RWY CONDITION WET. ACR MLG EXPERIENCES A NEAR RWY EXCURSION DURING LNDG PROC ROLLOUT.

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ON AUG/XX/94; FLT FROM ATL TO MDW HAD TROUBLE BRAKING AFTER LNDG ON RWY 31C IN MDW. THE CREW BELIEVED THE PROB WAS DUE TO HYDROPLANING ON THE RWY DURING THE LNDG ROLL- OUT. THE WT DURING LNDG WAS 88000 LBS; AND THE LNDG REF SPD WITH FULL FLAPS WAS 120 KTS. WX CONDITIONS AT THE TIME WERE 700 FT CEILING OVCST; VISIBILITY 1 1/2 MI IN RAIN; AND THE WINDS WERE 210 AT 15 KTS. THE CAPT TOUCHED DOWN AT THE 1000 FT POINT; AT A SPD OF 125 KTS (VREF PLUS 5 KTS). REVERSE THRUST WAS APPLIED UP TO 90 PERCENT N; AND BRAKES WERE APPLIED. THERE WAS NO BRAKE EFFECTIVENESS; I APPLIED MY BRAKES; AND THERE WAS NO EFFECTIVENESS EITHER. THE CAPT ASKED FOR THE ANTI-SKID TO BE TURNED OFF; THINKING GND SHIFT MECHANISM WAS IN THE INFLT MODE; PREVENTING ANTI-SKID (AND BRAKES) FROM OPERATING. THERE WAS STILL NO CHANGE IN EFFECTIVENESS. THE AIRPLANE HYDROPLANED DOWN THE RWY; AND AT APPROX 30 KTS. THE BRAKES BECAME EFFECTIVE. THE CAPT MANEUVERED TO AVOID RWY LIGHTS AT END OF RWY 31C; AND THE AIRPLANE CAME TO A STOP AT ABOUT A 60 DEG ANGLE TO THE RWY'S L EDGE. THE TWR SENT THE FIRE TRUCKS TO INSPECT THE AIRPLANE; AND O DAMAGE WAS OBSERVED TO ACFT; NOR ARPT PROPERTY. WE CALLED FOR TOW; AND WERE TOWARD TO THE GATE. DURING THE ENTIRE FLT; ALL CHKLISTS WERE CALLED FOR THE CAPT; AND COMPLIED WITH AT THE APPROPRIATE TIMES; IN ACCORDANCE WITH COMPANY PROCS. NOT SURE WHAT CAN BE DONE ABOUT HYDROPLANING; MAYBE TO MAKE A FIRMER THAN NORMAL LNDG; AND USE REVERSERS AND OTHER AERODYNAMIC BRAKING DEVICES TO MAX USAGE TO REDUCE THE AMOUNT OF WHEEL BRAKE USAGE NEED TO STOP THE ACFT AFTER LNDG. SUPPLEMENTAL INFO FROM ACN 281667: THE WX AT MIDWAY WAS LIGHT RAIN. THE PLANE NEVER SLID OFF THE RWY; IT REMAINED ON THE RWY UNTIL STOPPING.