Radar Wake Turbulence Separation Minima doesn't always guarantee that an aircraft won't encounter Wake Turbulence.
Once I had a Medivac C441 (twin turbo-prop) maintaining FL180. The preceding B737 on descent passed through FL180, 18NM ahead of the C441 (I know, I measured it at the time). About 4 minutes later the C441 flew through the same air that the B737 had descended through and reported encountering "severe wake turbulence".
Even though I had the minimum Radar Wake Turbulence Separation (5NM) the C441 still encountered severe turbulence. Ever since that day, whenever a light aircraft is going to pass through the same air as a medium or heavy I advise them: "caution wake turbulence".
