Fuel Leak After Topping Off

[Concerned Pilot]

About half an hour after having the tanks filled on an older 172 I discovered fuel dripping out of the right wing root. There was also a fuel smell inside the airplane with the door closed. I wrote up a squawk and switched airplanes.

The next day the maintenance department closed my squawk and the "resolution" was "Do not top off". A couple days later I checked the plane (which had not been run in the interim) and the dripping had stopped. I used the dipstick and found that the fuel level in the right tank had gone down by about a gallon.

So I guess it would be possible to take their advice and make sure the fuel truck driver puts at least a gallon less than the maximum in but I'm wondering if fuel could slosh into the wing during flight and create a fire hazard.

Any opinions, facts and advice welcomed.

 

[rk911]

I'm a low-hour Sport pilot but, to me, the leak is a hazard and I wouldn't fly it until it was properly corrected. When I was 18, I discovered a leak in the gas tank of my '65 Mustang coming from a hairline crack. Being 18 with a skull full of mush I took the advice of a friend and rubbed some bar soap on the crack rather than have the tank repaired or replaced. Amazingly, rubbing soap on the crack DID stop the leak but only for a day or so. Looking back that was a bone headed thing to do. I look at the advice to "not top off the tank" to also be bone headed.

 

[Brad W]

Back about 1992/1993 I was signed off for my multi checkride, and because of some logistic issue that I can't recall...and the examiner being located at a different airport...I was signed off to solo the old Piper Apache 150 to that airport for my exam.

during the pre-flight, I found fuel raining from the underside of the wings. Of course flight canceled. During the following week the shop inspected...and their direction was exactly that...don't fill past half tanks!

Turns out the bladders had dry rotted in the top halves, from too much time with all vapor and no liquid. It didn't sit well with me...just didn't smell right...and I did tons of research to see if I could find some indication that this was legal and kosher...I remember looking for minimum equipment lists, etc... I could find nothing that made me feel better about it and I refused to fly till they got it fixed. Long story short I never got my multi rating.

In hind site I've got to say that a big part of me wishes that I would have gone ahead and done it after I was for sure there was no more fuel running around loose inside the wings... I have no first hand memory, and I'm not sure if they ever used it for training again...but I suspect they did...and I am at least confident that someone flew it after that and before it was fixed.....or sold.

Anyway, without more explanation of the cause, I'm not sure I would fly your 172 either....but it seems much less of a fire risk than the old apache did with the engines out there in the wings

Regardless...sounds like a typical sad rental operation....pushing students into airworthy planes day after day.... well maybe practically speaking they're airworthy....but probably shouldn't be.

 

[benyflyguy]

thats a new place to sump fuel! 182H. Did my preflight. Got in and was just overwhelmed with fuel smell. Thought maybe where I dumped what I sumped. But when I was pulling door closed noticed this leak. Nice cracked dried out fuel line.

 

[Clip4]

A full fill followed by fuel expansion should be vented overboard, not into the wing. Like Benyflyguy posted, the likely source of the leak is a rotten rubber line connection.

 

[FlyingElvii]

Something similar happened in the Sundowner partnership I used to belong to. I was recently minted, and heading off on my first long cross country with a friend.
Club had a longstanding rule to only fill the tanks to the tabs after a flight, no one in the current group really remembered why.

Another Partner that flew it in the morning knew I had a long trip later that day, and graciously topped off the tanks for me, leaving it out on the ramp.

About an hour later I as was leaving after a long wait to take off in the 100% sunshine, 90+ degree day, I glanced out window just after rotation to see fuel pouring like a river around the fuel caps, and a stream from what I assumed was the overflow relief coming from underneath the wing.

It contuniued to flow out as I climbed to pattern alt and set up to land. By the time I was on final it had stopped.

Local Mechanic took a look and immediately identified it as expansion, he had seen the truck fueling it earlier. Ended up spewing out around 3-5 gallons, he estimated. Fuel comes out of the ground cool, and warms Up sitting in the hot sun, causing it to expand. Overflow valve couldn’t handle it, so it was pushing out around the caps On both tanks. Learn something new every day, I guess.

