IAP ILS APCH WHEN TERMINAL AREA WX RPT INDICATES ARPT WX MINS FOR LNDG DO NOT EXIST. MISSED APCH POINT AND PROC IN QUESTION DURING AN ALTDEV.

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INBOUND TO ONTARIO INTL; ATIS RPTED MARGINAL CEILING WITH VISIBILITY AT 3/4 MI. WE BRIEFED THE APCH WITH FO FLYING AS PER COMPANY PROCS TO ALLOW CAPT TO MONITOR APCH AND MAKE THE CALL TO LAND OR GO MISSED APCH. INSIDE PETIS NDB (APPROX 12 MI TO FIELD) WE WERE IN OUR DSCNT TO GS ALT WHEN WE WERE SWITCHED TO TWR (WE HAD ALREADY BEEN CLRED APCH). AT ABOUT 9-10 DME WHILE AWAITING GS CAPTURE; WE WERE SENT TO TWR. TWR ADVISED US THAT RVR WAS NOW BELOW OUR MINS SO I INFORMED THEM THAT WE COULD NOT CONTINUE THE APCH EXPECTING INSTRUCTIONS FOR A MISSED APCH TO A HOLDING FIX. I THEN LOOKED INTO MY FLT BAG FOR MY LOW ALT CHART TO BE PREPARED FOR A HOLDING FIX. IN THE MEANTIME; THE FO HAS RELEASED ALT HOLD AND DSNDED BELOW GS INTERCEPT ALT AND HAS FALLEN BELOW GS TOO! WE WERE ALREADY 400 FT LOW WHEN I LOOKED UP AND NOTICED. I INFORMED THE FO ONCE AGAIN THAT WE COULD NOT SHOOT THE APCH AND ORDERED A MISSED APCH. AT ABOUT THE SAME TIME; TWR GAVE US INSTRUCTIONS TO A BOX PATTERN AND A NEW ALT. TRANSMISSOMETERS WERE SUBSEQUENTLY NOTAMED INOP; AND WITH VISIBILITY MINS (3/4 MI AGAIN) WE SHOT THE NEXT APCH TO AN UNEVENTFUL LNDG. CONTRIBUTING FACTORS: NOT BEING KEYED INTO FLYING PUBLISHED MISSED APCH. WE ARE ALWAYS GIVEN INSTRUCTIONS IN THE SIMULATOR TO EXPEDITE APCHS. NOT EXECUTING MISSED APCH WHEN THE FIELD WAS ANNOUNCED VARIABLE DOWN TO '0' VISIBILITY ON THE RVR. THE 'CAN DO' ATTITUDE LEFT ME TRYING TO FIGURE OUT ANOTHER WAY TO MAKE A SUCCESSFUL APCH. NOT HAVING ALL MY CHARTS AVAILABLE PRIOR TO BEING CLRED FOR A MARGINAL APCH. I FELT '3/4 MI; NO PROBLEM!' NOT MONITORING APCH. IF I HAD ACCOMPLISHED THE 3 STEPS ABOVE I WOULD HAVE BEEN MORE ABLE TO MONITOR THE FO'S APCH AND CAUGHT THE ALTDEV PRIOR TO IT'S OCCURRENCE.