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No "BOTH" positions allowed on RV's

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vic syracuse

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Please, please, please don't do this. I was performing a prebuy on an RV-8 today and for the second time I found a fuel valve with a BOTH position. Huge NO-NO! A both position in a low wing aircraft will fool you into thinking it works correctly, especially when both tanks are either full or above the fuel pickup. But let one of the tanks get below a fuel pickup and even if you happen to have a full tank on the other side, it will suck air. Probably at the worst time, like when turning base to final. More accidents are caused by modifications to the fuel systems than any other modification. It was a even very nice Andair valve, too. 😓

 
Yikes! What did the owner/builder say?

Wasn't built by the current owner, who is now deceased. The airplane had been completely destroyed once, then rebuilt, and ground looped twice after that, so who knows who did this mod. :(

Vic
 
In spite of the overwhelming presence of the internet, and the push to have EAA Technical Counselor visits mid-build, I am still surprised/dismayed by how many "lone wolf" builders there are that aren't seeking technical help from anyone. The fuel valve error should have been caught....by someone....

In our area we always gently suggest a social visit by local builders when we find a new project under way. Multiple sets of eyeballs can prevent heartache further down the road.
 
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I can surely see this as an easy gotcha for someone. I didn't step in this pothole myself, but could see the simple overlook not thinking.

Vic (and others) please keep these things posted, as it helps everyone for sure.
 
When I was a DAR, I once would NOT issue the Airworthiness Paperwork to an RV-6A builder/owner/applicant till the builder changed the fuel selector to one that did NOT have BOTH. I explained how one tank could be full, the other sucking air with the switch in BOTH and the engine would quit running.

The builder / applicant changed the valve while I waited / watched.
 
it is my understanding that a L R fuel valve with no "BOTH" position still has one of you place the selector half way between the L and R. I has to be this way so that when switching tanks, the 2nd tank kicks in before the former tank is cut off. If so, be sure to place the fuel selector all the way to one position or the other and be aware of consequences for failing to do so on a low wing aircraft.

My opinions only. YMMV

Bevan
 
The Aero Commander 112 and 114 single engine low wing airplanes have fuel selectors with a both position AND a POH that recommends using the both position. I think some here are a bit extreme. Any empty tank will allow "sucking air". Doesn't matter if your fuel selector has only a Left and Right position or also includes a Both position. When the tank is dry you simply change from the empty tank. If the pilot understands his fuel system as he is supposed to then there is no surprise. He will know what happened and what is to be done to correct. It's a standard checklist item to place fuel selector on fullest tank when taking off or landing. The certified design accounts for running a tank dry and switching to a tank with fuel. What's the big fear here?
 
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Tech Counselor

When I was finishing my last build, a non RV, the closest tech counselor was 200 miles away. I did get one inspection early on but that guy did not have anything to offer despite being a Oshkosh Grand Champion builder. One EAA chapter eliminated the Tech position out of liability concerns. They are mostly a social group, only a couple of builders.
I know some outstanding tech counselors, all are a long ways away.
 
If the pilot understands his fuel system as he is supposed to then there is no surprise. He will know what happened and what is to be done to correct.

What's the big fear here?

The answer is in your question. Too many pilots do not fully understand their own fuel system, and when that becomes problematic it all too often becomes fatal. This is especially true of non-builder pilots that have never been exposed to the fuel plumbing.
 
...What's the big fear here?

As one who has been down this road before on VAF, I'll say that I agree in principle that a "both" selector is not inherently dangerous if managed properly. That said, that level of management is a human factors nightmare (strike 1), is nonstandard among a group of nonstandard airplanes (strike 2), and the consequences of getting it wrong is extremely hazardous (strike 3).

Yes, this forum has a distinct "...but think of the children!..." vibe to it, but in this case it's probably justified. There are plenty of non builder owners out there and there's no standardized POH to turn to for guidance.
 
If we did that, there would be no Rockets!
Are Rocket fuel systems different than the plans? Didn't Harmon offer plans?
Point being, if you dont know what your doing, (not you), do what the manufacturer of the kit recommends.
 
