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.
EdBC12D-4-85 liked this post
^^^^It's in CAR3 3.433 on.
Gary
Originally Posted by Steve Pierce
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."
N1PA
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
Last edited by Dog; 10-22-2022 at 02:00 AM. Reason: old grumpy guy g
mixer thanked for this post
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
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.
N1PA
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.
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
Life's tough . . . wear a cup.
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.
"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.
N1PA
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
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
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.
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
Life's tough . . . wear a cup.
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
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
N367PS
Psalm 36:7 "High and low among men find refuge in the shadow of His wing"
www.marty2plus2.com
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.
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
Life's tough . . . wear a cup.
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.
Last edited by Gordon Misch; 10-22-2022 at 04:46 PM.
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
Life's tough . . . wear a cup.
If lower (closer to the ground) than the lowest tank port in all pitch attitudes, then yes. But not merely at the "bottom" surface of the tank.
I get what you are talking about, sort of. I need to find a diagram that better shows the set up. My tanks differ from what are in my plans, more line the Cub Crafter tanks I suspect. I'm not opposed to skipping the header tank but also don't mind the extra fuel if it can work safely. I attached pictures to show what the plans call for and you can see my tanks are different than suggested in the plans. I have the simple caps on mainly to keep junk out of the tanks for now. Is this more like the Cub Crafter tanks? I guess I need to find a good diagram of the Cub Crafter set up; I know I have seen it here at some point. Will L, R, and both work with a header tank If I want to go that route?
Thanks,
Marty57
N367PS
Psalm 36:7 "High and low among men find refuge in the shadow of His wing"
www.marty2plus2.com
I agree, that's not a great drawing.
Here's the thing, as I see it - The fuel line from the aft port MUST run downhill from the port, such that fuel doesn't need to siphon over a hump in any pitch-up attitude. If it did need to siphon, flow would not restart in the event of any momentary unporting that could occur with turbulence or uncoordinated control input such as a slip. Similarly for the fwd port, that line must run downhill in any pitch-down attitude. It follows that the two lines MUST be teed together at a point that is always lower than the fuel surface (i.e. the lowest tank port) in any pitch attitude.
If you assure continuous gravity flow in any pitch attitude with the design, the header tank isn't needed. My -12 is plumbed like I've described, without a header, and I have never, ever, had a problem with fuel starvation. I suppose if one runs very close to empty, the header could help with intermittent unporting of a main tank due to sloshing, but my personal minimum is one hour reserve, so that's not a factor for me. And echoing somebody else in this thread, I would be very reluctant to count the header capacity as contributing to my plane's endurance.
I'm kinda verbose and insistent on this, I realize. I also realize that fuel system problems are a prominent, if not the most prominent, cause of EABl aircraft crashes.
When you are down to 5 gal of fuel and in a 3 point attitude or more how is that fuel going to run uphill past the front outlet of the tank?? It won't flow uphill from the back outlet and front tank outlet will only have air. Bring the rear outlet down to the floor than up to the volume tank. Put a sump at the low point of the rear fuel line on both sides. Trying to gauge how much fuel left in a tank with a site gauge is very hard. If you are wrong you have no reserve to fall back on.
DENNY
The is a digram below from a Bearhawk. It has the front fuel line installed running down the front windshield post, and the aft fuel line installed running down the aft door post. The tee near the floor. I include it to help visualize stuff discussed here. Your tank bungs are set up nicely for an install like this. The supercub headerless tank design seems to be similar.
But I also like a header tank system. It provides time to fill itself back up if flow stops. A 2 gallon header tank buys a lot of time especially on descent at idle..... and we don't often takeoff on a long duration climb with tanks less than 1/4 full. (That might be a good limitation) Plus your header will be filling from two sources though I guess 3/8" lines. That seems robust to me. I also like your header tank vent bungs. A liquid detector in that header tank could be installed that will light up anytime the tank is not full. https://www.radiantinstruments.com/p...iquid-detector
One principle of fuel systems design says that if a tank bung uncovers momentarily then an air bubble is created in that line. That bubble is a restriction that fuel has to flow around. I'd guess we might loose 50% fuel flow when it has to navigate a bubble. A longer bubble is probably way worse. Its very problematic. In the Bearhawk type of system when air enters the front line the rear line rear line will backfill the front line and push the air bubble back to the tank.
Lastly, when installing lines, any dip in a line or rise will trap water. Murphy is then sure to send a freeze your way.
Last edited by bcone1381; 10-23-2022 at 02:26 PM. Reason: added about two tanks in header system
One can use the cessna 206 fuelsystem as a good standard a headertank under the floor as the low spot with a waterdrain.
After that a fuel pump to feed a carb or injection on the 206.
The Maule uses a low spot with drains without the header tank on the left longeron bellow the pilot seat to achieve the same result without a header tank but still uses a pump even for the carb versions.
Both systems work well when managed properly and I have 29 years experience and many hours on both types they work perfect when managed correct.
Both have their share of maintenance problems but when maintained properly they are very save systems.
The CC system does not have a fuel pump but uses the forward facing fuel caps to maintain positive pressure to accomplish the same result.
If you use only Gravity feed the system needs to be perfect a bit of help from air pressure or a fuel pump go a long way to make fuel flow in appropriate amounts.
I've heard of 206 fatalities. Low level photo mission over urban area. Slipping and skidding for the photographer and a missunderstanding of which tank was low. No time to recover.
I have slipped toward the selected tank on final and had the 520 quit on roll out. Oops! Never again.