The guy that topped it off was just being nice to the new guy, but I was very hot about it.

 

[Concerned Pilot]

Back about 1992/1993 I was signed off for my multi checkride, and because of some logistic issue that I can't recall...and the examiner being located at a different airport...I was signed off to solo the old Piper Apache 150 to that airport for my exam.

during the pre-flight, I found fuel raining from the underside of the wings. Of course flight canceled. During the following week the shop inspected...and their direction was exactly that...don't fill past half tanks!

Turns out the bladders had dry rotted in the top halves, from too much time with all vapor and no liquid. It didn't sit well with me...just didn't smell right...and I did tons of research to see if I could find some indication that this was legal and kosher...I remember looking for minimum equipment lists, etc... I could find nothing that made me feel better about it and I refused to fly till they got it fixed. Long story short I never got my multi rating.

In hind site I've got to say that a big part of me wishes that I would have gone ahead and done it after I was for sure there was no more fuel running around loose inside the wings... I have no first hand memory, and I'm not sure if they ever used it for training again...but I suspect they did...and I am at least confident that someone flew it after that and before it was fixed.....or sold.

Anyway, without more explanation of the cause, I'm not sure I would fly your 172 either....but it seems much less of a fire risk than the old apache did with the engines out there in the wings

Regardless...sounds like a typical sad rental operation....pushing students into airworthy planes day after day.... well maybe practically speaking they're airworthy....but probably shouldn't be.

What I'm worried about is whether fuel in the wings but outside the fuel tanks could somehow be ignited by contact with wiring going out to the strobes for example.

 

[Tantalum]

About half an hour after having the tanks filled on an older 172 I discovered fuel dripping out of the right wing root. There was also a fuel smell inside the airplane with the door closed. I wrote up a squawk and switched airplanes.

The next day the maintenance department closed my squawk and the "resolution" was "Do not top off". A couple days later I checked the plane (which had not been run in the interim) and the dripping had stopped. I used the dipstick and found that the fuel level in the right tank had gone down by about a gallon.

So I guess it would be possible to take their advice and make sure the fuel truck driver puts at least a gallon less than the maximum in but I'm wondering if fuel could slosh into the wing during flight and create a fire hazard.

Any opinions, facts and advice welcomed.

yeah that's crap - ask yourself is there a limitation, with placard, in the POH that says 'do not top off' ?

 

[Tantalum]

I glanced out window just after rotation

is it more likely that the fuel expanded just enough to suddenly pour out during expansion, or, that the fuel tank now rotated 7-15 degrees submerged the cap and caused it to leak?

Note that this whole 'fuel expansion' thing, while true, is a little over used. Avgas only expands about 4% for a 40*C temperature rise.. in other words, if you get in the plane and it's 70 degrees when you fill it, about 21 C, then it warms to 100 F, or about 40 C, that's still only a 20 degree C rise and maybe a 2 percent expansion. Assuming they super duper topped off a 172's tanks at 43 gallons that's still well under a gallon it'll leak. Enough that you'd burn that off in taxi or run up.

If this was such a pandemic thing planes would be placarded accordingly or designed to account for this. I also doubt most planes encounter a 40 C or nearly 70 F temp rise from fuel fill up to takeoff.

I digress.

 

[baboss]

Not that uncommon on 172's with age on them when you gas it all the way up the filler neck. Look at the top of the wing. You might find streaks coming from the fuel quantity transmitter cover and/or filler neck. Have maintenance check screws for looseness but the gaskets eventually go bad. McFarlane has good Viton replacements and the transmitter screws they offer have Viton seals as well.
If that fails start looking for tank cracks. And don't be one of the dbags that rests the fuel nozzle against the filler neck while filling, that's how cracks start.

 

[wilkersk]

... And don't be one of the dbags that rests the fuel nozzle against the filler neck while filling, that's how cracks start.

