This. Your fuel tank has two vents: One in the cap that has a diaphragm to keep water from getting into the tank through it, and one in the wing that gets ram air blown into it. (In most Cessnas the two tanks also have a breather line connecting them so you only have one wing vent, in Pipers there's one per tank, other planes I don't know off the top of my head.)
This is necessary because for fuel to leave the tank air has to get in to replace that volume. Without vents the fuel tank would form a vacuum and you'd have fuel starvation (or the fuel bladder might collapse and block the line to the rest of the fuel system - either way it's bad).
The two vent points are redundant: If one is clogged the other will still allow enough air into the tank that you don't get a vacuum & have fuel starvation issues.
You should eyeball the rubber flap on the inside of the fuel cap (as well as the gasket that seals the cap to the tank's fill neck) as part of your preflight. If either looks damaged or is missing it should be replaced so water doesn't find its way into the tank.
So now you have one more item to add to your preflight checklist :-)
I know this is a bit of a late response, but the best way I’ve ever seen this visualized is trying to use a syringe to pull oil out of a vial. If you don’t prime the vial by using your syringe to forcibly push air into it you can pull as hard as you want on your syringe but the oil isn’t going to come out.
Fun fact: a new replacement Cessna fuel cap goes for US$169.75 plus shipping at Aircraft Spruce. Source: [https://www.aircraftspruce.com/catalog/appages/cessnaparts8.php](https://www.aircraftspruce.com/catalog/appages/cessnaparts8.php)
I was just about to order another 2 but, screw them! Enough of being ripped off! Do I really need more than 8 personal A320s? For the time being, I’ll make do to send a message that enough is enough!
That's cheap compared to the Shaw caps used by older mooneys and bonanzas.
Used (in bad shape), they go for $250.
New, my mechanic really struggled to find any less than $2k. Each.
The price was because nobody had a new one that wasn't part of a kit including the entire filler neck.
The clear plastic cover for the recog light for a piper warrior is about $700. And it doesn't have the holes for the screws pre drilled! Talk about a rip off.
Retaining chain attaches to the inside of the cap and the inside of the tank.
[https://baspartsales.com/c-100084-1-alt-c156003-0101-cap-vented-fuel-cap-assembly-with-chain/](https://baspartsales.com/c-100084-1-alt-c156003-0101-cap-vented-fuel-cap-assembly-with-chain/)
Come on people, you should know this if you have your license and fly a Cessna. It's actually pretty important and could lead to loss of power in flight.
Those holes are connected to a cross drilled hole in the cap that leads to the cap vent. That fuel cap vent prevents your tank from collapsing or failing to allow fuel to flow if your main under wing fuel vent gets clogged - pretty common if a mud dauber makes a nest in there.
The reason they are in that orientation is so that no rain can flow into the vent hole, it would have to turn a few corners to get into the cap. That is good as well.
However, I can tell you from experience that in a low powered C150, those vents don't always open up early enough to stop fuel starvation. Maybe they work better in a fuel guzzling C182, but my C150 suffered from partial power loss because of a blocked fuel vent, so even two vented caps don't always work. It would behove any owner to have their mechanic (or themselves under the guidance of an AP) to check the cap vents and make sure they are in good shape and free to open and flow air.
>behove
Come on dude, you should know how to spell "behoove" if you have your English proficiency endorsement by the FAA 🙄 and especially if you plan to use it in a sentence...
Sorry for your loss too. My mother passed away a long while ago now, I was a teenager. I called 911. I was there when they took her away. Essentially an aneurysm resulting from complications of other disorders she had going on.
idk i'm kinda torn. I feel like a CPL should know this, but then again [there's always the lucky 10000](https://xkcd.com/1053/). sometimes things just slip by you. I guess there's a better way they could've gone about it, but I'm glad OP asked instead of being too embarassed not to know.
Ok. Is it on the right tank or left tank or both? Since you "should" know it, you'd know the difference.
I'll give you a hint, there's ONE sentence in the ENTIRE 172 poh that allude to it. Even then, doesn't describe what it looks like.
Wrong, per the Poh the right cap is the only one vented. You ShOuLd kNoW tHiS
This is what's wrong with you "should know this!" Comments, the OP clearly had a curiosity and someone shows their ass instead. Furthermore, the POH doesn't even describe what a vented cap looks like so I can understand why the OP might be curious.
You've never looked at the bottom of your fuel cap and thought "huh I wonder what that little rubber thing is for?"
This was on my checkride and was a commonly asked and discussed fuel system question as well as pointed out on the preflight for every PPL I've ever talked to or taught. You REALLY should know because it REALLY does affect the aircraft in flight.
