DSNDING VISUAL APCH TO DCA; RPTR AND ACR Y COMMANDED ERRONEOUSLY BY TCASII RA.

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ACR X FLT 0001 ARRIVING FROM AML VOR ON 070 DEG HDG; DSNDING TO 4000 FT TO INTERCEPT POTOMAC RIVER VISUALLY. ACR Y FLT 0002 ARRIVING FROM BAL 255 DEG RADIAL DSNDING FROM 6000 FT TO 3000 FT TO INTERCEPT POTOMAC RIVER VISUALLY. BOTH ACFT WERE TO TURN S AND FOLLOW RIVER TO DCA. WHILE DSNDING THROUGH APPROX 4500 FT; WE GOT AN RA TO DSND AT 2000 FPM. MEANWHILE; ACR Y GOT RA TO CLB. MY FO SIGHTED THE ACR Y B-737 AND INSTRUCTED ME TO LEVEL OFF. THE ACR Y FO SIGHTED US AND INSTRUCTED HIS CAPT TO INCREASE DSCNT RATE TO AVOID A COLLISION. AFTER DISCUSSION WITH ALL PLTS INVOLVED AND DCA AREA MGR WORKING RADAR; IT WAS AGREED THAT; IF BOTH ACFT FOLLOWED THE COMMANDED RA; THERE WOULD HAVE BEEN A MIDAIR COLLISION. INITIAL RA CAME WHEN ACR Y WAS 100 FT BELOW US AS WE WERE BOTH DSNDING. OUR RATE OF DSCNT WAS 1000 FPM. B-737 WAS DSNDING AT 2500 FPM WITH GEAR; FLAPS; AND SPD BRAKES DEPLOYED. THEY WOULD HAVE BEEN UNABLE TO CLB IN THAT CONFIGN. IN THIS INSTANCE; THE TCASII RA WAS WRONG. JET ARRS FROM BAL VOR SHOULD BE LEVEL PRIOR TO INTERCEPTING THE POTOMAC RIVER AT 3000 FT IF WE ARE TO INTERCEPT AT 4000 FT.