2 older 172s my friend has taken on leaseback have come in with cracked filler necks. Owners squawk about the repair bill till we show them the damage that was easily missed on their pre-buys.

After we caught a young ramper hanging the nozzle on the filler neck of a different aircraft, our fueler concessioner seems to have done a good job of training their employees on the issue.

 

[Concerned Pilot]

Thankyou everyone for the suggestions and insights!

 

[KE5BM]

We have a hard rule on Coast Guard boats to only fill to 95% for this exact reason. In the summer, you know when someone goes over especially on the gasoline-powered boats because the fill port and vent turn in to pretty fountains. The club I'm in recommends filling to the bottom of the neck...allegedly it reduces the total fuel in a Skylane from 88 to 66.

But I'm a little more concerned about where OP discovered the fuel. He/she said there was fuel dripping from the wing root. Did that get there by making it's way down the wing from the cap or some other fitting? Or did it find it's way into the wing strut? That was never really cleared up. If fuel is getting INTO the strut that's a huge issue that needs to be addressed. Water and oils rarely drip out of where the leak actually is. Just something to consider next time OP checks out that plane.

 

[luvflyin]

About half an hour after having the tanks filled on an older 172 I discovered fuel dripping out of the right wing root. There was also a fuel smell inside the airplane with the door closed. I wrote up a squawk and switched airplanes.

The next day the maintenance department closed my squawk and the "resolution" was "Do not top off". A couple days later I checked the plane (which had not been run in the interim) and the dripping had stopped. I used the dipstick and found that the fuel level in the right tank had gone down by about a gallon.

So I guess it would be possible to take their advice and make sure the fuel truck driver puts at least a gallon less than the maximum in but I'm wondering if fuel could slosh into the wing during flight and create a fire hazard.

Any opinions, facts and advice welcomed.

Do you leave the selector on both? Leaving it on left or right might help isolating where the leak is. It could be in the cross over plumbing.

 

[Concerned Pilot]

We have a hard rule on Coast Guard boats to only fill to 95% for this exact reason. In the summer, you know when someone goes over especially on the gasoline-powered boats because the fill port and vent turn in to pretty fountains. The club I'm in recommends filling to the bottom of the neck...allegedly it reduces the total fuel in a Skylane from 88 to 66.

But I'm a little more concerned about where OP discovered the fuel. He/she said there was fuel dripping from the wing root. Did that get there by making it's way down the wing from the cap or some other fitting? Or did it find it's way into the wing strut? That was never really cleared up. If fuel is getting INTO the strut that's a huge issue that needs to be addressed. Water and oils rarely drip out of where the leak actually is. Just something to consider next time OP checks out that plane.

I checked the top of the wing behind the cap and it felt dry. It was dripping from the underside of the wing behind and near the sump valve. The valve itself did not appear to be leaking and looked new. Some of it may have been coming out through the hole that the valve sticks through.

The dripping was nowhere near the strut.

 

[Concerned Pilot]

Do you leave the selector on both? Leaving it on left or right might help isolating where the leak is. It could be in the cross over plumbing.

Not sure. I thought I had it on one tank but when I checked it two days later it was on both.

 

[luvflyin]

Not sure. I thought I had it on one tank but when I checked it two days later it was on both.

You might want to try L next fill up and then R on the next one. See what happens. It could maybe help isolate the problem. If it is the case then there's probably a leak in the vent lines. I had a 177 Cardinal. It was important to not leave it on both when parked and/or refueling. I don't think the 172 vent system is as complicated as the 172 though. Is the ramp level where you park/refuel?

 

[Concerned Pilot]

You might want to try L next fill up and then R on the next one. See what happens. It could maybe help isolate the problem. If it is the case then there's probably a leak in the vent lines. I had a 177 Cardinal. It was important to not leave it on both when parked and/or refueling. I don't think the 172 vent system is as complicated as the 172 though. Is the ramp level where you park/refuel?

The ramp looks level to me.

