INFLT ENCOUNTER WITH SEVERE TURB CREATES A TEMPORARY LOSS OF ACFT CTL. WINDSHEAR.

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I WAS PIC ON APR/XX/94; BOEING 737-300; SCHEDULED SVC FROM SRQ TO ORD. AFTER LEVELING AT FL310; OUR PLANNED CRUISE ALT; I TURNED OFF THE SEAT BELT SIGN. AS WE WERE IN AND OUT OF THE TOPS WITH A RIDE QUALITY THAT WAS SOMEWHAT UNPREDICTABLE; I LEFT THE SIGN OFF FOR ABOUT 15 MINS; BUT TURNED IT BACK ON WHEN THE RIDE WENT FROM MOSTLY SMOOTH TO OCCASIONAL/INTERMITTENT LIGHT CHOP. AFTER ABOUT 15 MINS; I TURNED THE SIGN OFF AGAIN. I TURNED THE SIGN BACK ON 10 MINS LATER AS THE RIDE WORSENED AND ZTL ADVISED OF MORE BUMPY AIR AHEAD. EACH TIME I TURNED THE SEAT BELT SIGN ON; I MADE AN APPROPRIATE CABIN ANNOUNCEMENT AS REQUIRED BY BOTH POLICY AND THE FARS. DURING THE FLT FROM APPROX THE ATL VOR TO THE IIU VOR ON J-73; THERE WEREN'T MANY COMMENTS FROM OTHER ACFT ABOUT THE RIDE QUALITY. BUT APCHING IIU; ZTL ADVISED US OF MODERATE TURB AHEAD; SO I REQUESTED CLRNC TO FL240. PRIOR TO THIS; ABOUT 20-30 MINS AFTER TKOF; CHICAGO DISPATCH SENT US AN ACARS MESSAGE ADVISING; 'MODERATE PLUS TURB FL240-FL260 OKK.' (OKK = KOKOMOMO; INDIANA VOR.) AFTER LEVELING AT FL240; JUST N OF IIU; AS I RECALL; THE FIRST FLT ATTENDANT ENTERED THE COCKPIT WITH A CABIN WRITE-UP ABOUT A BURNED OUT DOME LIGHT IN THE AFT R LAVATORY. SHE AND THE FO DISCUSSED THIS FOR A MIN OR 2 WHILE I MADE SEVERAL RADIO CALLS TO ZID; ATTEMPTING TO GET SOME RIDE RPTS AHEAD. I HEARD ZID CALLING SEVERAL ACFT ON THE RADIO; BUT DID NOT HEAR RESPONSES FROM ANY OF THE OTHER ACFT OR TO MY CALLS. AS THE FLT ATTENDANT TURNED TO LEAVE THE COCKPIT; ZID CALLED ME AND SAID 'TURB 10 MI AHEAD; DSND AND MAINTAIN FL190.' THIS STRUCK ME AS ODD; BECAUSE THE CTR HAD CLRED ME TO DSND FOR THE PURPOSE OF AVOIDING TURB WITHOUT MY REQUESTING IT. THIS WAS VERY UNUSUAL; BECAUSE CTR RARELY FLIES YOUR AIRPLANE FOR YOU. ATC WILL FREQUENTLY ASK ABOUT YOUR FLT/RIDE CONDITIONS; ADVISE YOU OF RIDE RPTS AT OTHER ALTS; AND OFFER/SUGGEST ALT CHANGES; BUT I DON'T RECALL EVER GETTING AN UNSOLICITED CLRNC TO CHANGE ALT SOLELY TO AVOID TURB. (TO AVOID TFC CONFLICTS CAUSED BY ADVERSE WX; YES; BUT THIS DIDN'T SOUND LIKE THAT KIND OF SIT.) IT STRUCK ME AS BOTH ODD AND A LITTLE FOREBODING. SO I TURNED TO THE FLT ATTENDANT AS SHE WAS LEAVING AND SAID; 'I THINK YOU GUYS OUGHT TO PUT YOUR STUFF AWAY; AND BE SEATED.' I HADN'T GOT THE LAST WORD OUT OF MY MOUTH WHEN WE ENCOUNTERED THE WORST INFLT TURB THAT I HAVE ENCOUNTERED SINCE FLYING INTO A TSTM OVER BERMUDA IN A P- 3 IN THE NAVY IN 1968! I IMMEDIATELY CLOSED THE THROTTLES; DISCONNECTED THE AUTOPLT AND AUTO