Narrative:

The incident occurred while in a descent from FL280 to 13000 ft while passing through FL180. We encountered moderate turbulence 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 flight attendants, to situation down. The radar was operating, and it showed no returns except for a rainshower 30 mi ahead and to our right. It had been perfectly smooth up to the encounter, and after 30 seconds it continued smooth all the way to our destination -- ord. It was dark, and although there were some clouds around they did not appear threatening. We had received no turbulence reports from ATC, and were not expecting any. Destination WX was good, and the landing was uneventful. Paramedics met the aircraft as 4 passenger and 2 flight 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 reports exist for turbulence. Possibly we should have a recommendation to have the seat belt sign on when leaving cruise altitude and throughout climbs and dscnts.

Related reports:

Google
 

Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.