I'm not sure there is going to be a next time as there are plenty of other planes to rent. The original squawk is still visible in the system and I followed up with a detailed account of what I observed, so I feel I have done my duty.

 

[luvflyin]

The ramp looks level to me.

I'm not sure there is going to be a next time as there are plenty of other planes to rent. The original squawk is still visible in the system and I followed up with a detailed account of what I observed, so I feel I have done my duty.

Ok. You could tell them the things you have heard here. It might help them trouble shoot it.'

 

[Dan Thomas]

Like Benyflyguy posted, the likely source of the leak is a rotten rubber line connection.

Not likely at all. The aluminum vent nipple on the 172's tank reaches almost into the cabin, and a leaking hose usually makes the cockpit stink mightily. Been there, seen it.

Local Mechanic took a look and immediately identified it as expansion, he had seen the truck fueling it earlier. Ended up spewing out around 3-5 gallons, he estimated. Fuel comes out of the ground cool, and warms Up sitting in the hot sun, causing it to expand.

No way that was all expansion.

Avgas only expands about 4% for a 40*C temperature rise.. in other words, if you get in the plane and it's 70 degrees when you fill it, about 21 C, then it warms to 100 F, or about 40 C, that's still only a 20 degree C rise and maybe a 2 percent expansion.

CAR3/FAR23 airplanes were required to have a 2% expansion space built into the tanks. The positioning of the tank vents on the tank achieves this.

Not that uncommon on 172's with age on them when you gas it all the way up the filler neck. Look at the top of the wing. You might find streaks coming from the fuel quantity transmitter cover and/or filler neck. Have maintenance check screws for looseness but the gaskets eventually go bad. McFarlane has good Viton replacements and the transmitter screws they offer have Viton seals as well.
If that fails start looking for tank cracks. And don't be one of the dbags that rests the fuel nozzle against the filler neck while filling, that's how cracks start.

The tank cover needs to come off. The tank bay and tank and plumbing are supposed to be inspected every 1000 hours, as recommended by Cessna. This is for the '77-'86 172s:

That "M" in the Special Inspections column:

And this is for the '69-'76 172s:

Note the "Attachment" in line 5. That's the hold-down straps. The "5" in the Special Inspections column is this:

Maybe 1% or 172s get this attention,and it showed when I opened that bay up. The tank hold-down straps are aluminum, and they have rubber strips on them ("bumpers," the manual calls them) that prevent the strap chafing on the thinwalled aluminum tank. Those rubber strips harden and crack and rot and crumble and the glue that holds them on the straps fails. They fall out, and now the straps rub on the tank at the corners and eats holes in it. Those same rubber strips are under the tank, too, on the spanwise hat sections that stiffen the floor of that bay. They fall out, too, and you get chafing of the tank bottom.
Then there's the fuel filler, as noted. Letting that fuel neck carry the weight of nozzle and hose flexes it, and the aluminum tank next to the neck weld cracks. Now a full tank will send fuel out of that. The gasket under the fuel sender also shrinks, and leaks develop.
I have seen an aluminum tank cracked across its top just from airframe vibration. It flexes.

Getting that cover off can be a nightmare. If it hasn't been off in a long time, the screws are often rusted into the anchor nuts and they'll break off or need drilling out. The nuts along the front edge are in the main spar, and you could cause spar damage while getting the busted screws and nuts out. It's one reason why Cessna says to do it regularly. Ignoring it does not save money.

Do you leave the selector on both? Leaving it on left or right might help isolating where the leak is. It could be in the cross over plumbing.

All the crossover plumbing is in the fuselage belly.

Fuel in the wing could easily create an air/fuel mixture within the combustible range. One spark from a chafed nav light wire or a loose fuel sender connection could set it off. Boom. No more wing. End of flight.

Edit: I added the inspection checklist items from the earlier 172 manual. The '77-'86 airplanes went to integral tanks somewhere along the production run.

 

[Gary Ward]

Not likely at all. The aluminum vent nipple on the 172's tank reaches almost into the cabin, and a leaking hose usually makes the cockpit stink mightily. Been there, seen it.

