MLG FLC CONTINUES APCH WHEN RVR DECREASES BELOW MINIMUMS.

Narrative

WE WERE PREPARING FOR THE ILS RWY 32L AT ORD. THE APCH CTLR ANNOUNCED THAT HE INTENDED TO START PUTTING ACFT ON THE ILS AND LET US DETERMINE FOR OURSELVES WHETHER OR NOT WE COULD COMPLETE THE APCH. THE MINIMUM RVR AUTH FOR THE APCH WAS 1800 FT; AND THE CURRENT RVR WAS 1800 FT. WE BEGAN THE ILS APCH; AND WHILE ON THE GS AT ABOUT 5000 FT; THE RVR DECREASED TO 1600 FT. AT ABOUT 4000 FT ON THE GS THE RVR DECREASED FURTHER TO 1400 FT. WE MISTAKENLY BELIEVED THAT IF THE RVR HAD BEEN AT LEAST AT THE REQUIRED MINIMUMS OF 1800 FT WHEN WE INTERCEPTED THE GS; THAT IT COULD DECREASE TO BELOW THE MINIMUMS AND WE COULD CONTINUE THE APCH TO DECISION HT. THIS IS WHAT WE DID; AND COMPLETED THE APCH AND LNDG. IN FACT; WE WERE REQUIRED TO DISCONTINUE THE APCH ANY TIME THAT THE RVR DECREASED BELOW 1800 FT AND WE WERE STILL ABOVE 2500 FT ON THE GS. SINCE WE DID NOT DISCONTINUE THE APCH; WE EXECUTED AN UNAUTH APCH. WE SHOULD HAVE REFED OUR COMPANY FLT MANUAL TO DETERMINE WHETHER OR NOT WE WERE AUTH TO FLY THE APCH; INSTEAD OF TRUSTING OUR MEMORY. WE WERE SIMPLY UNPREPARED FOR HOW QUICKLY THE WX DETERIORATED AT ORD. WE HAD CHKED THE WX BEFORE LEAVING; AND IT APPEARED FINE. WE ALSO BECAME RUSHED TO BE SEQUENCED ONTO THE APCH; AND MADE A DECISION BASED ON WHAT WE THOUGHT WE REMEMBERED FROM OUR FLT MANUAL.