the mod. See the end of page three of this thread.
Gary
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Re: Fuel Tank Vent - necessary???

Postby Green Bean » Thu Mar 15, 2012 12:46 am

The rest of the story::: From Cessna History while building the 1952 Cessna 180..the following is from the first Cessna's 180 project pilot William D. Thompson in 1952.

In accordance with standard practice at the time (prior to 1952), the forward-facing fuel vent tube was mounted on the top surface of the wing. In a naive attempt to provide emergency venting with an ice-blocked tube, a small bleed hole was drilled on the aft side of the 900 bend. This hole, of course, was in the low pressure field of the wing, and later we found that, with the vent blocked, fuel would stream out the vent bleed hole and over the wing at a surprising rate. Another problem was the splashing of fuel out of the vent (with a full tank) during heavy braking deceleration in the landing roll. In another misguided effort, we incorporated a simple ball check valve assembly at the extremity of the tube. This only aggravated the accumulation of impact ice and the loss of fuel through the bleed hole.

Fortunately, not many pilots were flying in icing conditions in those days. However, one daring pilot selected the C-180 for a New York-to-Paris flight to commemorate Lindbergh's historic flight. He carried auxiliary fuel tanks in the cabin with a separate (and less vulnerable) fuel venting system. As related later, his standard fuel vent iced up over the Atlantic Ocean, causing a surprisingly high "apparent" fuel consumption when using standard fuel tanks. We had alerted him to this possibility before his flight. However, operation on his auxiliary fuel was normal, and he was able to proceed to Ireland instead of Paris.

That experience prompted us to search for a "protected" location for a fuel vent. After much flight testing, we found a behind-the-wing-strut location that essentially "hid" the vent in all flight attitudes, while, at the same time, providing the required positive pressure in the air space above the fuel. An additional requirement was another increment of pressure to promote sufficient "gravity" fuel flow in steep best-angle-of-climbs at minimum flying weight. As one can appreciate, this meant that the positioning of the fuel vent became very important both in production at the factory and at overhaul facilities In the field.

A final requirement was the prevention of fuel loss through the vent from fuel sloshing in rough air, or when parked on a sloping ramp. A simple flapper valve was placed in the vent line in each fuel cell that closed when fuel moved toward the vent outlet. To permit expanding fuel (heated from the sun after a refueling) to escape, a bleed hole was added to each flapper valve assembly. This hole also served as a siphon-break in the event of a malfunctioning (open) valve. This redesigned fuel vent system proved to be so effective that it has been used in all subsequent Cessna models that use strut-braced wings.
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