Cub crafters simplified fuel stc

[RaisedByWolves]

First off, if you are calculating fuel load to include header tanks, ya might want to reconsider.

That being said, I like the headerless system. But, as pointed out above, it needs to be installed correctly. That means correct vent lines, fuel caps, selector, and front AND rear ports on the right tank. If it's done halfway, bad stuff will happen. I've heard of fuel starvation incidents but it was generally traced down to no front port installed on the right tank, or incorrect caps/vent lines that lead to a vacuum in one tank or other.

Web

I'm re doing a headerless install. they routed the lines how they seemed fit, and even used a cessna fuel selector. It's a nightmare

 

[skywagon8a]

Designed to retrofit. You can put a valve in the vent line and turn the fuel to a single tank when you park on a side slope. I don't park with one wing low full of gas very often myself so it hasn't been an issue.

What is the purpose of this modification and why was it supposed to be an improvement over Piper's system?

 

[wireweinie]

If a fuel cap vent clogs up, the vent line is supposed to allow air into the tank, from the opposite tank (which, hopefully, does NOT have a clogged vent).

I like your idea of venting from the outboard side of the tank. Still allows a second vent path but would be immune to fuel cross feed. Something to have welded on next time the tanks are out.

Web

 

[Gordon Misch]

The lowest point on the cross vent should be at the tanks, so that any condensation can't collect and freeze. For the experimental -12 I'm working on, the cross vent will connect to the outboard ends of the tanks to prevent side-hill issues, but will probably have to be lower in the middle. I'll run the gascolator vent to the lowest point.

 

[skywagon8a]

The lowest point on the cross vent should be at the tanks, so that any condensation can't collect and freeze. For the experimental -12 I'm working on, the cross vent will connect to the outboard ends of the tanks to prevent side-hill issues, but will probably have to be lower in the middle. I'll run the gascolator vent to the lowest point.

Gascolator vent?? IF your engine has a fuel pump it will draw air.
That crossover vent is very unlikely to ever collect any condensation. If that is a concern, you could incorporate a clear portion in the center for a visual check.

 

[Gordon Misch]

The fuel pressure will be the same at the vent line junction as at the fuel supply line at the gascolator. The head with fuel flowing will be less than the static head, but still non-zero. The gravity head is sufficient to generate excess fuel flow through the fuel supply lines at maximum power in adverse airframe attitude. So I don't see how the vent line would suck air. It's sure worth thinking about in detail though. Thanks.

Edit: A basic rule I think should be followed is to have every high-point vented and every low point drainable.

 

[Steve Pierce]

What is the purpose of this modification and why was it supposed to be an improvement over Piper's system?

Maybe less parts count when they were building Super Cubs around data plates or maybe to simplify the fuel system with a both position.

 

[DENNY]

What is the purpose of this modification and why was it supposed to be an improvement over Piper's system?

Gets rid of the header tanks and allows for both. I think I saw something in the regs about needing crossover with a both valve. I have been distracted and forgot to change tanks coughing a tank at 10 ft on both take off and on landing. I can change the tank usually before the second cough so it has never been an issue. If you have a lot going on I can see how the both would help. DENNY

 

[mvivion]

Gets rid of the header tanks and allows for both. I think I saw something in the regs about needing crossover with a both valve. I have been distracted and forgot to change tanks coughing a tank at 10 ft on both take off and on landing. I can change the tank usually before the second cough so it has never been an issue. If you have a lot going on I can see how the both would help. DENNY

Failure to attend to fuel level has bit a few folks, and killed a couple. One thing I’ve never been able to understand is why the same does not seem to be true of the Cessna 206, which also has a right/left/Off selector. Maybe 206 drivers just don’t talk about it.

MTV

 

[cubpilot2]

What is the purpose of this modification and why was it supposed to be an improvement over Piper's system?

As I recall the original selling point was to eliminate the header tanks especially the front as they had been known to rupture in a crash. They are very thin walled and easily crushed; splitting along the seams.
That is why the heavy duty tank was developed.

The thought of fuel dumping into your lap will cause you to loose sleep.

 

[cubpilot2]

Requiring the header tank, especially the front as I recall was to meet a certain fuel flow requirement to the carburetor inlet at a very high angle of attack. (Something like 45 degrees. ) And with something less than full fuel.
The aft fuel outlet alone was too low at that attitude to achieve the head pressure/ flow.