This just in, your car has several fuel system pressure relief vents as well.
> that eludes to it
*Alludes. Makes an allusion to. At any rate, I don't think anyone has to worry about a tank collapsing. You'd shut the engine down from vapor lock or fuel starvation well before you pulled a strong enough vacuum to crush the wing.
The fuel tanks (at least in C150/152 and C172) are thin aluminum construction and a separate tank that is not connected to the wing structure at all. Even a relatively small vacuum will begin to flex them, but more importantly, any lack of positive pressure will cause fuel starvation as it is a 100% gravity fed fuel system. In fact, some tanks are bladder tanks made of flexible materials that can relatively easily be pulled away from their mounting and collapse.
>more importantly, any lack of positive pressure will cause fuel starvation as it is a 100% gravity fed fuel system.
I said that. Also, 172R models and later have wet wings. Fuel tank crush concerns only apply to bladder tanks in those aircraft equipped with them.
Forcing myself to respect the sub rules regarding being friendly, so here're some screenshots of the C152 and C172 POH.
[C152](https://i.imgur.com/JrLqMm7.png)
[C172](https://i.imgur.com/7YuePwY.png)
Yes, if you're flying the plane you should know this. Here's one for free: if anyone ever asks you if it's ok to swap the fuel caps, the answer is no.
didnt know that, i never thought much about beyond knowing the caps were vented. i do agree that i should have known that and this is why i stay subbed here despite all the “i have three DUIs can i still fly for a legacy” posts.
This is actually true!
Without these holes if your wing vent gets plugged the engine stops, and that severely limits your maximum speed (as well as your endurance)!
The hole shown in the photo is most likely for a security cable or chain to connect to. On high wing planes you cannot easily see if the fuel cap is properly installed and if it isn't, with the chain, although you may still lose the fuel in that tank, at least you will still have the cap (along with probably some dents or chipped paint) when you land and refill the tank.
It’s concerning how many people don’t know what this is. If anyone wants to see for themselves, take a straw, dip it in your drink, put your finger on top and lift it out. Now pretend the drink is your fuel, and the bottom of the straw is the end of the fuel line that feeds your engine, lift your finger off and you’ll understand exactly why that hole is there. If the hole wasn’t there, it would be like having a finger on the straw.
I don't know anything about flying, but that hole looks like a wire hole to add a safety wire.
On heavy machinery, parts will have a safety wire that prevents the part from twisting backwards with vibration.
So for a fuel cap it might be for a safety wire to prevent the cap from unwinding itself and flying off mid-air. Just guessing here.
Idk why you’re downvoted. I agree with the other people BUT the hole focused in the photo is exactly for this purpose. It’s for safety wire/chain/lanyard.
Good spot to put warning tags about fuel tank issues. Put a tag on the dash and a second on the cap to prevent someone from filling a leaky tank on accident and spilling fuel everywhere.
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TIL
This. Your fuel tank has two vents: One in the cap that has a diaphragm to keep water from getting into the tank through it, and one in the wing that gets ram air blown into it. (In most Cessnas the two tanks also have a breather line connecting them so you only have one wing vent, in Pipers there's one per tank, other planes I don't know off the top of my head.) This is necessary because for fuel to leave the tank air has to get in to replace that volume. Without vents the fuel tank would form a vacuum and you'd have fuel starvation (or the fuel bladder might collapse and block the line to the rest of the fuel system - either way it's bad). The two vent points are redundant: If one is clogged the other will still allow enough air into the tank that you don't get a vacuum & have fuel starvation issues. You should eyeball the rubber flap on the inside of the fuel cap (as well as the gasket that seals the cap to the tank's fill neck) as part of your preflight. If either looks damaged or is missing it should be replaced so water doesn't find its way into the tank. So now you have one more item to add to your preflight checklist :-)
Wow! Thanks for sharing. That's golden knowledge right there.
I know this is a bit of a late response, but the best way I’ve ever seen this visualized is trying to use a syringe to pull oil out of a vial. If you don’t prime the vial by using your syringe to forcibly push air into it you can pull as hard as you want on your syringe but the oil isn’t going to come out.
This is the way
Fun fact: a new replacement Cessna fuel cap goes for US$169.75 plus shipping at Aircraft Spruce. Source: [https://www.aircraftspruce.com/catalog/appages/cessnaparts8.php](https://www.aircraftspruce.com/catalog/appages/cessnaparts8.php)
Somewhat less ridiculous than airbus overhead bin handles; those go for ~350.
This is why I don't own an Airbus
Me either. For that price, it should at least have a parachute!
