PC-12 Captain reported draining a small amount of fuel from the tank; to check for contamination; only to find the sump valve was stuck open. Maintenance was contacted but the leak could not be stopped and the flight was canceled. The Captain stated this is a common occurrence with this Pilatus aircraft.

Narrative

During pre-flight I attempted to sump the fuel in accordance with the Pilatus POH. The first point I tested was the outer sump on the left wing. I used the fuel tester to open the valve and then released the valve once I had an adequate fuel sample. The sump valve closed; but fuel continued to leak out. I attempted to reseat the valve by opening and then closing it again; but fuel continued to leak out. I then attempted to mitigate fuel being spilled on the ground using the fuel tester cup and a bucket and absorbent pads provided by the FBO. I also contacted our company maintenance department who then dispatched a local vendor to come out and look at the problem. While I waited for the maintenance provider to arrive; I attempted to use my body to block the wind that was blowing the fuel stream around. When the mechanic arrived; he also attempted to reseat the valve but with no success. He stated the valve would have to be replaced and that would require draining the tank. I also informed [the flight department] of the situation and the flights were moved to a different tail number. I then wrote up the aircraft per our company's maintenance departments instructions.The fuel sump seals are a recurring problem with this aircraft that I believe need to be addressed with Pilatus. Pilatus states that the tanks need to be check for fuel contamination; but when you actually follow their guidance; a leaking fuel sump is often the result. This sometimes cannot be remedied quickly; requiring costly de-fueling as well a disruption to the flight schedule as happened today. I had just topped off for the day with almost $1;000 in fuel that the company will now have to eat the cost of! More importantly; however; there is a safety culture problem that this recurring problem presents as well because it encourages pilots not to sump the tanks at all because of the possibility of a leak occurring. I have heard senior captains and even instructors actively discourage other pilots from checking for fuel contamination to save the hassle of a stuck valve. If Pilatus expects us to sump the tanks; then the valves and seals need to be up to the job. This aircraft is only 3 years old!Left wing outer fuel sump leaking after I attempted to test the fuel.