Flow had to exceed the engines max power consumption by a certain percentage and for a certain period of time.

 

[BC12D-4-85]

^^^^It's in CAR3 3.433 on.

Gary

 

[skywagon8a]

Designed to retrofit. You can put a valve in the vent line and turn the fuel to a single tank when you park on a side slope. I don't park with one wing low full of gas very often myself so it hasn't been an issue.

What is the purpose of this modification and why was it supposed to be an improvement over Piper's system?

As I recall the original selling point was to eliminate the header tanks especially the front as they had been known to rupture in a crash. They are very thin walled and easily crushed; splitting along the seams.
That is why the heavy duty tank was developed.

The thought of fuel dumping into your lap will cause you to loose sleep.

My question was about the cross vent line between the sight gauges which wireweinie explained as being for a plugged cap vent.
I fully understand the desire to remove the header tanks and for their original purpose.
Not only is the tank gauge cross vent a poor design for parking on a slope, it is also not good for a float plane where if one float leaked a bit, the fuel would also flow to that side exacerbating the situation. One should never leave a float plane in the water with any connection between the tanks. A vent connection between the inboard ends of the tanks and the fuel valve on both or off provides a flow path for fuel.
The optimum would be a cross vent with the connections at the outboard ends of the tanks and a fuel feed line from both the front and rear inboard corners of both tanks. In any case, the head of fuel must be above the carburetor or silence will happen during extreme nose up or nose down attitudes. Unless there is a fuel pump in the system which may not help in a nose down situation. Proper fuel system design is important. Band aids to fix a perceived issue sometimes opens other issues. I consider this gauge connection vent a band-aid fix.
For an in-depth understanding look at CAR 3.437 for "Determination of unusable fuel supply and fuel system operation on low fuel."

 

[Dog]

That is why there are the forward facing pressure fuel-caps especially with low fuel the compressed air will maintain positive pressure to force fuel to the carb, 2 of them connected by the vent line - for extra redundancy. My Beaver tip tanks have similar caps and with low tanks there is enough compressed air in there to feed pressurized fuel for a couple minutes after landing. With the correct installation the system meets and exceeds applicable standards you can second guess it all you want all the armchair engineering in the world will not change the fact that it passed the tests and met the regs. I just installed one in my latest project that will fly next week and you can trust in that I will give it a thorough test to make sure all is working as advertised. Now on the other side -- There are several Cessna crashes and emergency landings that come to mind where the pilots blamed the fuel system -- one where I had to deliver a fuel pump and bring some Gas just in case--guess what the pump was fine the story was the tank did not feed fuel to the engine. It was a save engine of landing so no one was hurt or the wiser.
On a 180 where the fuel system failed the chief pilot helicoptered in with 5 gallons of gas when the accident report came out there was fuel in the tank and it was a fuel system malfunction not pilot error, funny how that works sometimes. Know your plane and system and stay within the limitations avoid unusual attitudes when low on fuel. On the cross feeding through the vent line this can only happen if you have close to full tanks as long as you are switched on one tank. There is not much potential for much of a fuel imbalance as the tanks are almost full anyway. On the other hand if you leave here siting on both with halve tanks you will most definitely see your problem.
Again this is not the system it is the pilot problem. Sometimes it feels as if we are trying to find problems where there are none just to not deal with our shortcomings.
And just for the record I have done this for a long time and made my share of mistakes, while I do not like to admit them any more than the next guy at least I try to learn from them and not make the same one twice. Even better if I can save someone else from making the same mistake. How many here are actually doing annual training on type and configuration flown. Go up with a flying buddy and run through all the emergency practices than switch planes or seats and do the same for him this is a way better way to improve safety.

I guess I am a bit grumpy this evening

 

[mvivion]

My question was about the cross vent line between the sight gauges which wireweinie explained as being for a plugged cap vent.
I fully understand the desire to remove the header tanks and for their original purpose.
Not only is the tank gauge cross vent a poor design for parking on a slope, it is also not good for a float plane where if one float leaked a bit, the fuel would also flow to that side exacerbating the situation. One should never leave a float plane in the water with any connection between the tanks. A vent connection between the inboard ends of the tanks and the fuel valve on both or off provides a flow path for fuel.
The optimum would be a cross vent with the connections at the outboard ends of the tanks and a fuel feed line from both the front and rear inboard corners of both tanks. In any case, the head of fuel must be above the carburetor or silence will happen during extreme nose up or nose down attitudes. Unless there is a fuel pump in the system which may not help in a nose down situation. Proper fuel system design is important. Band aids to fix a perceived issue sometimes opens other issues. I consider this gauge connection vent a band-aid fix.
For an in-depth understanding look at CAR 3.437 for "Determination of unusable fuel supply and fuel system operation on low fuel."