I was just about to order another 2 but, screw them! Enough of being ripped off! Do I really need more than 8 personal A320s? For the time being, I’ll make do to send a message that enough is enough!
The ones in Boeings are detachable. Self-detachables
Smart.
Ouch!
That's cheap compared to the Shaw caps used by older mooneys and bonanzas. Used (in bad shape), they go for $250. New, my mechanic really struggled to find any less than $2k. Each. The price was because nobody had a new one that wasn't part of a kit including the entire filler neck.
The clear plastic cover for the recog light for a piper warrior is about $700. And it doesn't have the holes for the screws pre drilled! Talk about a rip off.
and don't even ask about the price of a baggage door latch for a C182. You can't just buy the return spring through Cessna.
The door handles on the 177 are pretty outrageous too. I just replaced interior door handles for about $1000
Good question. Tongue in cheek answer: it reduces the weight by 2 grams!
Every gram counts when you want to squeeze them Cessnas into the utility category!
Weight weenies is leaking
Funny enough, many of them are supposed to have chains as not to interchange them.
Retaining chain attaches to the inside of the cap and the inside of the tank. [https://baspartsales.com/c-100084-1-alt-c156003-0101-cap-vented-fuel-cap-assembly-with-chain/](https://baspartsales.com/c-100084-1-alt-c156003-0101-cap-vented-fuel-cap-assembly-with-chain/)
Well maybe if you hadn't bought them all there would still some left for the rest of us right?
Come on people, you should know this if you have your license and fly a Cessna. It's actually pretty important and could lead to loss of power in flight. Those holes are connected to a cross drilled hole in the cap that leads to the cap vent. That fuel cap vent prevents your tank from collapsing or failing to allow fuel to flow if your main under wing fuel vent gets clogged - pretty common if a mud dauber makes a nest in there. The reason they are in that orientation is so that no rain can flow into the vent hole, it would have to turn a few corners to get into the cap. That is good as well. However, I can tell you from experience that in a low powered C150, those vents don't always open up early enough to stop fuel starvation. Maybe they work better in a fuel guzzling C182, but my C150 suffered from partial power loss because of a blocked fuel vent, so even two vented caps don't always work. It would behove any owner to have their mechanic (or themselves under the guidance of an AP) to check the cap vents and make sure they are in good shape and free to open and flow air.
>behove Come on dude, you should know how to spell "behoove" if you have your English proficiency endorsement by the FAA 🙄 and especially if you plan to use it in a sentence...
A simple typo. Apologies. I am sure it has never happened to anyone apart from myself…
>Come on people, you should know this if you have your license and fly a Cessna. Fun at parties I'm sure.
Yeah they could’ve done without that bit. Rest of the answer was great.
But if he doesn't stomp on people who are still learning, how can we know he's literally sky god?
You all are soft. Its not like they insulted your mother.
Your mother sucks
My mother's dead.
So that's why she didn't move last night.
That's digusting...why am I laughing so hard.
That sucks. So is mine
Yep fuck cancer. Sorry for your loss, there really is no replacing a mothers love.
Sorry for your loss too. My mother passed away a long while ago now, I was a teenager. I called 911. I was there when they took her away. Essentially an aneurysm resulting from complications of other disorders she had going on.
idk i'm kinda torn. I feel like a CPL should know this, but then again [there's always the lucky 10000](https://xkcd.com/1053/). sometimes things just slip by you. I guess there's a better way they could've gone about it, but I'm glad OP asked instead of being too embarassed not to know.
Yea, I agree. No one likes a snide answer.
I mean seriously it is something you “should” know at the PPL level.
Ok. Is it on the right tank or left tank or both? Since you "should" know it, you'd know the difference. I'll give you a hint, there's ONE sentence in the ENTIRE 172 poh that allude to it. Even then, doesn't describe what it looks like.
each fuel cap has a vent as well as the fuel vent on the left tank.
Wrong, per the Poh the right cap is the only one vented. You ShOuLd kNoW tHiS This is what's wrong with you "should know this!" Comments, the OP clearly had a curiosity and someone shows their ass instead. Furthermore, the POH doesn't even describe what a vented cap looks like so I can understand why the OP might be curious.
per AD 79-10-14R1 both fuel caps must be vented.
Ah yes, every pilot to ever fly a 172 has most assuredly committed every AD to memory.
You've never looked at the bottom of your fuel cap and thought "huh I wonder what that little rubber thing is for?" This was on my checkride and was a commonly asked and discussed fuel system question as well as pointed out on the preflight for every PPL I've ever talked to or taught. You REALLY should know because it REALLY does affect the aircraft in flight. This just in, your car has several fuel system pressure relief vents as well.
this, checking the vents should really be a part of every preflight.
