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The single-engine Cessna 172 and 182 aircraft models have a dual gravity-fed fuel system, where you can draw from both the LH and RH tank simultaneously in flight.

I've heard it said that when you are securing the airplane after a flight to leave the tank selector on the RH tank to prevent crossflow if the ramp is uneven. (So you don't end up with all the fuel in one tank).

Is there any merit to this? Does it actually block the crossflow valve to do this?

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    $\begingroup$ Cessna 150 and 152 aircraft don't have a tank selector, just a cutoff valve. $\endgroup$
    – GdD
    Commented Nov 24, 2014 at 8:35
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    $\begingroup$ I believe you should follow the manufacturer's recommendation. What does it say in the POH? $\endgroup$
    – Skip Miller
    Commented Nov 24, 2014 at 16:15
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    $\begingroup$ For what it is worth, I have actually seen this happen in a Learjet when it was parked on a slope and the cross-feed was left open. They came out to the airplane the next day and found the right wing almost touching the ground and the left one high in the sky. $\endgroup$
    – Lnafziger
    Commented Nov 26, 2014 at 4:21

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The C172S manual I have says to set the selector to left or right as part of the "securing airplane" checklist:

  1. Fuel Selector Valve -- LEFT or RIGHT to prevent cross feeding

I was also taught to set it to right during my initial training. The idea is to prevent cross-contamination or leaks if one tank is compromised. Another reason is that if you park the aircraft on a slope, you could end up with an imbalance between the tanks, as you said.

So the POH confirms that it prevents cross feeding, at least in that model: you should always check the POH for your specific aircraft.

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    $\begingroup$ What would you do if you came out to the airplane and found that one tank had either contamination or a leak? Would you take off on the other tank? Of course not. It doesn't matter what position the fuel selector is in, you'd have to have it evaluated by a competent mechanic before flight. $\endgroup$
    – rbp
    Commented Nov 24, 2014 at 18:10
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    $\begingroup$ @rbp Of course you're correct, but by following the POH then only one tank can drain out onto the ground in the case of a leak, or (maybe) need to be emptied and cleaned in case of contamination. Limiting the impact of a problem is the goal, not flying on one tank. $\endgroup$
    – Pondlife
    Commented Nov 24, 2014 at 18:30
  • $\begingroup$ for the once-in-a-million times that it happens, I guess that makes sense. $\endgroup$
    – rbp
    Commented Nov 24, 2014 at 22:02
  • $\begingroup$ Yup, it is the once-in-a-million things that make the difference. When everything works correctly, there shouldn't be an issue. It is when that one thing you didn't plan for goes wrong that you are in trouble. $\endgroup$
    – J Sargent
    Commented Nov 25, 2014 at 21:07
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    $\begingroup$ @rbp Parking on a slope happens FAR more often than once in a million.... $\endgroup$
    – Lnafziger
    Commented Nov 26, 2014 at 4:20
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If you leave the tank on dual and you put fuel in the right tank by the time you are ready to put fuel in the left tank some of the fuel will have flowed into the left tank. The right tank would have more pressure from the weight of the fuel than the emptier left tank and as any liquid will do it will try to balance between the tanks. You will end up with less fuel than 2 full tanks. That is why the POH states either right or left tank. It doesn't matter which tank, just that they are not connected when refueling.

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  • $\begingroup$ This is the first time I've heard this reason. Do you have any idea how much of a difference that would make? It seems like it wouldn't be a material amount, and probably would sort itself out as the fuel can flow both ways (see the diagram here: i.sstatic.net/sZOXT.png) so it would just flow back to the other tank if you topped up the other tank. $\endgroup$
    – Canuk
    Commented Aug 21, 2018 at 19:54
  • $\begingroup$ In my experience with a Citabria (cross-flow fuel tanks without selector valve), this is a very minor effect. If I were trying to fill every last mL of fuel, then obviously this would interfere, but I suspect that the cross-flow is trivial compared to the flow-rate under cruise. The cross-flow tubes just aren't that big. $\endgroup$
    – Kenn Sebesta
    Commented Aug 6, 2021 at 14:25
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My aircraft (M20M) fuel selector has left, right, off. My technique (which would apply to any aircraft that doesn't have dual tank feed) is to turn the fuel off when it is parked. I taxi on the lower tank, then switch to the fuller tank to do the runup and take-off on. That assures that both tanks feed fuel, and I'm always departing on the fullest tank.

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  • $\begingroup$ Cessna 177 has the OFF selection blocked. Prevents OFF being selected when BOTH was intended. The Fuel selector knob and the plastic cover can be removed to access the OFF position for maintenance. My POH also to select RIGHT tank when parked or refueling, same as the 172 mentioned earlier. $\endgroup$
    – CrossRoads
    Commented Aug 20, 2018 at 15:49
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Is there any merit to this?

Possibly. Think of it like parking on a hill and leaving your car in gear/steering into the kerb, in case the handbrake fails.... it's a precaution, rather than a requirement.

In the case of a Cessna, the fuel tanks in the wings are very long, wide and flat - so it wouldn't take an excessive amount of bank to allow fuel to slowly move from one tank to the other. If that happened to a great enough degree, in theory the fuel could adversely affect handling.

Chances are that the ramp isn't off-level enough to require it, but assuming that it does block the valve (I'd assume it does, as I'm not sure how else it would work) then there's little harm to be done, and some potential benefit in some circumstances.

Does it actually block the crossflow valve?

No idea, I don't own a C172: it seems like a logical conclusion, but I'd check your manual or with the manufacturer.

Just remember to turn it back to dual feed when you set off.

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