Pete,
The Husky, a Part 23 aircraft, has crossover vent tube between tanks, much like the CC system being discussed. The Husky has individual tank vents at the wing tip. And, yes, if you park them on a slope with near full fuel, they’ll piss fuel out the down side vent. It’s a PITA, no doubt. When flying Huskys, I carried a plug for the down side vent. I know lots of folks used Thomas Dietrich’s “No Pee Valve” on their vents for this reason.

Point is, this is the result of FAA certification requirements. Poor design? Not a first.

As to seaplane rolling due to being parked with uneven fuel loads, many Cessnas, when parked, will port fuel through the selector valve. I was never taught to park them with fuel selector off or on one tank. Maybe bad practice on my part, but if your floats are damaged, get the thing out of the water and fix them.

But, while the crossover vent line can be a PITA as Dog says, it’s the operators responsibility to learn the system and its operation, and deal with it. I do agree that non certificated “band aids” like a shutoff on that line could cause some angst, again, if the pilot doesn’t understand proper operation and precautions.

MTV

 

[skywagon8a]

Pete,
The Husky, a Part 23 aircraft, has crossover vent tube between tanks, much like the CC system being discussed. The Husky has individual tank vents at the wing tip. And, yes, if you park them on a slope with near full fuel, they’ll piss fuel out the down side vent. It’s a PITA, no doubt. When flying Huskys, I carried a plug for the down side vent. I know lots of folks used Thomas Dietrich’s “No Pee Valve” on their vents for this reason.

Point is, this is the result of FAA certification requirements. Poor design? Not a first.
MTV

Mike,
I've not been involved at all with Part 23 certification. A quick look brings this:

"§ 23.2430 Fuel systems.
(b) Each fuel storage system must -
(3)Be designed to prevent significant loss of stored fuel from any vent system due to fuel transfer between fuel storage or supply systems, or under likely operating conditions;"

It seems to me that by your description, the Husky does not meet the requirements.

 

[frequent_flyer]

The Husky, a Part 23 aircraft, has crossover vent tube between tanks, much like the CC system being discussed. The Husky has individual tank vents at the wing tip. And, yes, if you park them on a slope with near full fuel, they’ll piss fuel out the down side vent. It’s a PITA, no doubt. When flying Huskys, I carried a plug for the down side vent. I know lots of folks used Thomas Dietrich’s “No Pee Valve” on their vents for this reason.

Point is, this is the result of FAA certification requirements. Poor design? Not a first.

The CubCrafters FX-3 (EAB) does not appear to have a crossover vent tube. There isn't one shown on the fuel system schematic and I have not seen one when I inspected the aircraft. The only vent tubes are between the inboard and outboard tanks in each wing. Maybe CubCrafters decided the crossover vent was a bad idea and deleted it for the experimental aircraft designs.

On a slope I park my FX-3 with fuel selector LEFT or RIGHT. There is no transfer between tanks. Still have to be careful if the tanks are close to full though as the vented caps are at the outboard end of the tank pair in each wing.

 

[wireweinie]

If you have a system with two tanks per wing, each with a vented cap, and inboard tank vented to outboard tank, you already have redundant vents on each wing. Therefore the crossover vent from one wing to the other would not be necessary.

As to Pete's idea of moving the vent ports to the outboard side of the tanks, what's everyone's veiw: major mod or minor? Ya'll know my veiw, lol.

Web

 

[frequent_flyer]

If you have a system with two tanks per wing, each with a vented cap, and inboard tank vented to outboard tank, you already have redundant vents on each wing. Therefore the crossover vent from one wing to the other would not be necessary.

There is only one vented cap per wing. It is in the outboard tank. Each wing's fuel tank system has only one vent to atmosphere.

