CLR AIR TURB ON DSCNT OVER MSN RESULTS IN INJURIES TO PAX IN 2 ENG HVT.

Narrative

THE INCIDENT OCCURRED WHILE IN A DSCNT FROM FL280 TO 13000 FT WHILE PASSING THROUGH FL180. WE ENCOUNTERED MODERATE TURB FOR ABOUT 5 SECONDS AND THEN IT GRADUALLY SUBSIDED TO SMOOTH OVER A 20-30 SECOND PERIOD. THE SEATBELT SIGN WAS OFF; AND I IMMEDIATELY TURNED IT ON AND AN ANNOUNCEMENT WAS MADE FOR EVERYONE; INCLUDING THE FLT ATTENDANTS; TO SIT DOWN. THE RADAR WAS OPERATING; AND IT SHOWED NO RETURNS EXCEPT FOR A RAINSHOWER 30 MI AHEAD AND TO OUR R. IT HAD BEEN PERFECTLY SMOOTH UP TO THE ENCOUNTER; AND AFTER 30 SECONDS IT CONTINUED SMOOTH ALL THE WAY TO OUR DEST -- ORD. IT WAS DARK; AND ALTHOUGH THERE WERE SOME CLOUDS AROUND THEY DID NOT APPEAR THREATENING. WE HAD RECEIVED NO TURB RPTS FROM ATC; AND WERE NOT EXPECTING ANY. DEST WX WAS GOOD; AND THE LNDG WAS UNEVENTFUL. PARAMEDICS MET THE ACFT AS 4 PAX AND 2 FLT ATTENDANTS HAD SUFFERED MINOR INJURIES TO LEGS AND ANKLES. I COULD HAVE TURNED THE SEAT BELT SIGN ON EARLIER; AND PROBABLY WILL IN THE FUTURE EVEN THOUGH NO RPTS EXIST FOR TURB. POSSIBLY WE SHOULD HAVE A RECOMMENDATION TO HAVE THE SEAT BELT SIGN ON WHEN LEAVING CRUISE ALT AND THROUGHOUT CLBS AND DSCNTS.