Narrative:

Aircraft was on a scheduled flight from lax to mdw. Forecast WX for mdw called for thunderstorms in the area, however the crew had talked to dispatch and determined lndgs were still in progress at mdw. A line of thunderstorms was east to west from about kirksville, mo, east to undetermined boundary. The alternate for flight was ord and was open. The fuel state was such that a diversion around the WX was imminent. ZAU indicated a break in the line from just east of quincy, il. The aircraft's WX radar also concurred so the decision was made to proceed through the break, and center reclred flight direct to rockford, il, northwest of chicago. The ride through the break in the WX was only light turbulence and aircraft was descended by center routinely and handed off to chicago approach control. As the aircraft proceeded from rockford to dupage to mdw, aircraft was descended to 3000 ft MSL, and asked to slow to 180 KTS. Between rockford and dupage the line of thunderstorms reappeared and lay just south of chicago. Mdw was landing on runway 31C, and was still reporting VMC and open for landing. At this point the aircraft WX radar showed virtually total attenuation. So deviations were asked for chicago approach and were granted to go north around the WX. Radar display stayed the same no matter what range scale or degree of tilt. Position of the aircraft was now about 10-15 mi west of mdw when violent turbulence was encountered. Captain was flying the aircraft with autoplt and autothrottles engaged. Suddenly the aircraft started lurching violently, the autoplt tripped off and ivsi (instantaneous vertical speed indicator) pegged out at 6000 FPM up. Airspeed went over 250 KTS and the altitude went to approximately 6500 ft in a few seconds (approximately 10-15 seconds). The captain was holding full forward on the control wheel calling for autoplt back on, altitude hold on and for flaps up and call for reclrnc from approach. The crew tried to call approach control but altitude deviation occurred before contact could be made. The aircraft exited the problem area at about 6500 ft and continued to fly normally. After communication with chicago approach was re-established, the decision was made to divert to ord where WX was VMC. A normal approach and landing was made at ord. On ground at ord captain called approach control and they said our aircraft 'looked like a ping-pong ball' on their radarscopes. A detailed ground inspection of the aircraft was accomplished at ord, and no damage to the aircraft was found.

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Original NASA ASRS Text

Title: ACR FLC ENCOUNTER SEVERE TURB FROM TSTM. VERT ALT EXCURSION IS 3500 FT. FLC RECOVER AND LAND WITHOUT FURTHER DIFFICULTY.

Narrative: ACFT WAS ON A SCHEDULED FLT FROM LAX TO MDW. FORECAST WX FOR MDW CALLED FOR TSTMS IN THE AREA, HOWEVER THE CREW HAD TALKED TO DISPATCH AND DETERMINED LNDGS WERE STILL IN PROGRESS AT MDW. A LINE OF TSTMS WAS E TO W FROM ABOUT KIRKSVILLE, MO, E TO UNDETERMINED BOUNDARY. THE ALTERNATE FOR FLT WAS ORD AND WAS OPEN. THE FUEL STATE WAS SUCH THAT A DIVERSION AROUND THE WX WAS IMMINENT. ZAU INDICATED A BREAK IN THE LINE FROM JUST E OF QUINCY, IL. THE ACFT'S WX RADAR ALSO CONCURRED SO THE DECISION WAS MADE TO PROCEED THROUGH THE BREAK, AND CTR RECLRED FLT DIRECT TO ROCKFORD, IL, NW OF CHICAGO. THE RIDE THROUGH THE BREAK IN THE WX WAS ONLY LIGHT TURB AND ACFT WAS DSNDED BY CTR ROUTINELY AND HANDED OFF TO CHICAGO APCH CTL. AS THE ACFT PROCEEDED FROM ROCKFORD TO DUPAGE TO MDW, ACFT WAS DSNDED TO 3000 FT MSL, AND ASKED TO SLOW TO 180 KTS. BTWN ROCKFORD AND DUPAGE THE LINE OF TSTMS REAPPEARED AND LAY JUST S OF CHICAGO. MDW WAS LNDG ON RWY 31C, AND WAS STILL RPTING VMC AND OPEN FOR LNDG. AT THIS POINT THE ACFT WX RADAR SHOWED VIRTUALLY TOTAL ATTENUATION. SO DEVS WERE ASKED FOR CHICAGO APCH AND WERE GRANTED TO GO N AROUND THE WX. RADAR DISPLAY STAYED THE SAME NO MATTER WHAT RANGE SCALE OR DEG OF TILT. POS OF THE ACFT WAS NOW ABOUT 10-15 MI W OF MDW WHEN VIOLENT TURB WAS ENCOUNTERED. CAPT WAS FLYING THE ACFT WITH AUTOPLT AND AUTOTHROTTLES ENGAGED. SUDDENLY THE ACFT STARTED LURCHING VIOLENTLY, THE AUTOPLT TRIPPED OFF AND IVSI (INSTANTANEOUS VERT SPD INDICATOR) PEGGED OUT AT 6000 FPM UP. AIRSPD WENT OVER 250 KTS AND THE ALT WENT TO APPROX 6500 FT IN A FEW SECONDS (APPROX 10-15 SECONDS). THE CAPT WAS HOLDING FULL FORWARD ON THE CTL WHEEL CALLING FOR AUTOPLT BACK ON, ALT HOLD ON AND FOR FLAPS UP AND CALL FOR RECLRNC FROM APCH. THE CREW TRIED TO CALL APCH CTL BUT ALTDEV OCCURRED BEFORE CONTACT COULD BE MADE. THE ACFT EXITED THE PROB AREA AT ABOUT 6500 FT AND CONTINUED TO FLY NORMALLY. AFTER COM WITH CHICAGO APCH WAS RE-ESTABLISHED, THE DECISION WAS MADE TO DIVERT TO ORD WHERE WX WAS VMC. A NORMAL APCH AND LNDG WAS MADE AT ORD. ON GND AT ORD CAPT CALLED APCH CTL AND THEY SAID OUR ACFT 'LOOKED LIKE A PING-PONG BALL' ON THEIR RADARSCOPES. A DETAILED GND INSPECTION OF THE ACFT WAS ACCOMPLISHED AT ORD, AND NO DAMAGE TO THE ACFT WAS FOUND.

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.