Please, please, please don't do this. I was performing a prebuy on an RV-8 today and for the second time I found a fuel valve with a BOTH position. Huge NO-NO! A both position in a low wing aircraft will fool you into thinking it works correctly, especially when both tanks are either full or above the fuel pickup. But let one of the tanks get below a fuel pickup and even if you happen to have a full tank on the other side, it will suck air. Probably at the worst time, like when turning base to final. More accidents are caused by modifications to the fuel systems than any other modification. It was a even very nice Andair valve, too. 😓

....I actually had a low wing aircraft with a fuel valve set up this way, and I will say it required extra care in use. It had a fuel circulation pump on each tank that could be operated interdependently from one another with switches. The intention was to enable you to select "both" on the valve and turn on one of the fuel pumps to transfer fuel to the other tank for balance purposes. This all sounds like a good idea, but in operation it required to much attention. Way to easy to forget a step and it would bite you. Simplicity is reliability, and the older you get the more important that is as C.R.S. sets in....:D
Thanks, Allan
 
Even Vans says to

I got an idea. How about we follow the plans!

Just as a data point, I was even directed by the mothership to not follow the plans and alter the fuel vent lines in an RV-8 to match a post here on VAF.

The mod was to create a fuel vent in the wing root to avoid running it through the gear towers.

I may be mistaken, but I think that mod originated in a Rocket.

Sometimes good ideas come along, but Vans can't keep changing the plans.

Build well, build thoughtfully, seek guidance, understand what you are doing, fly safe, be safe.
 
Are Rocket fuel systems different than the plans? Didn't Harmon offer plans?
Point being, if you dont know what your doing, (not you), do what the manufacturer of the kit recommends.

Don't want to cloud the message of this thread, but in the interest of trivia the first Rocket was built without plans.

And slightly more to the point, the typical Rocket fuel valve has a L - OFF - R actuation, which has you pass through OFF every time you switch tanks. This is a significant faux pas according to FAR 23.
 
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Any empty tank will allow "sucking air". Doesn't matter if your fuel selector has only a Left and Right position or also includes a Both position.

True

But it is SOP to select the fullest tank for landing. If a pilot does so, they have at least reduced the likelihood of an un-ported fuel pick-up to near zero.

If they ignore that procedure because they think "hey, I have a both position.... I am already feeding from the fullest tank" and then during landing they either run out of fuel on the lower tank or un-port the pick-up for some reason, the engine will quit.......
 
Silly me . I thought this thread was about fuel selectors.

It is. But even the plans sometimes violate established standards. No harm in questioning things - even Van the man.

Here's the deviation from the FAR often seen on Rockets (and maybe the -4?):

?23.995 Fuel valves and controls.
(g) Fuel tank selector valves must?
(1) Require a separate and distinct action to place the selector in the ?OFF? position; and
(2) Have the tank selector positions located in such a manner that it is impossible for the selector to pass through the ?OFF? position when changing from one tank to another.
 
I have to side with the OP. Couple of reasons.... 1) He's certainly an expert by most anyone's standards, certainly mine. 2) Why not "build out" any ANY potential gotchas and like he said they'll usually show their ugly head at the worst possible time.

The thought process of know your fuel system is of course important but at the same time, it's not a guarantee. Think of this... during one of the Saturn 5 rocket launches that took us to the moon, one of the main rocket engines shut down. Why, because despite all the checks built in by NASA and the rocket manufacturer they simply had two wires crossed! Their fix was to modify the wires so they would fit only one way thus building out any possible failure modes.

Because I know me, I built my plane as idiot proof as possible! :)
 
One issue with the "both" position that hasn't been brought up yet as far as I can tell is that, particularly with the analog fuel guages shown in the photo, it would be really hard to tell how much fuel really was in each tank. With the traditional setup, it's pretty easy to calculate fuel burn and subtract from a known quantity at the beginning of each flight so that at any given time, one should know exactly how much fuel remains in each tank without reference to the gauges. I'm not an authority, but i'd be surprised if the fuel flowed perfectly evenly from each tank with the selector set at "both" meaning that if one tank should run out, you could never be 100% sure how much fuel was in the remaining tank, only that it was "somewhere between 5 and 10 gallons" on the analog gauge. Digital instrumentation would certainly help, and it's definitely possible to forget to switch tanks on a L-off-R setup which would have the same result. In the end, I don't think it's way more dangerous to have the "both" setting in a low wing plane, but I don't see a single advantage of it either.
 
So when you are flying along with the selector set to both and the engine coughs from fuel mis management. Which tank are you going to switch it to. Oh wait, let me look at the gauges. I would like to think I will never be the one that runs out of fuel, but I bet no one has ever taken off on a flight and said this is the day I run the tanks dry.