 

[skywagon8a]

"Major alteration. An alteration not listed in the aircraft, aircraft engine, or propeller specifications—
(1) That might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness;" NO
"or
(2) That is not done according to accepted practices or cannot be done by elementary operations." NO

What is FAA's definition of "elementary operation"? There are only two prefixes under the E in the FAA's definitions. ET and EX.
What is an elementary operation for one mechanic may not be an elementary operation for another.

For me, welding a threaded bung to an aluminum tank is an elementary operation. For others using a good adhesive would be an elementary operation.

Adding additional venting to an already approved fuel system would have no adverse effects. Altering the fuel system itself would require FAA engineering approval. Probably room for argument here since we're addressing increasing a vent system.

My answer to Web ..... Minor Alteration.

 

[mvivion]

Mike,
I've not been involved at all with Part 23 certification. A quick look brings this:

"§ 23.2430 Fuel systems.
(b) Each fuel storage system must -
(3)Be designed to prevent significant loss of stored fuel from any vent system due to fuel transfer between fuel storage or supply systems, or under likely operating conditions;"

It seems to me that by your description, the Husky does not meet the requirements.

Pete,

Indeed, I should have noted that the vent lines in the Husky are equipped with check valves. In my experience, those check valves don’t seem to work well. Never met a Husky that wouldn’t piss fuel out the downhill vent when parked on a slope. A poor design, in my opinion, but obviously met Part 23 design criteria.

MTV

 

[mvivion]

If you have a system with two tanks per wing, each with a vented cap, and inboard tank vented to outboard tank, you already have redundant vents on each wing. Therefore the crossover vent from one wing to the other would not be necessary.

As to Pete's idea of moving the vent ports to the outboard side of the tanks, what's everyone's veiw: major mod or minor? Ya'll know my veiw, lol.

Web

I believe the installation of a second 18 gallon tank in each wing of the PA-18 was an STCd modification. I’ve only flown (that I can recall) two of those airplanes, and I’m trying to recall how they were plumbed. If memory serves, those tanks were connected together at the bottom, and outboard tank had no filler and thus no vent, which would have been in the fuel cap. There was no crossover vent line on either of those planes that I recall. There was a stock fuel selector, right/left/off,connected to inboard tanks. In effect, you just doubled tankage, but operating procedures were the same as stock.

Anyone else flown these only “Polar Bear Specials” and recall how they were plumbed better than me?

MTV

 

[frequent_flyer]

If memory serves, those tanks were connected together at the bottom, and outboard tank had no filler and thus no vent, which would have been in the fuel cap.

I don't see how it would be possible to get fuel into, or out of, a tank with only one port and no vent. CC long range tanks are filled from the outboard tank and one has to be very careful not to fill faster than the fuel can transfer from the outer to inner tank. The upper vent that links the two tanks gurgles signalling it's time to reduce fill rate. Ignore, or miss, that gurgle and the outboard tank will overflow long before the inner is full.

 

[wireweinie]

I believe the installation of a second 18 gallon tank in each wing of the PA-18 was an STCd modification. I’ve only flown (that I can recall) two of those airplanes, and I’m trying to recall how they were plumbed. If memory serves, those tanks were connected together at the bottom, and outboard tank had no filler and thus no vent, which would have been in the fuel cap. There was no crossover vent line on either of those planes that I recall. There was a stock fuel selector, right/left/off,connected to inboard tanks. In effect, you just doubled tankage, but operating procedures were the same as stock.

Anyone else flown these only “Polar Bear Specials” and recall how they were plumbed better than me?

MTV

Mike

It sounds like a simple plumbing system, but how do you verify that the outboard tank is actually full and not half full with air trapped in it?

Web

 

[mvivion]

I don't see how it would be possible to get fuel into, or out of, a tank with only one port and no vent. CC long range tanks are filled from the outboard tank and one has to be very careful not to fill faster than the fuel can transfer from the outer to inner tank. The upper vent that links the two tanks gurgles signalling it's time to reduce fill rate. Ignore, or miss, that gurgle and the outboard tank will overflow long before the inner is full.

Which is why I asked the question of others. Bear in mind, however that this was a mod developed by and for the polar bear hunters, who flew out with tons of gas, and 5 gallon cans in the back of one of the Cubs, client in the other Cub. Find a bear, Guide lands and he and the hunter stalk the bear. Assistant guide lands, and refuels both planes from cans. Getting cans out to outside fillers would be a BITCH.