THROTTLES; AND EXTENDED THE SPD BRAKES IN AN ATTEMPT TO EXECUTE A MODIFIED EMER DSCNT TO FL190 TO MINIMIZE THE TIME NEEDED TO GET DOWN. I WAS THINKING THAT POSSIBLY THE AREA OF SEVERE TURB MIGHT BE VERTLY NARROW; SO THAT BY EXPEDITING THE DSCNT; THINGS WOULD IMPROVE RAPIDLY. BUT THEY DIDN'T! THE TURB WAS FRONTAL; NOT JET STREAM RELATED. SO MY DSCNT TO FL190 RESULTED IN ALMOST NO IMPROVEMENT UNTIL I PASSED FL200; WHEN THINGS DID BEING TO IMPROVE. DURING THE DSCNT THROUGH FL200; THE AIRPLANE WAS ALMOST OUT OF CTL IN BOTH PITCH AND ROLL. I COULD HARDLY READ THE INST PANEL; AND THE FO BEGAN CALLING OUT THE ALTS EVERY 500 FT TO ASSIST ME. THE DSCNT COULDN'T HAVE TAKEN MORE THAN A MIN OR 2; BUT IT SEEMED LIKE A MONTH. WHEN WE FINALLY GOT TO FL190; I LEVELED THE AIRPLANE; RETRACTED THE SPD BRAKES; RE- ENGAGED THE AUTOPLT AND AUTO THROTTLES AND ASSESSED THE SIT. I CALLED THE FLT ATTENDANTS AND LEARNED THAT 2 WOMEN IN THE BACK OF THE AIRPLANE WERE ON THE FLOOR. THEY HAD BEEN OUT OF THEIR SEATS (DESPITE THE SEAT BELT SIGN) TO USE THE LAVATORIES AND BEEN THROWN TO THE FLOOR IN THE TURB. 1 WOMAN HAD A LACERATION ON HER HAND OR ARM; AND THE OTHER APPEARED TO BE UNINJURED; BUT SHAKEN UP. I CALLED DISPATCH AND NOTIFIED THEM BOTH OF THE TURB AND THE INJURIES; AND REQUESTED THAT PARAMEDICS MEET THE AIRPLANE AT ORD. I ALSO REQUESTED MAINT TO INSPECT THE AIRPLANE FOR DAMAGE. WE THEN PROCEEDED TO ORD AND LANDED UNEVENTFULLY. THE 2 WOMEN PAX WERE TAKEN TO A LCL HOSPITAL FOR EXAMINATION. SUPPLEMENTAL INFO FROM ACN 267704: THIS WAS BY FAR THE ROUGHEST RIDE I'VE EVER HAD. THE CAPT INITIATED AN EMER DSCNT; SPD BRAKES EXTENDED; AUTOPLT OFF; THROTTLES IDLE; BARBER POLE SPD. THE ACFT WAS VIRTUALLY UNCTLABLE FOR ABOUT 30 SECONDS AS WE DSNDED TO FL190. AS WE LEVELED AT FL190; THE AIR SMOOTHED OUT AND HE REGAINED CTL. WE THEN CALLED THE CABIN AND LEARNED OF 2 INJURIES AND POSSIBLE HEART ATTACK. WE IMMEDIATELY SENT A RPT TO DISPATCH; ATC; AND MAINT AND REQUESTED PARAMEDICS. UPON DEBRIEFING WE LEARNED THAT ZID AND ZAU MEET OVER OKK AND; AS YOUR HDOF IS ALT-DEPENDENT; YOU MAY NOT HEAR TFC IN FRONT. CTLR WORKS MULTIPLE SECTORS; LANDLINES; ETC. THIS ACCOUNTS FOR THE LACK OF RPTS AND OUR HEARING ABSOLUTELY NOTHING ABOUT THE SEVERITY OF THE TURB (IF ANYONE HAD INDEED EXPERIENCED IT BEFORE). FORTUNATELY; THE SEAT BELT SIGN HAD BEEN ON; BUT IT WOULD HAVE BEEN HELPFUL IF WE HAD KNOWN MORE IN ADVANCE OF THE PROB. (THE RPT WE HAD FROM DISPATCH WAS 3 OR MORE HRS OLD AND FOR ABOVE THE ALT WE ENCOUNTERED.) BETTER ATC SUPPORT (MORE CTLRS?) IN A BUSY SECTOR AND TIMELIER PIREPS/ABILITY TO HEAR OTHER ACFT ON FREQ WOULD HAVE HELPED.