Switching to a Single Tank After Shutting Down the Engine on a C172S

[172driver]

However there is a quirk in the 182 fuel system whereby it only vents on one side and then is a vent connection between each wing. When the tanks are fuelled to full then there is a tendency for fuel to flow through this as well.

Not only the 182. The 172RG has the same system (IIRC standard, older, 172s are the same, but haven't flown one in years, so not 100% sure). What in my experience is the case, however, is that uneven draw of fuel due to the venting system is more pronounced in the 182.

The POH of the RG recommends selecting 'left' or 'right' during refueling. Reason is quite simple - if the selector is on 'both' fuel from the tank that is being filled will migrate through the interconnecting vent line to the tank containing less fuel. Because you never fuel both tanks at once, you don't get 100% of fuel into the tanks, unless you move back and forth a couple of times. In my experience this is insignificant, but if you really, really need every last drop to be squeezed into the tanks, then don't fuel on 'both'.

The POH (again, this is an RG) also generally discourages flying in any selection other than 'both'.

Anyway, it is worth remembering that you must check the POH for the airplane you fly and NOT for some generic model. Mr. Cessna has built plenty of variants ! Be ever vigilant......

[palou89]

Vent lines are intended to vent air not fuel. Fuel will only be in the vent lines if the tank is so full the only way for excess fuel to escape is through the vent line.

If the fuel valve is left in the both position the fuel will travel from the high tank through the fuel selector valve to the low tank for the system in carbourated versions of the C172. The new build C172R and C172S have a small fuel reservoir tank between the fuel selector valve and the engine with the fuel shut off valve between the collector tank and the engine. Selecting left or right will stop fuel from the other tank from draining into the reservoir tank and then across to the other side. Unlike the earlier carb versions of the C172 pulling the fuel shut off on a C172R or S will not stop fuel from transfering from one tank to the other.

Sorry to be blunt but the fuel system is not complex and frankly you should not be flying the aircraft without understanding how it works.

I encourage you to re-read my post, go back to the 172S PIM/POH and tell me how selecting L or R will prevent fuel from flowing from one tank to another through the vent line in between them (NOT the line that goes through the fuel selector valve). I understand its ment for venting air, but are there any design features that prevent fuel from flowing trough it?

You mentioned the reservoir tank, which is something I have always wondered about: what's its purpose? estimated volume? location on the c172s?.

Thanks for your help

[Mark1234]

I understand its ment for venting air, but are there any design features that prevent fuel from flowing trough it?

I believe the vent line is somewhere near the top of the tank, whereas the fuel feed lines are necessarily at the bottom. Hence there will only be fuel in / near the vent line when the tanks are near full.

[palou89]

Asuming near dry tanks, you top one of the tanks first (valve on L position) the amount of fuel that will flow from that tank to the other will be practically negligible?

I simply did not know how high the line was

[Pilot DAR]

The "header" tank in the new 172 can be seen under the right side floor in this photo.



The tank in question can be seen with the two blue fittings in it's forward inboard corner. The red thing is the fuel pump. I'm guessing the tank holds about four liters. Generally such tanks are there to prevent interruption of fuel flow due to unporting of a wing tank when low, or during maneuvering. 185's also have them, though on the firewall.

The wing tank's airspaces are vented to each other. This has nothing to do with the fuel flowing from one tank to the other. That "cross feeding" happens through the fuel selector, when it is in the "both" position. If, however, liquid fuel was to flow from tank to tank through the vent line, nothing would prevent it from doing so.

[palou89]

Awesome, thank you

[Sultan Ismail]

Very informative picture. I would deduce that the horizontal round bar at the left is the push/pull Fuel Cut Off actuator.