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

Dhs-9 said:
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.
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You want an extra pump, then solenoids to operate check valves, automatically operated solenoids at that. Before your system is finished, several pages will be added to a POH, several pounds of extra crap will be added, NUMEROUS failure points will be introduced, etc...
Ever fly a modern jet or high performance turbo prop with all its "automatic" stuff? Its great, until there is a small problem, then lots of stuff stops working.
Its a small piston driven plane with minimal usable load, it is designed to be reliable. Reliable is most often achieved with simple.

If switching fuel tanks as needed, with need determined by looking at fuel gauges, is too much of a task, perhaps flying is not for you. I fly with tanks on both most of the time, but I still need to watch the fuel gauges and even switch to left or right now and then! In a caravan, you are switching a tank off about every half hour or so, there is even a reminder on the panel to balance the tanks.
 
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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Never needed the vibration watch ... had a Tiger before the RV7 and both will let you know you might want to think about changing tanks due to a heavy wing ...
 
Wagondriver said:
If switching fuel tanks as needed, with need determined by looking at fuel gauges, is too much of a task, perhaps flying is not for you.
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Yup. Training is far cheaper, more reliable, and increases safety a lot better than more gimmicks in the systems to "idiot-proof" them. And you get to keep the training when you move from airplane to airplane.

The people with the biggest problem with no Both position on the selector are, I think, those that learned in airplanes that had that Both position. They didn't learn to manage the tanks. It's no different than the pilots that learned to fly in trikes and now want to fly taildraggers but are terrified of the dreaded groundloop. Never mind that we have taildraggers that have been flying, since they were built 70 years ago, with no groundloop accidents. It's all about training.
 
The Ercoupe has two wing tanks and a header tank. One pump takes fuel from both tanks and pumps it to the header. There's a float gage in a tube right in front of your face on the cowling and when it starts to drop, you have about an hour before the engine quits.
 
When the answer is not “Bo”, default is “high-wing”. Solved.
 
The final indicator of forgetting to switch tanks is that big fan up front goes quiet. Hasn't happened to me yet, but I'm sure it would be an attention getter.

As far as John Denver goes, didn't he have some issues with his medical? Taking off with a known issue, trying to reduce the amount of fuel so he wouldn't need to pump as much out seems kind of dumb to me. Sounds more like it was a ADM issue than a fuel control issue.
 
Dhs-9 said:
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.
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Are you an actual pilot, student pilot or just an internet aviation enthusiast?
 
PaulS said:
As far as John Denver goes, didn't he have some issues with his medical? Taking off with a known issue, trying to reduce the amount of fuel so he wouldn't need to pump as much out seems kind of dumb to me. Sounds more like it was a ADM issue than a fuel control issue.
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FAA was in process of emergency revocation of his cert due to alcohol abuse.

Using his accident as a reason behind OP's campaign is quite silly since JD would probably found another way to kill himself if he had a 'both' fuel selector....
 
It is not just the electric pump, it is the engine driven pump. So you would need to add two pumps. And adding an engine driven mechanical pump will be the hard part.
 
PaulS said:
The final indicator of forgetting to switch tanks is that big fan up front goes quiet. Hasn't happened to me yet, but I'm sure it would be an attention getter.
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Only an attention getter if it happens when you aren't expecting it.....
 
As previously stated, Commanders have a both position and only one boost pump and one engine driven pump.
 
Dhs-9 said:
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.
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Ten to 15 pounds for a fuel pump? On what engine, an R-4360?
 
lmars said:
As previously stated, Commanders have a both position and only one boost pump and one engine driven pump.
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They have a very interesting fuel valve. You can also drain the left, right, or both tanks by pulling up the knob.

It appears the fuel gravity drains from the tanks to the fuel selector and then to the gascolator. It is from that point that the aux and engine-driven pumps suck it up into the carb or injection ssystem (depending on model).
 
Dhs-9 said:
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.
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Now you're making the airplane very electrically dependent. You sure you think that's a great idea?
 
kyleb said:
Now you're making the airplane very electrically dependent. You sure you think that's a great idea?
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Big airplanes, like airliners, do it. But they have multiple engines and multiple alternators and other backup stuff to make sure it keeps working. Little airplanes, the stuff most pilots can afford, cannot have all that stuff without adding much weight and cost, and aren't airplanes already too heavy and useful loads too small, and the whole thing too expensive, too? Do we want to fly or not?
 
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Einstein376 said:
Commanders all have a both on the fuel selector
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lmars said:
As previously stated, Commanders have a both position and only one boost pump and one engine driven pump.
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Yup. They have a Both position. But they did something different from Piper: the Commander's tank are well outboard in the wing, beyond the landing gear, and the the dihedral raises those tanks and there are check valves on the tank outlets so that uncoordinated flight isn't going to transfer fuel from one side to the other. The selector valve and fuel strainer and boost pump are down low in the belly, well below the tanks.

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The wheels are about 11 feet apart. The tanks start at BL82.5" from centerline, so well outside the gear.

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These are reasons why they can do a Both position that do not apply to many other low-wing airplanes.
 
Yup, the T-6ii had all forms of motive pumps into collector tank, with electric balancing sensor that would auto turn off whichever side's pump, which could also be actuated manually for manual balancing if desired. Relied on a bunch of sensors and electrical connections. Not the end of the world, just expensive and point-of-failure adding, which we of course didn't care as it was paid for by the taxpayers. A degree of electrics complexity and cost I prefer not to have in a recreational vehicle I just use to go upside down and do benign weather travel. I'm already over the whole retract thing as it is, going FG for the retirement bird. Now get off my lawn :D
 
Luscombe 8E models have two wing tanks but no "BOTH" selection. A guy once installed a selector with a BOTH position and he crashed.
 