No way that was all expansion.

CAR3/FAR23 airplanes were required to have a 2% expansion space built into the tanks. The positioning of the tank vents on the tank achieves this.

The tank cover needs to come off. The tank bay and tank and plumbing are supposed to be inspected every 1000 hours, as recommended by Cessna. This is for the '77-'86 172s:
View attachment 122500
View attachment 122499
That "M" in the Special Inspections column:
View attachment 122501

And this is for the '69-'76 172s:

View attachment 122534
View attachment 122533
Note the "Attachment" in line 5. That's the hold-down straps. The "5" in the Special Inspections column is this:
View attachment 122535
Maybe 1% or 172s get this attention,and it showed when I opened that bay up. The tank hold-down straps are aluminum, and they have rubber strips on them ("bumpers," the manual calls them) that prevent the strap chafing on the thinwalled aluminum tank. Those rubber strips harden and crack and rot and crumble and the glue that holds them on the straps fails. They fall out, and now the straps rub on the tank at the corners and eats holes in it. Those same rubber strips are under the tank, too, on the spanwise hat sections that stiffen the floor of that bay. They fall out, too, and you get chafing of the tank bottom.
Then there's the fuel filler, as noted. Letting that fuel neck carry the weight of nozzle and hose flexes it, and the aluminum tank next to the neck weld cracks. Now a full tank will send fuel out of that. The gasket under the fuel sender also shrinks, and leaks develop.
I have seen an aluminum tank cracked across its top just from airframe vibration. It flexes.

Getting that cover off can be a nightmare. If it hasn't been off in a long time, the screws are often rusted into the anchor nuts and they'll break off or need drilling out. The nuts along the front edge are in the main spar, and you could cause spar damage while getting the busted screws and nuts out. It's one reason why Cessna says to do it regularly. Ignoring it does not save money.

All the crossover plumbing is in the fuselage belly.

Fuel in the wing could easily create an air/fuel mixture within the combustible range. One spark from a chafed nav light wire or a loose fuel sender connection could set it off. Boom. No more wing. End of flight.

Edit: I added the inspection checklist items from the earlier 172 manual. The '77-'86 airplanes went to integral tanks somewhere along the production run.

Dan knows his stuff^^^^ I learn from him.

The hoses I found in the cockpit of my 1980 172 in 2020. It was in a tornado in 1990 and had the wings replaced.
So I don't think they were all originals? Some were in bad shape. None were leaking.

Loose fit up.

All new rubber.

Major amount of work to button it all up with structural screws. It took me months to get the old screw's out.

 

[Dan Thomas]

Good pic of the hold-down straps. The rubber that decays and falls off is visible. Once it's gone, especially at the corners of the tank where most of the stress is taken, they eat through that tank. It's not very thick stuff.

Those straps aren't tight yet.

Here we see two weak areas: 1. That weld at the base of the filler neck, where the tank cracks when the neck is stressed by the filler nozzle and hose. Yours has been reinforced by that square patch. And 2. The fuel level sender mounting. Several things there: The little U-shaped strip of aluminum that runs from under a screw and the airframe ground strap to the tank itself; it reaches so far under the gasket that it can let fuel out, so I used to trim it to a round end, and bend the edges down just a little so that positive contact with the tank is achieved. Positive electrical bonding of that tank is important, to avoid the risk of fire while refuelling. You don't want static building up in the tank.
Under the screw heads are tiny cork washers that are supposed to stop fuel spiralling up the threads from inside the tank and leaking out under the heads. This doesn't work well. I used a bit of Fuel Lube (now EZ-Turn) on the screw threads. Stopped those leaks dead.

The thick gasket under the sender will allow overtightened screws to warp the aluminum surface of the tank interface. A new gasket won't fix that, so mechanics need to stop retorquing the screws to stop the leaks, and remove the sender and install a new gasket. Do it right the first time. I preferred the Cessna cork gaskets, as they sealed better than the too-hard rubber gaskets McFarlane sold. Maybe they've fixed that. If that tank gets warped (wavy) around that hole, one has to make small wooden form blocks and use a pair of smaller water-pump pliers to lower the pulled-up anchor nut areas and raise the depressed places between the nuts.

