MD90 ENCOUNTERS WAKE TURB JOLT WHILE AT CRUISE ALT. SEAT BELT SIGN IS ON AND NO ONE HURT. SECOND ACFT OPPOSITE DIRECTION; 1000 FT ABOVE WAS A B747.

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WAKE TURB AT FL280. OUR FLT; A SCHEDULED ACR; MD90; WAS SWBOUND TOWARDS LVS; S OF FFU; AND ESTABLISHED AT OUR CRUISING ALT OF FL280 WHEN I NOTICED OFF TO OUR R A LARGE LIGHTED TAIL PASSING THE OPPOSITE DIRECTION 1000 FT ABOVE US; FL290. THIS FLT WAS AT NIGHT AND OUR TCASII CONFIRMED THE PASSING WELL WITHIN 1 MI WHICH WAS PROPER TFC SEPARATION; HOWEVER; THIS TFC WAS A B747. NOT TOO LONG AFTER PASSING BELOW THIS ACFT; WE ENCOUNTERED A MODERATE TO HVY JOLT AS WE PASSED THROUGH ITS VORTICES. OUR SEAT BELT SIGN WAS ALREADY 'ON' SO WE WERE FORTUNATE NOT TO HAVE HAD ANYONE STANDING OR MOVING ABOUT. OUR RIDE CONDITIONS WERE SMOOTH AT THE TIME JUST PRIOR TO ENCOUNTERING THIS BRIEF PASSAGE OF THE B747'S R WING VORTEX AND I WAS JUST ABOUT TO TURN THE SIGN 'OFF' WHEN WE FELT THE JOLT. NO ONE WAS INJURED AND THE FLT PROCEEDED TO DEST WITHOUT ANY FURTHER INSTANCES. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT THE FLC WAS AWARE THIS WAS A WAKE GENERATED TURB BECAUSE THERE WAS NO WAVE ACTIVITY OR EXTREME AIR MOVEMENT WHATSOEVER; VERY SMOOTH AIR. THEY FELT A VERY SHARP JOLT AND FORTUNATELY THE SEAT BELT SIGN WAS STILL ON. HE FEELS THE SEPARATION NEEDS TO BE INCREASED BEHIND HVY; WDB ACFT; NOT DECREASED AS SOME PROPOSALS INDICATE FOR THE FUTURE.