Dhs-9 said:
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.
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All of this no big deal in an experimental but requires reams of reports /approvals in certificated planes, right ?
 
There are many times engines have quit because of inattention to fuel, mostly at cruise. A little throttle back, swap tanks and they or it is running again. The guy wanting all the redundancy needs to buy a Howard 350 or 500. They had all the redundancy built in. If low fuel pressure was detected on either engine it would automatically draw from the opposite main tank. The system would be tested on taxi out, by putting both fuel selectors to off. The system would do its thing and illuminate two cross feed lights. Now if you managed to use all the fuel out of one of the mains it wouldn't keep the engine on that side running.

There have been airliners that have engines quit due to bad fuel management.

Learn your plane. Use good fuel management techniques. Basically stay awake and you probably won't have a problem for more than a few seconds.
 
Silvaire said:
Luscombe 8E models have two wing tanks but no "BOTH" selection. A guy once installed a selector with a BOTH position and he crashed.
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Uneven venting. The CARs/FARs demand that any system in which tanks are interconnected (as with a Both position), that the venting also be interconnected to keep the tank pressures equal. If the tanks have their own, non-interconnected vents, one could have a higher ram pressure than the other (or a fuel cap leaks and sucks on the fuel) and you get fuel transferring across through the selector valve. If the pressure is high enough, the tank drains both to the engine and into the other tank and out its vent, and that pressure keeps fuel from flowing out of the full tank to the engine. Things get quiet.
 
Randomskylane said:
All of this no big deal in an experimental
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Until it quits. Too many homebuilts have crashed because little or no thought was given to fuel flow and the factors that can affect it, including venting. One case I remember was an Aerosport Quail, single-seat high-wing that crashed when the engine quit not long after takeoff. The builder had the two wing tanks draining into a header that fed the engine. All three tanks had their own vents. The header's vent was a ram tube, and its pressure was high enough to force the fuel back out of it into the mains. The engine died and the airplane crashed. It had run perfectly well on the ground, with no airflow around the airplane. Beware.

Another one was a homebuilt that had a sight gauge on the tank behind the instrument panel. The tube of the gauge was connected to a nipple on the filler neck and to a tee on the fuel outlet. Worked fine, again, on the ground, but once in flight, as fuel got low, the fuel flow sucked fuel not only out of the tank but out of that gauge tube until air got to the engine. That tube should have had its own connection to the tank bottom.
 
Dhs-9 said:
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.
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your trying to solve a problem that does not exist by adding complexity and weight. two bad ideas in aircraft design.
 
I don't think there is any physical reason why a mechanical or electric fuel pump could not draw from a pooled inlet from both left and right tanks. In my aircraft the electrical pump is on the firewall, not in any of the tanks. The pumps are going to draw from whatever sump(s) is(are) connected. Selecting a single fuel source (left or right) offers some redundancy in the case of contamination, leakage, or air vent blockage. It also allows for more precise control of fuel remaining in each tank, which might be desirable if you want to have one tank much fuller than the other for approach and landing maneuvering. (It also helps manage fuel balance more precisely. I have flown in Cessnas that did not draw evenly from both tanks when the selector was set to BOTH.

Switching tanks is really not a big deal. First switch is at 1/2 hour, after that every hour, ending on the fullest available tank for final approach and landing so you don't have to switch tanks during a busy approach and landing phase.
 
The physical reason is that there's not always a point that's low enough below than the low point of the tank to use as a common feed point. If you're tanks are further up the wings and there's a lot of dihedral (like the Navion and the Commander mentioned) then it's not so big of a problem.
 
chemgeek said:
I have flown in Cessnas that did not draw evenly from both tanks when the selector was set to BOTH.
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There are multiple reasons for that:

If either fuel filler cap has a leak, the tank pressures will not be equal and so the flows will also not be equal.

When the tanks are full, the vent crossover line is submerged in the fuel, and since the vent inlet is on the left tank, its pressure will push fuel through the line from the left to the right until the level in the left tank is below the crossover.

Older 172s had a single outlet just aft of the center of the inboard wall. If the fuel sloshed around enough, air got into that line. The bubble got into the descending line behind the rear doorpost, and in cruise, with tanks on Both, that bubble rose at the same rate as the fuel descended, so it stayed put and acted as a dam that the fuel had to squeeze around. That tank would empty much slower than the other tank. It's the reason some airplanes had a placard that told you to switch to single-tank operation above 5000 feet. Altitude was a factor here. Drawing from one tank doubled the flow rate and carried the bubble through the system and into the carb where it left via the bowl vent. There was a service kit to modify the outlet lines at the tanks, with a spliced-in line to the vent crossover to send that bubble back to the tank. Later 172Ms and on had that line until the system was changed to two outlets and lines down both front and rear doorposts.

Most of these airplanes never get the plug in the fuel selector removed to drain accumulated debris. That's a 200-hour inspection item, on Cessna's inspection checklists. I have found plugs that have been in there for 40 years or more, and scary amounts of junk in that valve. That crap could cause uneven flow if there's enough of it.
 
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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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PA-25-235, D Model, two wing tanks, no fuel selector it is either on or off. Twin electric pumps in parallel. Two wing tanks gravity feed to one fuselage tank, one line forward splits to two electric pumps and then merges back to one line. A split off before the electric pumps to the mechanical pump. Both mechanical pump flow and electric pump flow combine down stream for one line to a carburetor. Only one electric switch for the electric pumps, so you don't know if one quits.
 
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