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.
