A B737 HAS A RWY EXCURSION WHEN BRAKING ACTION PROVES TO BE LESS THAN RPTED.

Narrative

FLT XXX ORIGINATED IN TJSJ WITH AN INTERMEDIATE STOP IN MIA AND A FINAL DEST OF MDW. MDW HAD BEEN EXPERIENCING SNOWY CONDITIONS THROUGHOUT THE DAY. SNOW HAD BEEN RPTED AS LIGHT. I MADE MY FIRST INQUIRY ABOUT FIELD CONDITIONS IN SAN JUAN BEFORE I LEFT THE HOTEL AND WAS INFORMED BRAKING ACTION WAS GOOD AND THE SNOW WAS NOT STICKING TO THE SURFACE. THE FLT FROM TJSJ TO MIA WAS UNEVENTFUL. I MADE MY SECOND INQUIRY ABOUT FIELD CONDITIONS WHILE ON THE GND IN MIA. I WAS TOLD BRAKING ACTION WAS GOOD BY A B737. ENRTE; I MADE MY NEXT REQUEST FOR A FIELD CONDITION RPT AND CURRENT MDW WX VIA COMRDO CONNECTION WITH DISPATCH. RPTED WX WAS WINDS 070 DEGS AT 10 KTS; VISIBILITY 1 MI; LIGHT SNOW AND MIST; CEILING BROKEN 500 FT OVCST 900 FT; RVR 5500 FT WITH RWY 4R BRAKING ACTION FAIR. MDW ATIS AT XA53Z RPTED WINDS 080 DEGS AT 10 KTS; VISIBILITY 3/4 NM; LIGHT SNOW AND MIST; BROKEN 600 FT OVCST 1000 FT; TEMP 0 DEGS. WAS GIVEN RVR OF 6000 FT AND BRAKING ACTION ON RWY 4R AT FAIR AFTER I REQUESTED CURRENT BRAKING ACTION AND RVR FROM APCH CTL. MAX LNDG WT FOR A WET RWY CONDITION WAS 136000 LBS; OUR LNDG WEIGHT WAS APPROX 122000. WE INITIATED A NORMAL COUPLED APCH OF ILS RWY 4R INTO MDW. TOUCHDOWN WAS WITHIN THE TOUCHDOWN ZONE APPROX 1500 FT DOWN THE RWY. I APPLIED MAX BRAKING AND MAX REVERSE AT TOUCHDOWN. INITIAL BRAKING WAS FAIR. AT APPROX 3000 FT REMAINING; BRAKING ACTION WENT FROM FAIR TO POOR AND LAST 2000 FT BRAKING ACTION WAS NIL. I INITIALLY BEGAN TO REDUCE REVERSE THRUST AT 60 KTS PER COMPANY FLT MANUAL WHEN IT APPEARED WE WERE GOING TO BE ABLE TO EXIT THE RWY BY THE LAST TXWY. THE BRAKING EFFECTIVENESS DECREASED AND I RETURNED TO MAX REVERSE UNTIL THE ACFT CAME TO A FULL STOP. IT BECAME APPARENT THAT THE ACFT WAS NOT GOING TO STOP PRIOR TO THE BLAST FENCE AT THE END OF RWY 4R. I TRIED TO STEER THE ACFT TO THE R HOPING TO MAKE THE LAST TXWY. I WAS UNABLE TO MAKE A CTLED TURN BUT THE ACFT VEERED TO THE R WHICH RESULTED IN THE NOSEWHEEL LEAVING THE RWY SURFACE JUST PAST THE LAST TXWY WHICH HELPED BRING US TO A FULL STOP. THE ACFT CAME TO A SMOOTH STOP AND THE PAX AND FLT ATTENDANTS DID NOT REALIZE THAT ANYTHING ABNORMAL HAD OCCURRED. WHEN THE ACFT STOPPED; THE MAIN GEAR WAS ON PAVED SURFACE AND THE NOSE GEAR WAS APPROX 25 FT OFF THE PAVED SURFACE. THERE WERE NO INJURIES TO PAX OR CREW AND NO DAMAGE TO THE ACFT. A MECH SAID A R MAIN TIRE MIGHT NEED TO BE REPLACED DUE TO DAMAGE RECEIVED. THE PAX WERE DEPLANED BY AIRSTAIRS USING THE R AFT SVC DOOR AND TAKEN TO THE TERMINAL BY BUS. THE EVENT OCCURRED DUE TO NIL BRAKING ACTION WHICH HAD BEEN RPTED AS FAIR. AFTER THE EVENT; THE ARPT GND CREWS CLRED RWY 31C TO TRY TO REOPEN THE ARPT. THE GND CREW THEN RPTED RWY 31C BRAKING ACTION AS NIL.