Has anyone had fuel transfer from a main tank to a tip tank? The other day I filled both my mains, I keep about 5 gals in both tip tanks incase I need some fuel. I went for a 40 minute flight today, selector on "Both", and the right tank went down fast, left tank stayed near full. When I got back to the hangar I opened the right main, it was about 3/4 empty, opened the right tip it was almost full. Left tank almost full. Fuel totalizer said I used about 11 gallons. It would appear that fuel transferred from the right main to the right tip all by itself. The vents are clean and the pumps / gauges work.

Do you have auxiliary electric fuel pumps for the aux tanks? Supposedly there's no way to get fuel to pump from the mains into the aux tanks since the electric aux fuel pumps only pump it from the aux tanks to the mains one way. I guess it could be possible that the impeller in one of the aux fuel pumps might disintegrate enough to let fuel go the other way but it sounds unlikely.

Never heard of fuel going outboard like that.

Are you sure the aux transfer pump in question is pumping fuel when turned on and not just making noise?

This is the closest I could find for a photo or diagram of the aux transfer pump. Not the exact model, but looks identical to the Facet pumps on the Maule.




Kirk

I know the Facet pump allows fuel to pass through it when its off, this is the same pump that's used in Pipers for a boost pump, but I don't know if it allows fuel to flow both directions freely. If there's a check valve in it perhaps it's stuck. The pump seems to function when I use it as intended. I would also think that the fuel transfer tube is at the top of the main tank and the bottom of the aux tank ( but I don't know that for sure ).

Could be a check valve rather than the piston that stops reverse flow. Aircraft Spruce sells non certified versions of that pump with a myriad of valving options, some including check valves.

I don’t know the exact model on the Maule.

Kirk

Yes, have seen these many times. It's transferring through the vent system which is an open line from aux tank to main tank if you have the underwing fuel vent check valves, then through the fuel valve and or with the newer birds through the overhead vent line main tank to main tank then to the other main tank to the other aux tank. If you park on a slop sideways or have a low tire it will transfer fuel.

I looked at the parts diagram and my M5 has two vent (per wing), one for the main and one for the aux. The aux, bottom side, feeds into the pump that feeds into the top side of the main. So, no vent connection to crossflow. I guess the fuel is being pushed from the main tank through the pump to the aux. Next I'll have to see of the pump has a check valve that's not working, and while I'm there I'll replace the filters. There's no model number for the Facet pump in the parts book, so no reference.

Do a keyword search for Facet here. There's a few threads on the part number somewhere.

The 12 volt aux pumps have built in check vales but the 24 volt do not have built in check vales and have an external check valve installed in the vent line.