Sent from my SM-G965U1 using SuperCub.Org mobile app
The heavens declare the glory of God; the skies proclaim the work of His hands. Psalms 19:1
Parked isn't an "operating Condition"!
Don't recall offhand which regulation it is, but when you have a selector with a "Both" position, there is a requirement that the two tanks be vented together. I suspect that is one reason Piper didn't put a both position in their airplanes, it would have meant another $0.05 of aluminum tubing and they were cheap! Another reason for not having a both position is on some aircraft, there would not be sufficient flow and with the sun beating on the wing right over the fuel lines it could bet hot enough to cause vapor lock. By having only left or right, the flow would be sufficient to cool the tubing so vapor lock would not occur even at the highest operating temperatures.
Elementary operation is simple hand tools, wrenches, screwdrivers, pliers. That's what we were taught in A&P school back when I went in the 1970s.
So you would consider moving the vent ports to the outboard a major?
Web
Life's tough . . . wear a cup.
True, except Cessna didn't open up access to those belly drains coming out of the factory, at least on some 206s. I remember a 206 crashing at Merrill Field, with header tanks full of water, and frozen. Access ports to the drains had never been opened, and apparently, pilots and mechanics didn't realize they were there.....falling into the category of GOOD GRIEF!!!! I forget how old that plane was, but it'd been in service for a while......
As I noted earlier, any fuel system can kill you if you don't understand it and respect it's characteristics.
MTV
Oh, there's no doubt pilots have managed to screw the pooch in the 206 because of fuel mis management. The 206 I flew most was an F and had hard tanks replacing the bladders, so had 77 gallons of fuel useable. Another pilot who was used to flying a G model, borrowed the plane for a few days. I stressed to him the difference in fuel available, he said "yeah, yeah" and left. Three days later, I prepped the plane for a trip, and needed full fuel. It took 75 gallons, through a certified meter......had a little chat with the other pilot the next day, but said I was just trying to make him look bad. I told him I was just trying to keep him from killing himself and others......grrrrr!
They're out there, and some have pilots certificates.
MTV
Back in late 70's still lots of Cubs with " Polar Bear" tanks around Anchorage, not sure what was considered " standard" but I do remember a couple of different ones I flew had 4 Imperial selectors, same as a standard PA-12 would have 2 of them. There was no way to monitor either outside tank except to run them dry first. Using your watch to time them. I remember old Don Johnson telling me "I could run all four tanks dry and know within a minute of when she would finally quit"........ Not sure on that one: But I do know his son Warren, left one 'out on the ice' with Gordon Eastman (of Wildlife Photography fame) and was rescued ( barely) after 4 days at -25 below and they both were very very lucky, to survive it!Flying a Cub with 72 gals in the wings is weird for sure; Then add a 32 gal belly tank, and your getting into serious fuel situation!!! Ron Hayes says he used to comonly throw 4/6 cans in as well as the 104 gals he already had on board; to dump in later, over on the Russian coast! ( Approaching 850 lbs fuel) so add 5/600 lbs of people, guns, survival gear, and your at 1500 lbs of payload, and this was before Borer props! Most " polar bear Cubs" had 74/56 thru 60 pitch props on them!!
They worked em: for all there is in one ....
E
We should not loose sight of the fact, all this drama about front porting and headerless systems is really just a basic copy of how Piper did the Pacer fuel system. I have my left tank plumbed exactly like a Pacer with a front port that is already in your 18 left tank. Just remove the plug and plumb it out same as a Pacer, down and T in before left selector valve. Because my RT tank is NOT front ported: I will run the right tank dry first ,( and probably skid the plane to hold fuel up against inside of tank) till it quits, then simply switch, to my 'left tank with the front port' and figure if that won't limp us in; I was just cutting it toooo close.. No fault of my fuel system, jmho
Last edited by TurboBeaver; 10-27-2022 at 02:33 AM.
Currently deciding on the fuel line routing and valves for our Legend Cub build. Legend typically builds headerless systems.
We have Atlee tanks with the pickups front and rear. 3140-Installation-Instructions.pdf (fadodge.com)
I prefer the single L,R,Both,Off valve instead of two On/Off valves, but that's only because I've flown with that in the Cessnas. Not opposed to the two valve system, particularly if it's more robust and poses less safety issues.
Q's:
1) Any advice on whether to include the crossover tube in the system or not?
2) Tips for ease of maintenance down the road (tube routing tricks, etc.)?
Locate airframe sumps so not to interfere with a pod. I like them on the left lower longeron. My Wildcat sumps are in the center and they’re a pain to reach, but they are the low point and I don’t need a pod.
With good vents there’s no need for a balance tube.
I like my Andair L-R-Both-Off valve. I’d use it again.
We've moved the rearmost sump behind the pod.
Steve,
With the Dakota headerless, one valve system, do you recommend a crossover vent tube or do you feel the vented caps on each tank are adequate? This is for the Atlee 30.5g tanks with pickups front and rear on both.
Thanks,
Johnny
Last edited by JohnnyR; 11-21-2022 at 10:04 AM.
My E-AB Cub has a single Left-Right-Both-Off valve, no header tanks, dual tank outlets, 24 gallon tanks, snorkel vented caps and no cross vent line. Never had any type of issue nor saw any need for a cross vent line. I usually run on both, with both tanks always draining evenly. The size of the tanks should have no bearing on the operation of the system.
N1PA
Understood.
see my Post #28 for detailed question on potential risks of this config. Maybe I’m off base?
https://www.supercub.org/forum/showt...stem-decisions
See my answer in Post #29 https://www.supercub.org/forum/showt...stem-decisions
N1PA
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