Major amount of work to button it all up with structural screws. It took me months to get the old screw's out.

Yes. Exactly. And you did right in using the structural screws as Cessna demands in the parts manual. That cover is a structural panel, and with cheap screws that stretch it can come a bit loose, and the wing loses some of its torsional rigidity. I've found smoking areas around such screws. The panel has been moving.

 

[Gary Ward]

View attachment 122551
Good pic of the hold-down straps. The rubber that decays and falls off is visible. Once it's gone, especially at the corners of the tank where most of the stress is taken, they eat through that tank. It's not very thick stuff.

Those straps aren't tight yet.

View attachment 122554

Here we see two weak areas: 1. That weld at the base of the filler neck, where the tank cracks when the neck is stressed by the filler nozzle and hose. Yours has been reinforced by that square patch. And 2. The fuel level sender mounting. Several things there: The little U-shaped strip of aluminum that runs from under a screw and the airframe ground strap to the tank itself; it reaches so far under the gasket that it can let fuel out, so I used to trim it to a round end, and bend the edges down just a little so that positive contact with the tank is achieved. Positive electrical bonding of that tank is important, to avoid the risk of fire while refuelling. You don't want static building up in the tank.
Under the screw heads are tiny cork washers that are supposed to stop fuel spiralling up the threads from inside the tank and leaking out under the heads. This doesn't work well. I used a bit of Fuel Lube (now EZ-Turn) on the screw threads. Stopped those leaks dead.

The thick gasket under the sender will allow overtightened screws to warp the aluminum surface of the tank interface. A new gasket won't fix that, so mechanics need to stop retorquing the screws to stop the leaks, and remove the sender and install a new gasket. Do it right the first time. I preferred the Cessna cork gaskets, as they sealed better than the too-hard rubber gaskets McFarlane sold. Maybe they've fixed that. If that tank gets warped (wavy) around that hole, one has to make small wooden form blocks and use a pair of smaller water-pump pliers to lower the pulled-up anchor nut areas and raise the depressed places between the nuts.



Yes. Exactly. And you did right in using the structural screws as Cessna demands in the parts manual. That cover is a structural panel, and with cheap screws that stretch it can come a bit loose, and the wing loses some of its torsional rigidity. I've found smoking areas around such screws. The panel has been moving.

Exactly right as I did the same thing the first time I had it out on the original tank while replacing the gasket hoping it was the leak on the old tank and discovered that. I trimmed mine also.
The McFarlane sender unit gasket did take about 3 retorks to stop it from leaking. But I was really conservative tightening those screws at first.
There was a damaged tank in my wing that I suspect was in a tornado?
It did not lay flat in the fuel cell area in the wing. It would oil can every time I filled it.

The crack in the bottom. Bruce Hartwig and his guys are experts on these cessna tanks. They saved my core which'd saved me a ton of money. Some of these 172 tanks are very rare and hi dollar now. They sent me a rebuilt pressure tested tank that fit perfect fast. Not cheap but they are the best. I am aware of the filler neck weakness.
Eventually this crack formed from the oil canning on the bottom.

They were able to repair/save that tank. First I was told it might be junk. It had a lot of dents and leaks around the seam which was welded once before. They are honest people.

 

[Gary Ward]

I spent months soaking those screws and loosing them. Soak em some more, loosen and re tighten and go fly some more. I also used a pair of vampire pliers that work well to grab the heads when they get stripped out. I am currently working on my other side, terrified what I might find in there. I also have a custom McFarlane vent line I want to install that has a hump in it to prevent drips when taxing with full fuel tanks. Having that cover off would help to install it.

I fantasized about installing long range tanks, only fantasized once I realized how much work that would be. My buddy had one so I borrowed it to play with it.