Why is there not a both fuel selector in low wing GA aircraft

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Dhs-9

Filing Flight Plan
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DHS-9
I have read many post in a lot of forums that Low wing do not have both in case one tank runs dry the pump would pull from the dry engine not the one with fuel.

First except for a leak, clog etc or just turning in circles all day I would think a low wing aircraft running properly would have pretty even fuel burn between tanks

I know the FAR indicate no one fuel pump can draw from two tanks at a time.

So why wouldn't this work so pilots don't have deal with diverting attention to switching tanks just use two fuel pumps one in each wing with a selector left both right and off

Each tank having its own pump does not violate FAR. So you are on both and notice a significant fuel imbalance on your fuel gauge something is probably wrong (again I dont see why burn rate port and stbd sould differ that much) so you switch tanks now you are using the pump in that wing only. With both selected left and right pumps run at same time.

If somehow you manange to run one tank dry maybe pilot error in not topping off both tanks (or leak) you would think the pump in the wing with fuel would keep you flying even without selecting the wing with the fuel in it. If the pump senses it is sucking air employ a sensor to shut it off, again your other pump will still be running. It is pretty easy just to look at your fuel gauge to see if you have uneven burn or one tank is empty, then select the tank with more fuel in it if you wish.

Where is this logic incorrect it seems too simple that this type of system is not used.
 
Two pumps is twice the chance to go wrong? Two the cost? Twice the weight?

It's a design decision. If you're worried about forgetting to change tanks, get an apple watch, turn on the vibrate mode, and set a repeating timer for 30 minutes when you start the airplane up.
 
bflynn said:
Two pumps is twice the chance to go wrong? Two the cost? Twice the weight?

It's a design decision. If you're worried about forgetting to change tanks, get an apple watch, turn on the vibrate mode, and set a repeating timer for 30 minutes when you start the airplane up.
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Exactly what I do.
 
I imagine a lot has to do with head pressure. There is a lot of “free” pressure when you lift that much gas above your head in a Cessna, even if the engine driven pump gives up. In fact, the 150s & 172s I trained in did not have fuel pumps at all (engine driven or electric boost) & were plumbed direct to the carb.
 
Almost all pumps will fail if run dry. In a self system the possibility of one running fry is fairly high - even in high wings you’ll frequently see 28 one one side and 4 on the other). But a sensor to turn it off is problematic. What if the sensor fails in the wrong position and shuts off ? Ok let’s run a indicator light that then has to turn off. Well that’s starting to need to be managed no different than switching tanks….
 
Dhs-9 said:
I have read many post in a lot of forums that Low wing do not have both in case one tank runs dry the pump would pull from the dry engine not the one with fuel.

First except for a leak, clog etc or just turning in circles all day I would think a low wing aircraft running properly would have pretty even fuel burn between tanks

I know the FAR indicate no one fuel pump can draw from two tanks at a time.

So why wouldn't this work so pilots don't have deal with diverting attention to switching tanks just use two fuel pumps one in each wing with a selector left both right and off

Each tank having its own pump does not violate FAR. So you are on both and notice a significant fuel imbalance on your fuel gauge something is probably wrong (again I dont see why burn rate port and stbd sould differ that much) so you switch tanks now you are using the pump in that wing only. With both selected left and right pumps run at same time.

If somehow you manange to run one tank dry maybe pilot error in not topping off both tanks (or leak) you would think the pump in the wing with fuel would keep you flying even without selecting the wing with the fuel in it. If the pump senses it is sucking air employ a sensor to shut it off, again your other pump will still be running. It is pretty easy just to look at your fuel gauge to see if you have uneven burn or one tank is empty, then select the tank with more fuel in it if you wish.

Where is this logic incorrect it seems too simple that this type of system is not used.
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There are Cessna models (182RG and 182T) certified under Part 23 with engine driven and electric fuel pumps that normally deliver fuel from both tanks. They are simple systems requiring no special valves or metering.
 
The Navion (if I remember correctly) is low wing with "both" setting. So it is possible. Not sure of the design for the Navion.
 
I suspect @bflynn is close to the answer. Two auxiliary fuel pumps add weight and cost and complexity. Keeping off unnecessary weight is a common answer to design "why?" questions.

Kind of like the answer to, "why are a 172's ailerons corrugated?"
 
Dhs-9 said:
I have read many post in a lot of forums that Low wing do not have both in case one tank runs dry the pump would pull from the dry engine not the one with fuel.

First except for a leak, clog etc or just turning in circles all day I would think a low wing aircraft running properly would have pretty even fuel burn between tanks

I know the FAR indicate no one fuel pump can draw from two tanks at a time.

