B737-800
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
ATC Issue All Types; Conflict NMAC
Air Traffic Control Issued Advisory / Alert; Flight Crew Took Evasive Action
Procedure; Human Factors
[We were] cleared for the visual approach to Runway 23R at destination. Approach Control advised VFR traffic; 10 o'clock; 5 miles; 500 FT above you has you in sight and is maintaining visual separation. We reported that we had the traffic on TCAS but not visually. Approach responded that 'he has you in sight and is maintaining visual separation. He will pass above and behind your aircraft. Maintain best forward speed; you are number one'. Approaching the glide slope; the TCAS target started descending. I informed the First Officer (pilot flying) that it appeared the traffic was attempting to 'duck under' our flight path; rather than pass overhead; as instructed. I got a visual on the aircraft at the same time we received a TA from the aircraft. Within seconds; we received an RA to CLIMB. We leveled our approach; maintained visual with the traffic as he passed under us by approximately 200 FT.[Issue was due to] reliance of ATC to allow a VFR pilot to be the only one responsible for separation in Class Bravo airspace. If both aircraft cannot maintain visual separation; in this case we did not initially see the other aircraft; perhaps a vector away from the potential conflict could have been issued; even though the weather was VMC.
B737 Captain reports a NMAC with VFR traffic during visual ILS approach. The traffic was pointed out and expected to maintain altitude 500 FT above the B737 but started descending causing a TCAS RA.