We are all free to build the plane we want, but some of our fellow aviators have paid the price with blood. Mistakes happen, but we can learn from others and hopefully not ever repeat a mistake.
 
Lots of good comments, and I want to be the last one to stifle creativity EXCEPT with fuel systems. A good friend of mine just lost his plane because the engine quit at 12,500' and they rode it all the way to the ground and totaled it without ever switching tanks. The EFIS data clearly shows the tank ran dry.

So how is that relevant here you ask? Well, there were a few posts about how a BOTH system can work in a low wing with extra care and precaution. Ever hear of tunnel vision that some pilots get when faced with an emergency? In this case, they were focused on the turbocharger becasue it had a prior failure a few years back, and the engine just happened to quit as he was adding some boost. Purely coincidental, and lots of failure points after that that we can all armchair quarterback
.
But don't judge until you have been faced with the emergency. For me, I'd rather have a fuel system that DOESN'T require some extra precaution at the worst possible time.

Vic
 
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I have a "both" fuel tank selector option on the 6A that I just bought, and I've found myself using it less and less as I fly my plane more. I was reminded during transition training in my airplane to always have the selector on the fullest tank (not on "both") for takeoff and landing, which is what I've always done. I'd then go to "both" once established in cruise. Most of the time though I'd still eventually see uneven fuel levels in the tanks after a while at cruise, so I'd just revert back to feeding from an individual tank and switching when the GRT Sport interval alarm would go off anyway. :)

I have much to learn about RVs and airplane ownership in general, so I really appreciate the discussion and various opinions on here. I now feel a bit of confirmation in my thinking pertaining to how I'm using my fuel tank selector, and really see no good reason to utilize "both" in the future!
 
And now for a slightly different perspective... I'm building a Glasair Sportsman - a high-wing aircraft with fuel in both wings and gravity feed to the engine. A perfect candidate for a "both" fuel selector, right?

Think again!

The default design from the factory has the fuel lines from L and R tanks coming to a Tee, then to a simple On/Off selector. Great! Simple, cheap, easy, light... Except that some aircraft would develop significant fuel imbalances because the fuel tank vents could produce different air pressures in the tank, essentially force-feeding fuel from one tank while the other tank remained pretty much full. Not good!

As a result of the fuel imbalances experienced in the field the standard fit now is a 4-position valve (L-Both-R-OFF) which allows us to even out fuel burn if we need to.

As with many lessons in aviation, the "no Both position on low wing aircraft" rule of thumb was written in blood. While the Both position may work, it might also bite you at a very inopportune time. Better not to have the Both position than to have the risk of it being misused.
 
I have a "both" fuel tank selector option on the 6A that I just bought, and I've found myself using it less and less as I fly my plane more. I was reminded during transition training in my airplane to always have the selector on the fullest tank (not on "both") for takeoff and landing, which is what I've always done. I'd then go to "both" once established in cruise. Most of the time though I'd still eventually see uneven fuel levels in the tanks after a while at cruise, so I'd just revert back to feeding from an individual tank and switching when the GRT Sport interval alarm would go off anyway. :)

I have much to learn about RVs and airplane ownership in general, so I really appreciate the discussion and various opinions on here. I now feel a bit of confirmation in my thinking pertaining to how I'm using my fuel tank selector, and really see no good reason to utilize "both" in the future!


Thank you! I am very proud of you. :)

Consider putting a placard over the BOTHposition to blank it out.

Vic
 
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Thank you! I am very proud of you. :)

Consider putting a placard over the BOTH position to blank it out.

Vic

Good idea, Vic.

Some may view taking that step as unnecessary, reactionary, or even over the top, and that's fine. But for me, if I have the option to eliminate a certain risk (not using "both" at all, and placarding it as such), rather than just decrease the risk (by "proper" use of "both"), in this case I would rather get rid of that risk altogether. Why chance a fuel starvation situation by using "both" when it can just be avoided in the first place?
 
I'm building a Glasair Sportsman - a high-wing aircraft with fuel in both wings and gravity feed to the engine.

The default design from the factory has the fuel lines from L and R tanks coming to a Tee, then to a simple On/Off selector. Great! Simple, cheap, easy, light... Except that some aircraft would develop significant fuel imbalances because the fuel tank vents could produce different air pressures in the tank, essentially force-feeding fuel from one tank while the other tank remained pretty much full. Not good!

