ACR-LGT AND MLG HAVE POTENTIAL CONFLICT WHILE FLYING PARALLEL ILS APCHS.

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ON 330 DEG HDG AT 6000' TO INTERCEPT RWY 30L LOC AT STL; XING THE MISSISSIPPI S OF THE ARCH; WAS ADVISED OF MLG TFC AT 2:30-3 O'CLOCK ON A PARALLEL APCH TO RWY 30R. BELIEVE HIS ALT WAS 5000'. SIGHTED AND ADVISED TFC IN SIGHT. FLT VIS HAZY AND ABOUT 3-5 MI. GND CONTACT; BUT ARPT NOT IN SIGHT. CLRED FOR RWY 20L ILS TO MAINTAIN 2200' OR HIGHER UNTIL ESTABLISHED ON LOC AND TO MAINTAIN 190 KTS TO THE OM. VISUALLY SIGHTED RWY IN DSCNT AND TURNED SO AS NOT TO INTERCEPT LOC; BUT TO REMAIN PARALLEL AND S OF THE LOC/RWY CENTERLINE. KNOWING THAT APCH PROBS EXIST AT STL DUE TO CLOSE PROX OF RWY 30L AND 30R CENTERLINES; I ELECTED NOT TO CENTER LOC NEEDLE UNTIL CONFIRMING POS OF THE MLG. AS I LEANED FORWARD AND TO THE RIGHT TO CHK ON THE POS OF THE MLG; THE F/O WAS ASKING IF I HAD THE MLG AS HE WAS GETTING TOO CLOSE. LOOKING OUT HIS SIDE WINDOW AND FRONT WINDSHIELD; I OBSERVED THE MLG AS IT WAS IN AN APPROX 20-30 DEG BANK TO THE RIGHT CORRECTING BACK TO THE 30R CENTERLINE. I ESTIMATE HIS POS TO BE ABOUT 100' HIGHER AND NOT MORE THAN 500' HORIZ. THIS INCIDENT COULD HAVE BEEN PREVENTED IF THE INTERCEPTS TO THE PARALLEL LOCS HAD BEEN STAGGERED; IE; NOT PARALLEL WING TIP TO WING TIP APCHS. AS ANY AUTOPLT CAN EVEN TAKE YOU THROUGH THE LOC CENTERLINE BEFORE CORRECTING BACK ON TRACK; SUPPOSE BOTH ACFT ARE ON AUTOPLT AND BOTH GO THROUGH THEIR RESPECTIVE LOC CENTERLINE--ANOTHER CLOSE CALL. ALSO; WX AND VIS IN FLT CAN DIFFER GREATLY FROM THAT ON THE GND. A CTLR MAY PREDICATE HIS ACTIONS BY THE RPTED GND VIS; FOR EX 10 MI; WHEN FLT VIS MAY BE ONLY 3-5 MI. FLT VIS--ANOTHER IMPORTANT FACTOR IN PARALLEL APCHS. FOR AN ARPT LIKE STL; WHERE PARALLEL RWYS ARE CLOSE TOGETHER; SIMULTANEOUS APCHS SHOULD ONLY BE MADE WITH STAGGERED INTERCEPTS--NOT WING TIP TO WING TIP. IT'S MUCH EASIER AND SAFER TO MAINTAIN VISUAL CONTACT WITH YOUR TFC AND FLY THE APCH IF THAT TFC IS AHEAD OF YOU AND NOT AT 9 OR 3 O'CLOCK.