Narrative:

Cruise at FL310 effectively smooth entire flight. Seat belt sign off. Entered clouds a few mins before event. Clouds appeared to be layered. Radar showed no returns within 100 NM. No center or aircraft reports of turbulence. Sudden severe turbulence for no more than 3-5 seconds. 4 flight attendants in aft galley area hit ceiling and injuries occurred ranging from head bumps to sprained/broken ankle. Fortunately, all passenger were seated. No trauma injuries reported by passenger. This appears to be the classic 'unavoidable' encounter. If one leaves the seat belt light on for full flight it is ignored. On the other hand, this type of event can occur without warning. I don't think there is anything that can be done to prevent encountering unexpected turbulence, but I do believe that seat belt use can be strongly encouraged and that flight attendants can be trained to be seated and belted when not attending passenger. Supplemental information from acn 370132: once in clear air, I called the flight attendants to check for possible injuries. The a-line, very emotional said they were not ok. The captain worked with them and determined all 4 flight attendants were injured but passenger appeared ok. We notified center we needed an emergency descent and needed to expedite to mcc. Emergency medical assistance was requested through center. Emt's met the airplane at the gate. There were no reports of turbulence in the area, no cloud formations to determine the cause of the turbulence.

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Original NASA ASRS Text

Title: FLC OF B737-200 ENCOUNTERS SEVERE TURB AT CRUISE WITH NO WARNING. FLT ATTENDANT INJURIES RESULT.

Narrative: CRUISE AT FL310 EFFECTIVELY SMOOTH ENTIRE FLT. SEAT BELT SIGN OFF. ENTERED CLOUDS A FEW MINS BEFORE EVENT. CLOUDS APPEARED TO BE LAYERED. RADAR SHOWED NO RETURNS WITHIN 100 NM. NO CTR OR ACFT RPTS OF TURB. SUDDEN SEVERE TURB FOR NO MORE THAN 3-5 SECONDS. 4 FLT ATTENDANTS IN AFT GALLEY AREA HIT CEILING AND INJURIES OCCURRED RANGING FROM HEAD BUMPS TO SPRAINED/BROKEN ANKLE. FORTUNATELY, ALL PAX WERE SEATED. NO TRAUMA INJURIES RPTED BY PAX. THIS APPEARS TO BE THE CLASSIC 'UNAVOIDABLE' ENCOUNTER. IF ONE LEAVES THE SEAT BELT LIGHT ON FOR FULL FLT IT IS IGNORED. ON THE OTHER HAND, THIS TYPE OF EVENT CAN OCCUR WITHOUT WARNING. I DON'T THINK THERE IS ANYTHING THAT CAN BE DONE TO PREVENT ENCOUNTERING UNEXPECTED TURB, BUT I DO BELIEVE THAT SEAT BELT USE CAN BE STRONGLY ENCOURAGED AND THAT FLT ATTENDANTS CAN BE TRAINED TO BE SEATED AND BELTED WHEN NOT ATTENDING PAX. SUPPLEMENTAL INFO FROM ACN 370132: ONCE IN CLR AIR, I CALLED THE FLT ATTENDANTS TO CHK FOR POSSIBLE INJURIES. THE A-LINE, VERY EMOTIONAL SAID THEY WERE NOT OK. THE CAPT WORKED WITH THEM AND DETERMINED ALL 4 FLT ATTENDANTS WERE INJURED BUT PAX APPEARED OK. WE NOTIFIED CTR WE NEEDED AN EMER DSCNT AND NEEDED TO EXPEDITE TO MCC. EMER MEDICAL ASSISTANCE WAS REQUESTED THROUGH CTR. EMT'S MET THE AIRPLANE AT THE GATE. THERE WERE NO RPTS OF TURB IN THE AREA, NO CLOUD FORMATIONS TO DETERMINE THE CAUSE OF THE TURB.

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.