Maybe there was a vent on the outer tank, and maybe that tank had a filler neck and vented cap.

It's been thirty years or so since I flew one of those planes, and I never flew either a lot, so memory may have faded. I just don't remember outboard fillers and caps, though. If they did, those caps would also have been vented, like standard Cub caps.

I'm also trying to recall how the Sierra tanks on one C-206 we had were plumbed, but I don't think there were outboard fillers there either.

Ah, to have the memory I THOUGHT I had when I was twenty....

MTV

 

[Marty57]

I've been reading and trying to digest all this great info here about fuel systems; without much clarity. Anything wrong with this set up plan; nothing plumed yet for my Wag 2+2/PA14 scratch build but close to needing to dive into it. My custom built wing tanks are about 16 gallons each and will have site gauges on the wing root. I had the tanks built with both a front and rear port on each tank along with sump and site gauge fittings. The plan has the two ports "T" together than run into the header tank. Each tank currently has simple vented caps. I have a header tank for under the panel. The plan is to have both tanks running into the header than a simple brass on/off valve before the gascolator on the fire wall. Fuel will be simple on/off from header tank. I know many here don't like a header tank for safety (how much different is that over a J3 tank btw) so let's leave that out of the equation for the time being. Any reason to not use such a simple set up? I'm really confused after reading all the posts on the Cub Crafter STC, and other fuel systems. My engine is an O290D2. Thanks

Marty57

 

[frequent_flyer]

I've been reading and trying to digest all this great info here about fuel systems; without much clarity. Anything wrong with this set up plan; nothing plumed yet for my Wag 2+2/PA14 scratch build but close to needing to dive into it. My custom built wing tanks are about 16 gallons each and will have site gauges on the wing root. I had the tanks built with both a front and rear port on each tank along with sump and site gauge fittings. The plan has the two ports "T" together than run into the header tank. Each tank currently has simple vented caps. I have a header tank for under the panel. The plan is to have both tanks running into the header than a simple brass on/off valve before the gascolator on the fire wall. Fuel will be simple on/off from header tank. I know many here don't like a header tank for safety (how much different is that over a J3 tank btw) so let's leave that out of the equation for the time being. Any reason to not use such a simple set up? I'm really confused after reading all the posts on the Cub Crafter STC, and other fuel systems. My engine is an O290D2.

I claim no expertise in fuel system design but maybe worth thinking about:

How would you stop cross feed and fuel spillage if you ever have to park with the wings not level?
What happens if you forget to put a gas cap back on? - Is all the fuel from both tanks lost
What about a leak in the feed from one or other tank? Is all fuel lost?

I'm used to systems in which left and right wing tanks can be isolated from each other and I'd not want to be without that capability.

 

[wireweinie]

Here are diagrams from the PA18 service and parts books. Notice the diagram from the service manual does not include the vent line on the front header tank. I find it interesting that the lines from the front tank doesn't tee together until the header tank. Anyone have insight into this?

Web

 

[Gordon Misch]

Marty, alarms are ringing and flashing.

Where would the tee be located? Unless it's well below the tank you could still unport in nose high or low attitude.

Imagine the following scenario. Tee by the front port, low on fuel but "enough" for the flight, such that in a pitch up attitude the fuel surface is below the forward port. The fuel line in that case would be at or close to the same height as the fwd port, above the fuel level. The system would draw air from the fwd port as the fuel in the line to the engine is drawn down.

This would be worse than the scenario Piper encountered, and for which I think an AD was issued for rear-port only installations. In that scenario fuel starvation could occur in a pitch-down attitude. But fuel flow would be restored upon pulling up. Fuel starvation in a pitch-up attitude could be horrific.

The tee needs to be well below the tank to avoid this. I would suggest the tee be right at the fuel valve(s).

FWIW, I like the simple two-valve system that Piper used on my -12, but a more modern valve, such is Andair with L, R, Both, off would be my choice for an Exp. That's what we're using on the Exp -12 I'm working on. Valve on the aft side of the firewall with an extension to the handle just under the panel. Similar to what Stinson did.

 

[wireweinie]

So as long as the tee is lower than say the bottom of the tank, fuel from either front or rear port will always displace air?

Web