SIMULTANEOUS ILS APCHS WERE IN PROGRESS AT ORD ON THE RWY 27'S. DC10 OVERSHOT ILS RWY 27R LOC AND SET OFF THE TCASII RA ON A B747 ON LOC RWY 27L. B747 TCASII IN RA MODE; ISSUES CLB COMMAND. RPTR CONCERNED OVER FACT THAT TCASII OPS ORDER SAYS TO USE TA IN THESE TYPE APCHS. IF FOLLOWED; THE TFC SEPARATION WOULD HAVE DECREASED.

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FLT WAS A SCHEDULED B747 FREIGHTER SVC BY A LARGE INTL CARRIER (FOREIGN REGISTERED). ATC INSTRUCTIONS WERE TO INTERCEPT ILS RWY 27L AT 5000 FT; MAINTAIN 5000 FT TILL 'WAVIE;' CLRED ILS RWY 27L; SPD 170 KIAS TILL 'WILTT' (LOM). CAPT (PF) WAS ON COURSE; SLIGHTLY L OF CTRLINE. TCASII WAS STILL IN 'TA/RA' MODE. 1 MI FROM 'WAVIE' (WHERE GS INTERCEPT SHOULD OCCUR). TCASII TA SOUNDED. OTHER ACFT APCHING FROM THE N; TURNING TO INTERCEPT RWY 27R CTRLINE. ACFT SYMBOL ON TCASII DISPLAYED -400 FT (4600 FT MSL). SHORTLY AFTER TA; WE RECEIVED AN RA. TCASII COMMANDED 'CLB' WITH APPROX A +1500 FPM COMMAND. CAPT FOLLOWED TCASII COMMAND; AND I ALERTED ATC OF 'TCASII CLB.' I ACQUIRED THE OTHER ACFT VISUALLY; A MAJOR UNITED STATES CARRIER; A DC10 AIRPLANE. OTHER ACFT APPEARED TO OVERSHOOT THE RWY 27R LOC BEAM. I OBSERVED IT IN A R TURN; ABOUT 1200-2000 FT TO OUR R AND 400 FT BELOW. ATC ADVISED US THAT HE WAS ON FINAL RWY 27R. HAD WE BEEN IN 'TA' ONLY; AS MANY PUBS RECOMMEND FOR CLOSELY- SPACED PARALLEL APCHS; THE 'RA' WOULD NOT HAVE SOUNDED. POTENTIAL FOR COLLISION WOULD HAVE BEEN MUCH HIGHER; AS GS INTERCEPT OCCURRED AS 'RA' SOUNDED. DSCNT ON GS WOULD HAVE REDUCED VERT SEPARATION. OTHER ACFT DID NOT APPEAR TO DSND SO I SUSPECT THE CREW HAD ALREADY SELECTED 'TA ONLY.' HUMAN FACTORS: IT IS VERY DIFFICULT TO PICK OUT TFC THAT IS BELOW YOUR ALT; AT NIGHT; OVER A BRIGHTLY-LIT CITY. THOROUGH VISUAL SCAN IS REQUIRED -- DO NOT ALLOW SCAN TO BECOME LAX IN TCASII EQUIPPED ACFT. ATC: RECOMMEND 'STAGGERED' POS ON PARALLEL APCHS. EXAMPLE: ACFT ON L SIDE 1-2 NM AHEAD OF ACFT ON R SIDE. MOST APCHS ARE FLOWN AT A COMMON (ATC-DIRECTED) AIRSPD; SO 'STAGGERING' WOULD BE MAINTAINED. PROCS: RECOMMEND TA/RA UNTIL FAF/LOM; VERY LITTLE MANEUVERING WILL OCCUR INSIDE THAT POINT. EQUIP: PUT TCASII IN ALL LARGE ACFT OPERATED UNDER FAR 121; INCLUDING FREIGHTERS. I AM NOW A BIG SUPPORTER OF TCASII.