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I've been trying to rack my brain around this and asked a few CFI's who couldn't give me more than "because that is how it was done", but a good number of GA aircraft save for Cessna and maybe a few others only have a Left/Right fuel selector:

Left Right Fuel Selector Image Credit: Make Time For Flying Blogspot

To me, as an engineer, it makes great sense to have a Left/Right/Both selector such as found in most Cessna's. This lets the pilot focus on flying the aircraft rather than having to switch tanks every 30 minutes to balance fuel out. A Left/Right/Both selector isn't sigificantly more complicated than a Left/Right selector, so that leaves me wondering...

Why do GA manufacturers opt to use just a Left/Right selector valve given that a majority of GA accidents are fuel related and off-airport landings with fuel in a tank is still relatively common? Is there a safety issue I'm not seeing?

I understand that even with a "Both" selection, fuel may not drain from both tanks equally, but you can always switch to the fuller tank for a little while then back to both in that case.

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    $\begingroup$ Given that Cessna produced nearly 100k 150, 172, and 182 series aircraft compared to the roughly 33k PA-28 aircraft Piper produced, your title would better reflect reality if "most" were changed to "many" or similar. $\endgroup$
    – J W
    Dec 17, 2016 at 5:10
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    $\begingroup$ @JonathanWalters When I say "most" I mean by model, not necessarily production numbers. $\endgroup$
    – Ron Beyer
    Dec 17, 2016 at 5:21

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A quote from the book of regulations, Chapter 14, part 23, verse 951(b):

(b) Each fuel system must be arranged so that—
    (1) No fuel pump can draw fuel from more than one tank at a time; or
    (2) There are means to prevent introducing air into the system.

On a high wing aircraft satisfying #2 on that list is pretty easy: If you connect both tanks to the "top" of the fuel selector as in a Cessna 172 or similar then as long as you have liquid in at least one tank gravity will ensure there will be liquid at the top of the fuel selector to feed through into the engine:
High Wing Fuel System

The design of the system provides a passive "means to prevent introducing air" because gravity is always trying to pull fuel downward out of the wings and to the fuel selector.

On a low-wing aircraft like the Piper in your picture we don't have gravity to help us: There's no natural "head pressure" at the fuel selector (or at least much less than in a high-wing with the tanks mounted well above the valve), so we're relying on the fuel pump to draw fuel through the system.
This means we can no longer guarantee item #2 above: If the fuel selector were in the "Both" position and one tank ran dry the fuel pump is going to happily try to suck air through the line from the empty tank, and that could cause the engine to quit.

It's possible to guarantee item #2 in a low-wing fuel system using float valves that seal the tank outlets when the fuel is below a certain level - an active means to prevent introducing air into the system - but that adds complexity and weight.
Since the aircraft already needs to have a fuel selector valve anyway it's easier to just design the valve in such a way to satisfy item #1 on the list: Make sure the fuel pump can only draw from one tank at a time - i.e. "Don't provide a "Both" position on a low-wing aircraft."

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    $\begingroup$ I didn't think about the introduction of air, which is a good cause. I'd be willing to bet though that the reason for not using mechanical float valves to shut the empty tank off automatically may be more of "another point of failure" more than weight and complexity. Float valves have been around quite a long time and don't weigh more than a pound. I guess the other side is that it would introduce more unusable fuel (maybe a gallon at most) but I still would think that having the float valve is better than an embarrassing landing with fuel in one tank. $\endgroup$
    – Ron Beyer
    Dec 17, 2016 at 1:47
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    $\begingroup$ Another potential safety issue is if the selector was at "both" and one tank developed a leak. In that situation you don't want any means for the levels to equalize through the selector valve when it is set to "both" - especially if the purpose of the "both" setting is to reduce the amount of attention given to the fuel gauges! $\endgroup$
    – alephzero
    Dec 17, 2016 at 4:10
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    $\begingroup$ Where Cessna does use a fuel pump, that pump is often designed to draw fuel from the header tank (for example in the C177), thereby also satisfying #1. $\endgroup$
    – J W
    Dec 17, 2016 at 16:59
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    $\begingroup$ @JonathanWalters Indeed, IIRC the fuel-injected C172s are also designed this way (there's a "fuel reservoir" tank after the fuel selector, and before the electric boost pump), because in theory with the pump on and one tank dry you could draw fuel at a rate that exceeds the supply from the other tank (thereby introducing air). Depending on the size of the header tank this may satisfy the letter of the regulations while missing the spirit though: Running that header tank dry will still wake up the pilot in a hurry! $\endgroup$
    – voretaq7
    Dec 23, 2016 at 22:26
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    $\begingroup$ On C177s, there is a plastic enclosure around the fuel tank selector. If the selector knob is removed, the plate removed, and the knob replaced, then an Off position can be selected, which can be useful for maintenance. The plate makes sure that can't be selected normally. $\endgroup$
    – CrossRoads
    Apr 2, 2019 at 17:57
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Comment found on RPX Aviation about Left, Right, Both and OFF