As a result of the fuel imbalances experienced in the field the standard fit now is a 4-position valve (L-Both-R-OFF) which allows us to even out fuel burn if we need to.

Yes, but given the fuel design of the Sportsman, you can drain one tank completely dry and it will still pull from the other one unless your pressure imbalance is about 1.5 PSI or more, which is darn near impossible to achieve in a sub 150 knot airplane ;)

So the left-right selector is for the annoyance of a heavy wing imbalance, and the pilot discomfort over seeing 1 gallon in one tank and 6 in the other, not the chance of an engine actually stopping due to the imbalance.

I have personally been in a Sportsman on "BOTH" with 0 gallons in one tank and between 6 and 2 in the other over half an hour with no issues. It's a functionally safe fuel system with only an ON-OFF selector.
 
it is my understanding that a L R fuel valve with no "BOTH" position still has one of you place the selector half way between the L and R. I has to be this way so that when switching tanks, the 2nd tank kicks in before the former tank is cut off. If so, be sure to place the fuel selector all the way to one position or the other and be aware of consequences for failing to do so on a low wing aircraft.

My opinions only. YMMV

Bevan
You are exactly right. I have a spare fuel valve and checked this out... Three dedents - Left, right, off. If you stop between left and right, it feeds from both tanks.
 
Jordan - I was merely pointing out that, even in an aircraft which should be reasonably ideal for the use of a "Both" selection, it's still not necessarily ideal, and that the lessons learned through the preponderance of experience in certain aircraft types should be heeded when making your fuel system.
 
What about the next owner?

We mostly build the plane WE want. But barring a catastrophe, our aircraft will eventually end up in the hands of someone not intimate with its construction. Shouldn't there be some responsibility for meeting acceptable standards?

I've heard/read various versions of the accident but we know the loss of John Denver was related to non standard placement of the fuel valve. What I haven't read is what that builder went through afterwards.

Just sayin......
 
What did Burt Rutan go thru afterwards??

This was a case of not following the plans. Burt did NOT build John's plane or design the fuel switch location where it was installed on the plane. I am sure Burt had lots of grief over the loss of John, but no more than Van would have if someone did not follow the plans with a modification that later turned fatal.
 
Please, please, please don't do this. I was performing a prebuy on an RV-8 today and for the second time I found a fuel valve with a BOTH position. Huge NO-NO!

Vic, it may be possible that the "BOTH" position is not plumbed. Did you ask the owner if he actually gets fuel out of the "BOTH" port. It may just be a capped port. In my aircraft I have the same valve but the centre port is capped and labelled "AUX". It is available only for a temporary ferry tank.

Mind you, if the central port is indeed capped it is certainly misleading and dangerous for it to be labelled "BOTH" (for obvious reasons).
 
Vic, it may be possible that the "BOTH" position is not plumbed. Did you ask the owner if he actually gets fuel out of the "BOTH" port. It may just be a capped port. In my aircraft I have the same valve but the centre port is capped and labelled "AUX". It is available only for a temporary ferry tank.

Mind you, if the central port is indeed capped it is certainly misleading and dangerous for it to be labelled "BOTH" (for obvious reasons).


An "AUX" input port is different than a "BOTH" position on the selector. The BOTH position is an internal configuration that allows the tanks to feed simultaneously.

Vic
 
Perhaps someone can clarify for my benefit how the Beech Skipper fuel system differs from that of an RV.

The low wing Skipper training aircraft only had a BOTH or OFF position. It did have left and right fuel gages, plus a low fuel alarm and annunciator light, which I believe was supposed to go off at 3 gallons remaining.

This system completely removed the issue of fuel L-R switching from student pilot training in the aircraft, which could be viewed as either good (student pilot never loses engine on solo cross country due to failure to switch tanks), or bad (student pilot never learns fuel tank management). Never had an issue with left or right heavy wing that I recall.

I'm particularly interested in how it worked, such that the empty tank sucking issue described here never occurred. Perhaps just a low tube cross-connecting both tanks, such that they always had essentially the same level?

I'm a fan of the simplest possible systems in an aircraft, so it could be something useful in an RV it if can be made to work reliably as in the Skipper.

-Paragon
Cincinnati, OH
 
IIRC, The Skipper has a small header tank in the lower fuselage that acts as an accumulator that damps out most of the problems of unporting a single tank.
 
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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
 
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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.
 
...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.
 
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.
 
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.
 
:D
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.....:D
 
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:D

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.....:D

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. ;)
 
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.
 
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