No "BOTH" positions allowed on RV's

[Larry DeCamp]

I fly a carbed engine with the Vans issue fuel valve circa 2008. The valve travels through OFF to switch tanks and the carb bowl accomodates an uneneventful process. My new project is fuel injected and I assume the supply pressure will drop immediately when the valve crosses OFF. What should I expect and is the Andair valve a better option for FI ?

[cccjbr6]

Originally Posted by jliltd

Vic. Thank you. Thank you.

One of the things that gets my attention in homebuilt aircraft more than anything else is proper fuel system design and implementation.

With respect to a BOTH selector, the same rules apply whether low wing or high wing.
The physical location or height of the fuel tank location or whether gravity or pump feed do not matter.


Vic is dead on target. The whole BOTH problem can be traced back to the 1920's and it has everything to do with balanced vent pressures above each tank.

The CAA certification regulations were revised to state that in order for a fuel system to be approved for a simultaneous feed valve position (i. e. BOTH) the air space above each tank's fuel level must be interconnected in order to balance the vent pressures. This requirement was written in blood. There is no reference to high wing or low wing.

If two or more tanks without common vents were simultaneously fed there is a chance for fuel starvation and in extreme cases one of the tanks to run dry and physically collapse depending on the venting. In other cases the pressure difference would transfer all the fuel into one tank and vent vast amounts of fuel overboard until empty. The original Cessna 120/140 aircraft have a left/right/off selector just like the Sportsman mentioned above, all without BOTH despite all being high wing aircraft. This is because they don't have a tank vent air interconnect of the space above each tank's fuel. The later Cessna 140A model (and most subsequent high wing Cessnas) had a BOTH position due to having a cross vent tube between the tanks inside the headliner. This satisfied the tank air spaces having equal pressure and when combined with vented caps or per tank vents added more safety by allowing one or more tank vents being plugged while still providing continued normal operation via the cross-venting.. One had to be careful to use fully vented caps in the Cessna 120/140 aircraft rather than those with rubber flap check valves integrated within the cap. In the case of the 140A an extra ram air vent on the roof tee'd into the cross vent line The earlier non-BOTH Cessnas had quite a few fuel starvation accidents attributed to one clogged fuel vent.

Lots of gotchas in the older fuel system configurations.

High wing aircraft lend themselves to the BOTH selector due to it being extremely simple to interconnect and balance the vents as described above. However, since it's all about interconnected balanced venting and not wing position, a low wing aircraft could in fact be allowed a BOTH position if the fuel system has properly interconnected venting. Unfortunately as a rule low wing aicraft geometry provides for a difficult configuration to accomplish this in a practical manner. Who wants a vent line snaking from the left wing root to the right wing root across the seats or following a canopy bow? This impracticality is the reason the rule of thumb is simplified to a straight admonition to never have a BOTH position in a low wing aircraft.

It should rather be the more correct, and wordier: "Never have a multi-tank feed fuel selector in an aircraft unless the vent air in the top of each tank is physically connected."

I have never seen an RV with the appropriate interconnected vents for a BOTH position but that doesn't mean it couldn't be done with an appropriate amount of effort, weight, aesthetic hit and inconvenience.

This particular rule isn't an example of beaucratic overkill. It was devised by airframe and systems designers to keep good folks from being killed.

Excellent thread Vic.

Jim

Jim (and everyone else),
Let me run this issue by you (actually two related issues). Anyone else with ideas please weigh in. As will become apparent below, I am a flyer, not a builder.

On my RV-6A I have a pair of removable Farn Reed integral 8.5 gallon aux tanks that do not have cross connected vents. Each tank has its own vent within the tank assembly. They bolt onto the spar inboard of the wing tip. I rarely need or use the aux tanks for trips, but I keep them on the plane just in case. They usually are empty.

When I purchased the plane it had two fuel selectors. The main selector had L, R and AUX settings. The AUX position was fed from a separate selector just for the aux tanks, marked L and R. So when in the AUX position I had to switch between the L and R aux tanks in flight.

The A&P I had upgrade the plane right after I bought it did not like that fuel set up. He removed the selector for the Aux tanks, and connected both aux lines so that they fed the directly in to the AUX port on the main valve (essentially a BOTH setting for the aux tanks).

Since that change, on the rare occasions I use them, the aux tanks feed unevenly and do not drain fully. At most I would use 6-8 gallons (per the Garmin fuel flow) out of 17 before the engine became fuel starved. On occasion I also would be missing fuel from the aux tanks upon landing. For example they would take 12 gallons when the fuel flow said I only used 6. I also see significant but uneven fuel loss from both aux tanks even when I don't use them. I checked both of the aux tank vent check valves to make sure they were working. One was not moving but was easily fixed. That did not solve the problem.

