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The C172 POH checklists say to set the fuel selector to "right" or "left" when securing the plane but "both" while operating the plane.

What is the point of changing the fuel selector when the engine isn't running? IOW, what practical (not legal) problems could it cause to leave it on "both" while secured?

[Edited to remove incorrect understanding of the vent connection between tanks, which is cleverly designed to not cross-feed fuel.]

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It's pretty much to limit imbalance when parked on a slope, and if the tanks are full, stop fuel from the high side tank from forcing fuel out of the low side tank's vent or seeping out the fuel cap if it isn't completely fluid tight.

On the '68 Cardinal I used to own, the vents went straight to the wing tip on each tank's side. If parked on a side slope with fuel on both and full tanks, you might end up with a third of the high side's tank volume on the ground under the left wing tip (later they were cross vented to prevent this).

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  • $\begingroup$ This diagram from the POH shows the tanks are cross-fed separately from the selector, so it seems like that wouldn't solve the imbalance issue? $\endgroup$ – StephenS Jan 27 '19 at 18:26
  • $\begingroup$ It doesn't show the internal valving. If the selector is set to Left say, the left tank is connected to the engine and the right tank is completely isolated, so yes it will. $\endgroup$ – John K Jan 27 '19 at 18:34
  • $\begingroup$ The line at the top (directly between the tanks, not going through the fuel selector) has valves on it? How do they work? $\endgroup$ – StephenS Jan 27 '19 at 18:47
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    $\begingroup$ It just connects the tops of the two tanks together. Allows the right tank to vent to the left tank but they actually connect to each tank on the outboard top of each so fuel can't migrate through. The venting fuel caps are used as additional protection against a wasp or whatever blocking the main vent. $\endgroup$ – John K Jan 27 '19 at 19:04
  • $\begingroup$ Ah, it all makes sense now. Thanks! $\endgroup$ – StephenS Jan 27 '19 at 19:09
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On most C172s, the fuel selector cross-feeds in the BOTH and the OFF position. The LEFT or RIGHT position do not crossfeed, which eliminates cross flow of fuel, except when the tanks are nearly full. Most older C172s can do a small amount of cross feed between the tanks through a vent line connecting the tanks. The external vent is near the strut on the left wing. If the plane is parked nearly full, with the right wing high, the vent will discharge fuel as the fuel level rises in the left tank.

So a topped L tank can transfer some fuel to the R tank, and reduce the fuel in the L tank. Something to watch out for when fueling and managing fuel.

Probably a safer parking configuration would be to park with the fuel OFF, but unfortunately, on most C172s this means that the tanks can cross-feed. Therefore, the LEFT or RIGHT setting are preferred.

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    $\begingroup$ Thanks for pointing out the fuel selector can cross-feed in the "off" position; that's an unexpected (mis)feature. $\endgroup$ – StephenS Jan 28 '19 at 17:30

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