So why wouldn't this work so pilots don't have deal with diverting attention to switching tanks just use two fuel pumps one in each wing with a selector left both right and off

Each tank having its own pump does not violate FAR. So you are on both and notice a significant fuel imbalance on your fuel gauge something is probably wrong (again I dont see why burn rate port and stbd sould differ that much) so you switch tanks now you are using the pump in that wing only. With both selected left and right pumps run at same time.

If somehow you manange to run one tank dry maybe pilot error in not topping off both tanks (or leak) you would think the pump in the wing with fuel would keep you flying even without selecting the wing with the fuel in it. If the pump senses it is sucking air employ a sensor to shut it off, again your other pump will still be running. It is pretty easy just to look at your fuel gauge to see if you have uneven burn or one tank is empty, then select the tank with more fuel in it if you wish.

Where is this logic incorrect it seems too simple that this type of system is not used.
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Get a glass of water and two straws. Put straws in water but with the exposed ends staggered. Now suck with both straws in your mouth. Then suck on just the longer one leaving the shorter one exposed. The shorter one represents the tank that ran dry. The longer one is the tank that still has gas in it. But you aren’t going to get any gas out of it.
 
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jsstevens said:
The Navion (if I remember correctly) is low wing with "both" setting. So it is possible. Not sure of the design for the Navion.
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There's a small tank down next to the fuel selector (it's where the quick drain is as well) that both mains drain into and it's pumped from there. In fact, there's only one fuel filler cap and one fuel guage for both wing tanks. The stock Navion fuel selector is ON (both) and OFF. If you have an underseat aux, it will be MAIN - AUX - BOTH. I have a baggage tank that drains into the main (simple valve) and two tip tanks that can be seleted with the fuel selector.
 
flyingron said:
My plane is a low wing and hit has a both selector (in fact, it's just OFF and BOTH). There's a small tank down low by the fuel selector that both main tanks drain into and it's pumped from there.
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Ron has a Navion.

So the main tanks gravity into the sump tank, which is pumped to the engine. Gravity (head pressure) makes sure the sump has fuel, as long as the main tanks have any...
 
luvflyin said:
I think the Tomahawk did to.
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You’re probably thinking of the Beech Skipper instead. They were one of the few low wing aircraft I’ve seen that will feed from the main wing tanks at the same time.
 
Ask john denver about this... Oh you cant. Many accidents ocur due to failure to switch tanks or beleive it or not faiing to do it properly including not moving switch fully to one position or another look at NTSB reports on this. A fuel pump only weights about 10-15 pounds each I am sure and extra 15 pounds isn not going to affect performance in any way. As for failures of sensors sure it can happen. I am not even sure an auto cutoff sensor would be neccessary in a two pump system. The dry pump sucks air while the good pump still sucks fuel independently. The mixture would be off and you would see this right away but I am not sure the engine would quit (not an engineer). And yes every forum speaks of the two straw example which in this case proves my point for two pumps.
 
If you have the capability for an extra pump. A lot of engines don't have an extra place to put a second engine-driven pump. Yeah, you could make it electric, but you've now added another failure point to fix the first one.

Part of John Denver's problem is he was about to have his balky fuel valved worked on which is why I believe he was flying with such a small amount of fuel. He was burning off what he had so it didn't have to be drained. I have to admit I did the same prior to having fuel system work done on the Navion. But I did it in a situation that I could get down safely if I managed to screw up the burn.
 
He didnt top off becuase of a short flight after making a few touch and goes. His fuel valve was not being repaired it was very hard turning and he knew this by trying of all things to put a vice grip on the selector. The problem according to NTSB he was pushing right rudder while turning 90 to turn selector over his and behind his left shoulder and had uncoordinated flight at 500 feet (a bad altitude to be switching tanks anyway). The plan for this particular aircraft was was to put aircraft on AP and then make the change...he didnt do this. As for fuel pump again not an engineer but I dont think a pump has to go on the engine per se. I would rather have two pumps than one. If you have one in a low wing and it fails your done. If you have two and one fails your flying. Like any airliner redundant system which also at the same time takes one more thing off the pilots plate.
 
Dhs-9 said:
A fuel pump only weights about 10-15 pounds each I am sure and extra 15 pounds isn not going to affect performance in any way.
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Ever notice that older airplanes of the same model weigh substantially less than newer ones? And that the owners of the newer ones complain about not having enough useful load? A little extra here and there adds up.
 