So I have two aux fuel problems, uneven feeding and loss of fuel in flight. I have looked carefully for a fuel mist in flight but have seen nothing. I have not seen fuel leaking on the ground. I checked for fuel stains and found them on both wings aft of the fuel cap above and aft of the aux tank vents below. The plane is dark blue and they are hard to see.

To troubleshoot the fuel loss, last week I replaced the o-rings on the aux fuel caps then I completely blocked both aux fuel vents, filled the aux tanks up and flew about 2.5 hours on the mains only. The aux tanks were still full when I landed. So I appear to be losing significant amounts of fuel from the aux tanks in flight, most likely from the vents. I think I would have seen it had it come from the fuel caps.

I would appreciate anyone's thoughts on what is going on and options to fix it.

One more thing. Apart from solving these issues, I have considered simplifying the fuel system by feeding the aux tanks directly to the adjacent main tank and doing away with the aux lines to the fuel selector altogether. I would greatly appreciate opinions on this idea as well.

[BillL]

Originally Posted by cccjbr6

The A&P I had upgrade the plane right after I bought it did not like that fuel set up. He removed the selector for the Aux tanks, and connected both aux lines so that they fed the directly in to the AUX port on the main valve (essentially a BOTH setting for the aux tanks).

If there was not a demonstrated issue with the original configuration, I might consider returning to it. It is a tough thing tracking another builders thinking and development.

Here are Bob's comments on his configuration - no comment on performance. There were some other threads at the time discussing the connection of tip type tanks, venting and issues with some of the proposals.

[lr172]

Originally Posted by Larry DeCamp

I fly a carbed engine with the Vans issue fuel valve circa 2008. The valve travels through OFF to switch tanks and the carb bowl accomodates an uneneventful process. My new project is fuel injected and I assume the supply pressure will drop immediately when the valve crosses OFF. What should I expect and is the Andair valve a better option for FI ?

My valve has an OFF position between L and R. I also have fuel injection and have never observed a single hiccup in 250 hours, even when I turn it slowly.

Larry

[rvbuilder2002]

Originally Posted by vic syracuse

However, I do wish we would standardize the way the fuel valve is installed in our RV's. Just today I performed a prebuy on an RV-8A that had the fuel valve installed such that the LONG arm of the selector points to the chosen tank. The little point on the small side of the selector handle had been ground off.




In my hangar currently undergoing a CI, I have an RV-10 that has the same valve installed such that the little pointer on the small end denotes the selected tank.

Good idea Vic
One standard could be to install valve per the plans.
The top photo is per plans with the little pointer removed as specified in the instructions. In the bottom photo, the physical installation of the valve is the same but the pointer was left in place and the placarding was reversed to index with the pointer instead.
If people don't like the design of using the handle as the pointer... blame me. It was my design. A lot of people fly RV's at night... an easily tactily sensed valve position is important. Painting a bit of read paint on a tiny pointer with red cockpit lighting does not cut it for me.

Originally Posted by JonJay

As you know, but others may not, in "legacy" RV''s like my 6, the fuel valve is plumbed in a "cross over" configuration. In order for the fuel selector to be correct, the long side of the handle becomes the pointer. Hence, the pointer was ground off to avoid confusion. Painting it red would be a good idea.

Originally Posted by Sam Buchanan

I agree. Guess my 1999 RV-6 is legacy but it is plumbed so the little pointer indicates the tank in use. The little pointer was painted red while the plane was still under construction.

I've done EAA inspections on RVs where I couldn't tell how the valve worked due to the lack of a clearly marked indicator. This became a point of discussion...maybe a dot of red paint will prevent a tragic accident by a future owner.

The early (legacy) RV-6 plans called for the valve to be installed like Sam's is configured. If built per plans, it also required passing through off between L and R (not ideal)

Originally Posted by airtractor8

I did the same thing. Here is a pic of the "Imperial" valve that came in my 1993 kit. It has a slightly different handle to the others shown in this thread and doesn't have the little pointer as such. Red pointed to the tank in use.
I removed it when it became so difficult to turn I was frightened the handle would break off halfway in between switching tanks It has a brass on brass type cone arrangement and despite several attempts to fix it nothing seemed to work. Could not stop the gland from seeping fuel either. Changed it out with an Andair. Problem solved

Good example of the early (legacy) valve. The sticking problem was the primary reason for the change in valve style sometime in the mid 90's, but the early valve could/can be made to work reliably if it is lubed with fuel lube (yea, that is one of the primary reasons the stuff exists) at each condition inspection. The original RV-6A prototype is still flying with one of these original valves.

Originally Posted by Larry DeCamp

I fly a carbed engine with the Vans issue fuel valve circa 2008. The valve travels through OFF to switch tanks and the carb bowl accomodates an uneneventful process.