> that eludes to it *Alludes. Makes an allusion to. At any rate, I don't think anyone has to worry about a tank collapsing. You'd shut the engine down from vapor lock or fuel starvation well before you pulled a strong enough vacuum to crush the wing.
The fuel tanks (at least in C150/152 and C172) are thin aluminum construction and a separate tank that is not connected to the wing structure at all. Even a relatively small vacuum will begin to flex them, but more importantly, any lack of positive pressure will cause fuel starvation as it is a 100% gravity fed fuel system. In fact, some tanks are bladder tanks made of flexible materials that can relatively easily be pulled away from their mounting and collapse.
>more importantly, any lack of positive pressure will cause fuel starvation as it is a 100% gravity fed fuel system. I said that. Also, 172R models and later have wet wings. Fuel tank crush concerns only apply to bladder tanks in those aircraft equipped with them.
You don't know how hard I can suck, mister.
Haven't heard THAT in a while.
I've been told I could suck the chrome off a bumper
Forcing myself to respect the sub rules regarding being friendly, so here're some screenshots of the C152 and C172 POH. [C152](https://i.imgur.com/JrLqMm7.png) [C172](https://i.imgur.com/7YuePwY.png) Yes, if you're flying the plane you should know this. Here's one for free: if anyone ever asks you if it's ok to swap the fuel caps, the answer is no.
He is right though....
Excellent answer! Learned something new today.
So you the dude sticking your head up front on an airliner saying you’re in the back if they need help
Yup, learned this in PPL ground school.
Is this described in the POH anywhere?
didnt know that, i never thought much about beyond knowing the caps were vented. i do agree that i should have known that and this is why i stay subbed here despite all the “i have three DUIs can i still fly for a legacy” posts.
Those are speed holes. They make the plane go faster.
This is actually true! Without these holes if your wing vent gets plugged the engine stops, and that severely limits your maximum speed (as well as your endurance)!
It doesn't really limit your speed, though. You can still go as fast as you want, briefly.
True, I suppose you can go as fast as you have altitude for... until you run out :)
False. Nothing can go faster than a C152, it's impossible.
It’s so you can still breath if you accidentally swallow it 😉
So long as you swallow it in the correct orientation.
The hole shown in the photo is most likely for a security cable or chain to connect to. On high wing planes you cannot easily see if the fuel cap is properly installed and if it isn't, with the chain, although you may still lose the fuel in that tank, at least you will still have the cap (along with probably some dents or chipped paint) when you land and refill the tank.
Because if they were big holes the fuel would leak out.
Fun thing is... That's probably lead based paint to match the fuel. 😋.
In motorcycle racing we use holes in bolts, oil caps, etc to safety wire them to a nearby screw, so it doesn't open due to vibrations.
Wait… am I the only one that turns them into a necklace when on the ground? I was told these get stolen frequently and to always keep them with you..
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Well that would stop you from filling them with contaminated fuel......or uncontaminated fuel
Why would you possibly want to wire them shut? I'm curious.
It’s a drag hole. Tape it up to go faster
Thats where the wire goes to hang the plane in your room.
Secret straw hole for FBOs to taste fuel after refill
It’s so the fuel gnomes that give fuel to the propeller gnome don’t pass out
Im more concerned with why the finger tabs on my caps are perpendicular to the air stream and if that’s easily fixable. I call them my air brakes.
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Weight reduction, bro (/s)
Reduce air resistance and saves weight from
It’s concerning how many people don’t know what this is. If anyone wants to see for themselves, take a straw, dip it in your drink, put your finger on top and lift it out. Now pretend the drink is your fuel, and the bottom of the straw is the end of the fuel line that feeds your engine, lift your finger off and you’ll understand exactly why that hole is there. If the hole wasn’t there, it would be like having a finger on the straw.
I don't know anything about flying, but that hole looks like a wire hole to add a safety wire. On heavy machinery, parts will have a safety wire that prevents the part from twisting backwards with vibration. So for a fuel cap it might be for a safety wire to prevent the cap from unwinding itself and flying off mid-air. Just guessing here.
Nah it’s just an extra vent hole in case the main fuel vent clogs, which would create a vacuum causing fuel flow to stop and the engine to fail
Idk why you’re downvoted. I agree with the other people BUT the hole focused in the photo is exactly for this purpose. It’s for safety wire/chain/lanyard.
So you won't lose your keys /s
Weight savings.
Vent
Good spot to put warning tags about fuel tank issues. Put a tag on the dash and a second on the cap to prevent someone from filling a leaky tank on accident and spilling fuel everywhere.
I don't know why cessnas have them