UNAUTH LNDG MADE BY ACR MDT PIC WHEN BRAKING ACTION RPTED NIL.

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DURING APCH TO RWY 05 ORF; ENCOUNTERED LIGHT; CLR ICING. ON ILS; AT APPROX 1000 FT; 3 MI FROM RWY; TWR TOLD US THAT DC-9 JUST LANDED; RPTED BRAKING NIL. PREVIOUS RPT HAD BEEN POOR. WE WERE JUST DSNDING THROUGH OVCST AS THIS NEW BRAKING RPT WAS GIVEN. THE FO ASKED ME IF WE WERE LEGAL TO LAND WITH THIS NEW RPT. I TOLD HIM I WASN'T SURE. I CONSIDERED MY OPTIONS: MAKE GAR; FLY IN ICING CONDITIONS WAITING FOR COMPANY DISPATCHER TO MAKE DECISION AS TO WHETHER WE SHOULD LAND IN ORF; FILED ALTERNATE OR SOME OTHER STATION...OR CONTINUE AND LAND ON RWY; NOW IN SIGHT. RWY LOOKED FINE. (?) SURFACE WIND WAS WITHIN 10 DEGS OF RWY AND A STEADY 17 KT. I FELT COMFORTABLE MAKING THE LNDG AS THE DASH 7 HAS EXCELLENT STOPPING/DIRECTIONAL CTL WITH FULLY REVERSIBLE PROPS....BRAKES ON THIS AIRPLANE ARE ONLY NEEDED FOR STOL LNDGS AND PARKING. I ASKED THE FO IF HE FELT COMFORTABLE MAKING THE LNDG; HE REPLIED IN THE AFFIRMATIVE. I LATER LEARNED THAT MY COMPANY OPS MANUAL SAYS NO OPS ALLOWED ON RWY WITH NIL BRAKING. THE LNDG WAS SMOOTH; UNEVENTFUL. CTL OF ACFT WAS TRANSFERRED TO ME; I TESTED BRAKING AND FOUND IT TO BE POOR (IN MY OPINION). PROB: I BROKE COMPANY RULES. WE PLTS NEED A MORE PRECISE MEASUREMENT OF BRAKING CONDITIONS...'NIL' TO A DC-9 PLT MAY BE 'POOR' OR BETTER TO A DASH 7 PLT...I NEED TO REVIEW MY OPS MANUAL.