If you carefully read the POH for early Cessna 172s, the POH cautions you about flying on both tanks when cruising at altitudes over 5000 ft. Doing so may cause "power irregularities" (A.K.A. engine failure). But I guess this is okay if it is in the POH. Just switch to one tank, ignore the sputtering, glide for 60 (very long) seconds, then switch to the other tank and hope the vapor has cleared and the engine restarts. That is why many early Cessna 172s have a placard on the fuel selector (between the seats, out of sight) that says something to the effect of "Use single tank above 5000 ft.

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Another thought to add to the discussion. Some high wing aircraft are known for fuel to drain from one tank into the other when both tanks are selected. Airplanes seldom fly perfectly level all the time. This creates uncertainty for the pilot. He/she may keep a mental a note of how much gas they have, but incorrectly assume which tank it is in. Yes the engineer could install check valves to prevent gas from unintentionally cross flowing back into a different tank, but that would require the introduction of check-valves which would add cost, weight and another failure mode. KISS is a good approach. Best to provide a fuel system that is simple, reliable and allows the pilots to know for sure where their fuel is stored.

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  • $\begingroup$ This is definitely true of the Cessna 182s. You could set the fuel selector on my 182 on one side and go flying and when you land you need to put fuel in both tanks. You also had to be careful when fueling if the plane was not level. Fuel will flow from one tank to the other even if the fuel selector is set to off. $\endgroup$
    – JScarry
    Dec 14, 2019 at 23:54
  • $\begingroup$ @JScarry The Cessna fuel selector cross-feeds in the Off position just like in the Both position; it’s just missing the connection to the engine. In some models, the Off position was replaced with a separate fuel cut-off valve, so you could prevent cross-feeding. $\endgroup$
    – StephenS
    Feb 1, 2022 at 19:29
  • $\begingroup$ @StephenS I just watched a Blancolirio video when a brand new C172 got stuck in CA because of the winter storms. The new ones have left, right, both, and a red knob above the fuel selector for fuel cutoff. youtu.be/ZlJ4d2pIAh4 $\endgroup$
    – JScarry
    Feb 2, 2022 at 1:56
  • $\begingroup$ @JScarry That’s what I was referring to. $\endgroup$
    – StephenS
    Feb 2, 2022 at 2:22
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Just a thought...in my experience aircraft without the "BOTH" selector position tend to have a higher wing dihede. For example a Cessna 177 is about 1.15 degrees and a PA-28 is about 7 degrees. So the BOTH might be used to address fuel unporting concerns.

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  • $\begingroup$ Of course you have to take into account that low wing aircraft almost always have more dihedral than high wing aircraft of similar performance and handling, so we could state this alternatively as "low wing aircraft are more likely to lack a "both" position than high wing aircraft" -- not saying your answer is wrong, but just looking at it from a different angle -- $\endgroup$
    – quiet flyer
    Feb 1, 2022 at 19:33
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    $\begingroup$ @quietflyer, you might re-read my comment. $\endgroup$
    – mongo
    Feb 2, 2022 at 19:37

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