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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 Walters Dec 17 '16 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 '16 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 '16 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 '16 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 Walters Dec 17 '16 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 '16 at 22:26
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    $\begingroup$ Good engineering would avoid the introduction of additional potential failure modes, so a method of isolating a nearly empty tank might not be such a good idea. $\endgroup$ – Anthony X Aug 17 '17 at 2:58

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