Was it aluminum, stainless, a copper primer line or a brazed fitting on the end of a fuel injection line?
This was an aluminum line coming out of the firewall that twisted around to the gascolator which was also mounted to the firewall but on a bracket that allowed just the slightest amount of flex. Slightest.
So the story goes that I'd read a comment by Van that RV pilots never really explored stalls or low speed handling but rather were content to just go blast through the sky. Unrelatedly, I also realized that he I'd never really thought about emergency procedures in my RV-4, so one day I spent a few minutes in the cockpit, thinking about same.
One evening, I went out to do a few stalls and no big deal, power off. I added power to climb back up and the engine quit, prop still windmilling. Emergency procedures to the fore! Hit the boost pump, engine runs, happy boy.
Oh, what the hey, let's go back home and check it out. On that flight, I was also looking to burn off some fuel so I could remove the left fuel tank for maintenance, so I *knew* from the dipstick that I had four gallons in the left and 13 in the right when I took off, and I was burning from the right tank.
Went home, nice landing, taxied to the hangar, engine running, wondering what test I could do before shutdown when the engine shut down all by itself. Was going to restart it but, no, let's take a look first.
All seemed good but the right gear leg was black and blue. There are no black and blue fluids in the plane...
On further examination, the aluminum fuel line had cracked at the firewall. Just a little crack. When this happened, the engine-driven pump was sucking air and the engine quit. When I hit the boost pump, the fuel line was pressurized on the other side of the crack and the engine ran.
But here's the scary part -- when the boost pump was running, I was pumping a gallon a minute overboard inside the engine compartment, exhaust stacks ten inches to each side. At the hangar, the engine quit due to fuel starvation.
The black and blue was blue avgas and black was grime cleaned out of the engine compartment.
One other poster to this thread pontificated about hard lines being the standard in other parts of aviation, but you had to read the fine print, several paragraphs down, that his comments were for properly supported and properly installed hard lines. There's more...
Aluminum hard lines have no place when one end is rigid and the other can move. In my case, it was firewall to gascolator. In all RVs, it's firewall (or gascolator) to engine. And don't forget that a prototype F-14 was lost when hydraulic lines cracked and leaked -- and I think those were stainless steel.
"When things work right, it's recreation. When they don't, it's education."
Be safe, be well, wash your hands...