ACR LGT INFLT ENCOUNTER WITH TSTM ACTIVITY.

Narrative

FLYING PARALLEL TO A FRONT THAT EXTENDED FROM TX TO CANADA. RADAR WAS PAINTING ISOLATED TSTMS FOR CHICAGO TO HOUSTON. CEILING VARIED DURING FLT BUT WAS IFR DURING OCCURRENCE. NO CELLS PAINTED ON RADAR; BUT SUDDENLY EXPERIENCED SEVERE TURB AND LIGHTING. LOST 500 FT IN SECONDS. LASTED APPROX 30 SECONDS - 1 MIN. DURING THIS TIME A FLT ATTENDANT WAS THROWN TO FLOOR BREAKING PELVIS IN 2 PLACES. RADAR WAS ON DURING MOST OF FLT IN CONJUNCTION WITH TCASII (IN AUTO POS). BELIEVE THE RADAR DID NOT PAINT TSTMS. HAVE HEARD OF RADAR IN CONJUNCTION WITH TCASII NOT PAINTING TOPS OF TSTMS! AFTER SEVERE TURB OF 30 SECONDS - 1 MIN; AIR WAS ONCE AGAIN SMOOTH. CAN ONLY ASSUME THIS WAS THE TOP OF A TSTM. CENTER WAS NOT PAINTING STORM NOR WAS OUR RADAR PAINTING ANYTHING IN IMMEDIATE AREA; ALTHOUGH IT WAS PAINTING OTHER ISOLATED STORMS IN THE AREA. I THINK MORE INVESTIGATION OF THIS RADAR USED IN CONJUNCTION WITH TCASII IS IN ORDER. COMPANY HAS EXPRESSED INTENT TO FOLLOW-UP. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE COMMENTS ABOUT TCASII INTERFERENCE WAS UNSUBSTANTIATED HOTEL LOBBY TALK. THE SEAT BELT SIGN WAS ON AND THE FLC WAS LOOKING FOR TSTM ACTIVITY WITH RADAR TILT BUT GOT NO 'RETURN' IN THEIR IMMEDIATE AREA. SUPPLEMENTAL INFO FROM ACN 191401: THERE WAS NO TFC IN OUR AREA THAT COULD HAVE POSSIBLY MASKED THE CELL.