Sheez! You people need a real fuel system to mull over!
The real nasty is the Partenavia. The fuel valve has several selections and even with a new aircraft the valve operation is abnormally stiff to operate. Eventually it seizes and the problem is in the linkage at the engine end. Because few pilots turned the fuel valve off, the inevitable jamming in the linkage goes unnoticed. Then it becomes impossible to select fuel off or crossfeed.
During the servicing of one Partenavia in Melbourne the LAME attempted to turn off the fuel valve of each engine . It was impossible. The defect was reported to CASA who issued an AD which basically said pilots must write up the defect if the fuel valve was abnormally stiff to operate as it would steadily get worse. In addition the fuel valve must be checked for normal operation at each 100 hourly. Either the LAME's use both hands to turn the valve (and then say it is serviceable) or they just tick the box, I can't say. But the prevalence of seriously jammed fuel valves in the cockpit of Partenavias is proof that some pilots and engineers are ignoring the point of the AD.
In UK, a female pilot had an engine failure in a Partenavia while over the sea. She was unable to move the fuel valve because it was jammed so badly. She was forced to ditch and two passengers lost their lives.