No "BOTH" positions allowed on RV's

[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

[JonJay]

Excellent explanation. Th Fiat G.46 is plumbed exactly as you describe with interconnected vent lines. The vent sits atop the fuselage behind the rear canopy. I was wondering why they went to such trouble.
Both tanks also feed into a small header tank.

[Toobuilder]

Originally Posted by jliltd

...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."...

There is more to this than the vent balance however. The unporting issue will manifest itself easily with a low fuel, uncoordinated flight scenario. A high wing has the advantage of higher column pressure as well as the larger "reservoir" in the form of longer fuel supply lines.

This assumes that the pilot does not take the procedural step of selecting the fullest tank for landing.

[Pat Falley]

If using a BOTH system and tanks are vented properly, in uncoordinated flight, fuel will merely transfer from the high wing to the low wing, so unporting is not an issue until all the fuel from the high wing transfers.

If you have BOTH system, you can make a good case it would be safer on final in BOTH than selecting a tank and slipping the wrong way. Has anyone ever flamed out on final because they slipped with the selected tank low and then having it unport? I routinely use full rudder stop slips to get my wood prop RV4 down out of the break, but always consider which tank is selected vs. direction of slip.

[Toobuilder]

OK, so lets say you have managed a balanced fuel load with a "BOTH" selector and you are landing after a max effort cross country with the legal 30 minutes of fuel remaining. Let's call it 4 gallons total. That's 2 gallons per side. It's not going to take much of a slip to uncover one of the pickups, suck air and cease all fuel flow. OTOH, if you have a L/R system then you should have managed it so that one tank is empty (or so close as to be unusable) and the other one has 4 gallons on top of the pickup. Sure you could still unport, but the severity of the slip would be much higher because the one tank is more full.

A "BOTH" fuel selector could be managed in the same way, but it does add some extra steps - which is why it's possible technically, but sub optimal from a human factors perspective.

[Sam Buchanan]

Originally Posted by Toobuilder

OK, so lets say you have managed a balanced fuel load with a "BOTH" selector and you are landing after a max effort cross country with the legal 30 minutes of fuel remaining. Let's call it 4 gallons total. That's 2 gallons per side.

Moot point...I would have already been on the ground for 45 minutes....

Landing with two gallons in each tank? Wow....ok...I'm a wimp.....

[N941WR]

Originally Posted by Sam Buchanan

Moot point...I would have already been on the ground for 45 minutes....

Landing with two gallons in each tank? Wow....ok...I'm a wimp.....

When I had the O-290, four gallons was an hour of fuel. Two met the requirement to have a 30 minute reserve. Flying with extra fuel is overrated.

[Sam Buchanan]

Originally Posted by N941WR

Flying with extra fuel is overrated.

May be true...if you know precisely how much fuel is in the tanks....but I'm a wimp.

[BillL]

Originally Posted by Sam Buchanan

May be true...if you know precisely how much fuel is in the tanks....but I'm a wimp.

Me too. I was trained in a 182 with 110 gal tanks, "both" was standard, but since I everyone I worked with was a professional pilot of some sort, the drill was - - don't fly on "both" under a quarter tank, pick one and then be ready to switch to the better tank when the engine quits. That left out some guess work.

With gages like my 1950 Plymouth, they were not trustworthy.

Even with a totalizer, it is total, not precision by tank.

[Toobuilder]

Originally Posted by Sam Buchanan

Moot point...I would have already been on the ground for 45 minutes....

Landing with two gallons in each tank? Wow....ok...I'm a wimp.....

Yes, two in each tank is sketchy, but 4 in one is much better.