Sure it does but I would give up 15 pounds of payload for a safety feature such as this
 
I dunno -- switching tanks isn't that big of a deal IMO. I do it in my low wing airplane on every flight over an hour in duration and without looking. The issue with John Denver was his setup (which he didn't build) was truly wonky and the fact he chose to fly with it that way. That said I do have 2 fuel pumps: the engine's mechanical one and an electric boost pump (which is off except for startup, takeoff, and landing).
 
Best idea yet. But this goes to my original Q why isnt it done routinely on low wing. Couldnt cost that much more and it would be a selling and safety feature.
 
Dhs-9 said:
Best idea yet. But this goes to my original Q why isnt it done routinely on low wing. Couldnt cost that much more and it would be a selling and safety feature.
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Simple answer is because it's a non-issue. The overwhelming majority of fuel related accidents have nothing to do with the fact there's not a both setting or a boost pump for each tank on low wing aircraft.
 
Dhs-9 said:
Best idea yet. But this goes to my original Q why isnt it done routinely on low wing. Couldnt cost that much more and it would be a selling and safety feature.
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It wasn't done simply because it was a cost efficient method to meet the certification requirement. Just like a number of other designs decisions that were made like the use of non-pullable CBs. As to the cost of an STC, you would money ahead to simply buy an aircraft with a fuel selector with "both" than develop an STC. However, I doubt the market would not be much larger than you and maybe a dozen others. I dont recall it being much of discussion over the years.
 
ateamer said:
Left or Right - no Both position.
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Bell206 said:
It wasn't done simply because it was a cost efficient method to meet the certification requirement. Just like a number of other designs decisions that were made like the use of non-pullable CBs. As to the cost of an STC, you would money ahead to simply buy an aircraft with a fuel selector with "both" than develop an STC. However, I doubt the market would not be much larger than you and maybe a dozen others. I dont recall it being much of discussion over the years.
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non-pullable CBs ????
 
Even two pumps does not solve the issue of introducing air into the system. You would need a series of check valves to avoid air and avoid pushing fuel into the opposite tank in the event of a pump failure or fuel tank leak. Next time you are at a restaurant ask for two straws. Put one in your drink and the other outside the glass. Put both in your mouth and try and drink!
 
I’m guessing high wing is more gravity driven than a low/mid wing and just how they designed it. I used to be a little worried about that changing from a Cessna to a Piper but it’s really never been an issue.

On approach to an airport you have your GUMPS (CGUMPFS) check which is G for gas, check that fuel selector is on the fullest (or desired) tank, after awhile it just becomes mental habit, and if you fly the airplane and forget to the change the fuel selector, it gets out of balance and starts to pull to the fuller tank (due to the extra weight) and then it’s like riding a bicycle, if you want to go straight you are like let’s balance this out and switch fuel tanks.

I’ve talked to some pilots about longer flights and the concern of running out of fuel, winds aloft can suddenly change and stuff happens, in that case you could run a tank dry, the engine starts to get rough, flip on the fuel pump and switch tanks, then you know you’ve used all possible fuel instead of swapping over with a couple gallons unusable remaining (which probably gives you an extra 15 min). Not a general practice to attempt but if crap hits the fan, optimizing your options / endurance.
 
Everyone talks about the two straws which furhter supports the case for two pumps. Yes check valves would be needed but these are minor engineering issues easily done with solenoids which would auto activate upon switching Pumps from the left or right or both depnding on what you are doing to isolate the shut off pump.
 
luvflyin said:
non-pullable CBs ????
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Non-pullable (manual trip) CB on the left and pullable on the right. With the pullable about 50%+ more expensive which is the reason some OEMs use those cheaper types as it meets the certification requirement. Same as no "both" for the fuel selector. Cheaper.
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Jeff767 said:
Even two pumps does not solve the issue of introducing air into the system. You would need a series of check valves to avoid air and avoid pushing fuel into the opposite tank in the event of a pump failure or fuel tank leak. Next time you are at a restaurant ask for two straws. Put one in your drink and the other outside the glass. Put both in your mouth and try and drink!
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This. Everyone keeps suggesting costs or weight or ignorant engineers or whatever, but this suction thing is the right answer. We've had several threads before on this.

Sure, one could install sensors or other stuff to make a Both position work on a low-wing that has no header tank, but that there DOES add cost and weight and more failure points. So we have the systems we do.

The fact is that the fuel from a low-wing airplane has to be lifted to the engine if the tanks are in the wings, and lifting requires suction, and suction only works if there is no way air can get into the system from anything other than the tank in use.

Yup, one could put boost pumps in both tanks and run them 100% of the time. You want to replace those pumps on a regular, preventive basis? Or risk pump failure if the alternator quits and the battery can't keep up until safely landed?
 
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