Fuel imbalance

[marcush]

Has anyone experienced a fuel imbalance before?

Departed today for a local area flight with just over half full wing tanks. Both gauges reading exactly the same before departure.

Didn't notice the imbalance in flight, but once the engine was shut down the left tank gauge showed half full and the right showed a quarter full. Big difference!

Basically the left tank had hardly supplied the engine at all.

Dipped the tanks and the gauges were reading correctly.

Both left and right fuel valves remained on for the duration of the flight. This is my SOP and I guess is correct?

This is my 6th flight since taking delivery of the aircraft and although there has been a slight imbalance before, nothing as marked as this.

Did a test on the ground by taking the fuel pipe off and switching electric fuel pump on and then opening one tank at a time. Both tanks had no problem suppling fuel at the same pressure.

Not sure what's going on here, but any advice appreciated. Thanks.

Tecnam Sierra, 100 hp Rotax.

[John Crone]

The fuel system sends the overflow back into the left tank - one reason why you should never fill it to the top.
I expect it is worse during T/O & landing when you have the electric pump on - you didn't leave it on longer than usual did you?
However I've have observed that it not always consistent and sometimes they go down evenly.

I also fly with both valves open unless I'm deliberately trying to even the tanks up.

But if parking on uneven ground I always turn one off.

[marcush]

Thanks very much for your reply.

I should have been aware of the the overflow going back into the main tank.

Yes I'm pretty sure we did leave the electric pump on inadvertently for a good part of the flight. Mainly because I was practising PFLs and touch and goes etc and some of my checks went out of the window.

I guess this would explain the imbalance.