FLC OF ACR MLG ACFT INADVERTENTLY LINED UP WITH THE WRONG ARPT DURING A VISUAL APCH.

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ON A DAYLIGHT VFR DSCNT INTO RAPID CITY; SD; REGIONAL ARPT (COPLT FLYING) MY FLT (ACR MLG ACFT) WAS GIVEN A VISUAL APCH CLRNC BY ELLSWORTH AFB APCH CTL. THE AREA WAS BLANKETED WITH SNOW. WE HAD 'RANCH NDB' THE LOM FOR RWY 32 AND THE ILS FOR RWY 32 TUNED AND IDENTED. WE WERE ANGLING FROM THE E TO FINAL. AT A POINT APPROX 5 MI FROM THE RWY; WE REALIZED WE WERE LINING UP FOR RWY 31 AT ELLSWORTH. AT THAT SAME TIME; RAP TWR ADVISED US OF THE SAME. I TOLD TWR WE WERE MAKING A L TURN BACK TOWARDS RAP. (WE LEVELED AT 1100 FT AGL.) WE WERE REISSUED A VISUAL APCH TO RWY 32 AND WE LANDED. I FEEL THE LESSON FOR ME IS TO BE MORE CONSERVATIVE ON VISUAL APCHS WHEN GND/FLT CONDITIONS ARE LESS THAN PERFECT. THIS PARTICULARLY SO WHEN 2 ARPTS ARE IN CLOSE PROX WITH THE RWYS ALIGNED. ALSO; MY COPLT DID NOT HAVE THE FIELD IN SIGHT WHEN I FELT I DID. I WAS PREMATURE IN CALLING 'FIELD IN SIGHT' WITHOUT HIS HAVING IT AS WELL. WE EVEN DISCUSSED ELLSWORTH BEING JUST N OF RAP AND WITH A SIMILAR RWY ALIGNMENT; BUT STILL LINED UP ON IT. THE SNOW COVERED GND CONDITIONS CAUSED ME TO SEE ELLSWORTH FIRST AND I BECAME LOCKED ONTO IT.