Then it is another example of an RV who's fuel system was not installed per the plans. If done per plans the valve gets installed so that for left tank the handle points 45* to the left and 45* to the right (like Vic's first photo above). The valve never moves through off switching between tanks.

[PilotjohnS]

Please correct me if I am wrong, but didn't the FAA recently mandate that for certified planes the larger part of the fuel valve must point to the tank in use?

[rvbuilder2002]

Originally Posted by cccjbr6

Jim (and everyone else),
Let me run this issue by you (actually two related issues). Anyone else with ideas please weigh in. As will become apparent below, I am a flyer, not a builder.

On my RV-6A I have a pair of removable Farn Reed integral 8.5 gallon aux tanks that do not have cross connected vents. Each tank has its own vent within the tank assembly. They bolt onto the spar inboard of the wing tip. I rarely need or use the aux tanks for trips, but I keep them on the plane just in case. They usually are empty.

When I purchased the plane it had two fuel selectors. The main selector had L, R and AUX settings. The AUX position was fed from a separate selector just for the aux tanks, marked L and R. So when in the AUX position I had to switch between the L and R aux tanks in flight.

The A&P I had upgrade the plane right after I bought it did not like that fuel set up. He removed the selector for the Aux tanks, and connected both aux lines so that they fed the directly in to the AUX port on the main valve (essentially a BOTH setting for the aux tanks).

Since that change, on the rare occasions I use them, the aux tanks feed unevenly and do not drain fully. At most I would use 6-8 gallons (per the Garmin fuel flow) out of 17 before the engine became fuel starved. On occasion I also would be missing fuel from the aux tanks upon landing. For example they would take 12 gallons when the fuel flow said I only used 6. I also see significant but uneven fuel loss from both aux tanks even when I don't use them. I checked both of the aux tank vent check valves to make sure they were working. One was not moving but was easily fixed. That did not solve the problem.

So I have two aux fuel problems, uneven feeding and loss of fuel in flight. I have looked carefully for a fuel mist in flight but have seen nothing. I have not seen fuel leaking on the ground. I checked for fuel stains and found them on both wings aft of the fuel cap above and aft of the aux tank vents below. The plane is dark blue and they are hard to see.

To troubleshoot the fuel loss, last week I replaced the o-rings on the aux fuel caps then I completely blocked both aux fuel vents, filled the aux tanks up and flew about 2.5 hours on the mains only. The aux tanks were still full when I landed. So I appear to be losing significant amounts of fuel from the aux tanks in flight, most likely from the vents. I think I would have seen it had it come from the fuel caps.

I would appreciate anyone's thoughts on what is going on and options to fix it.

One more thing. Apart from solving these issues, I have considered simplifying the fuel system by feeding the aux tanks directly to the adjacent main tank and doing away with the aux lines to the fuel selector altogether. I would greatly appreciate opinions on this idea as well.

Chris,
For it to work properly, it needs to be plumbed the way Bob had originally done it. It does make it a bit more complicated from a pilot work load standpoint but that is sometimes what happens when you add complexity to a system.
Your two tip tanks now don't work properly because they are being effected by the exact same influences mentioned in this thread that make a both position on a fuel selector in a low wing airplane a bad idea.

[Toobuilder]

Originally Posted by Larry DeCamp

I fly a carbed engine with the Vans issue fuel valve circa 2008. The valve travels through OFF to switch tanks and the carb bowl accomodates an uneneventful process. My new project is fuel injected and I assume the supply pressure will drop immediately when the valve crosses OFF. What should I expect and is the Andair valve a better option for FI ?

The Rocket has never seen any issues with passing through "OFF" between tanks either. I only brought it up because this configuration is SPECIFICALLY prohibited per FAR 23, yet a "BOTH" position for low wing aircraft is not.

I just find it curious that people will categorically rally around the admonition against the use of a BOTH position for low wings (which is OK per FAR 23), but accept passing through OFF when switching tanks.

The above said, I am in agreement that the as designed RV fuel system does not support a direct retrofit of the fuel valve with a BOTH without adding some significant human factors risks, but that is not to say that the same valve cant be made to work if you have the appropriate design expertise and system execution. And considering how often Homebuilts are brought down by fuel starvation issues, it is prudent to really know what you are getting into before making the change.

[vic syracuse]

I thought I posted this but it didn't seem to go. I also like the tactile feel of using the large pointy end of the handle. That's the way I've done mine until the Andair valves came out. I fly at night and trying to see a red pointer with red cockpit lighting is not possible.

Vic

[Sam Buchanan]

The little red pointer on my fuel valve is primarily for a pilot who isn't familiar with the plane. I am very accustomed to how the valve handle feels and which way it rotates and it is second nature to operate without looking since it goes from 9:00 o'clock to 3:00 o"clock.

I don't fly at night.