Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Conflict NMAC
Flight Crew Took Evasive Action
Weather; Human Factors
Our fight was on arrival to the Bedford airport; receiving vectors from Boston Approach for a right downwind visual approach to Runway 29. During the event in question we were in IMC with no meaningful visibility outside the aircraft. From 4;000 feet we received a descent clearance to 3;500 feet. As we initiated the descent; Approach issued a traffic advisory for opposite direct VFR traffic [around] 3;000 feet. Descending through about 3;700 feet Approach advised that the traffic appeared to be climbing and instructed us to stop the descent. Almost simultaneously; the TCAS began issuing aural traffic alerts. I estimate that from the time Approach issued the 'stop descent' instruction until we were able to arrest the descent of the aircraft we had descended another 150 feet. Once I had identified the conflicting traffic on the TCAS screen I found that it was very close (in front of us) laterally and converging vertically. I initiated a climb; which approach instructed a moment later. At closest approach on the TCAS screen; I could discern no lateral separation and the target icon reported the other aircraft as 400 feet below us. Once clear of the traffic; we received clearance for a descent to 3;000 feet and emerged into VMC around that altitude.When I later informed Approach that our TCAS had indicated a very close pass with only 400 feet of vertical separation; the controller indicated that the traffic had departed BED westbound; was squawking VFR and not in contact with Approach. (I find myself wondering if the other aircraft ever realized how close 20;000 pounds worth of our airplane had come traveling nearly head-on.)More notes regarding the event:-The screen of the model of TCAS installed in this airplane is fairly small and dim. At maximum brightness I find it satisfactory in night operations; but sometimes difficult to read in the day. The airspace around Boston is congested; and the conflicting target did not begin to stand out to my eyes until the commencement of aural advisories and the icon changed color. Needless to say; however; it would still pay to check the TCAS screen carefully prior to initiating altitude changes.-Also; possibly due to the volume of radio traffic; my PF First Officer did not evidently hear the 'stop descent' instruction from the controller. A moment was required before I realized this and took action. As a result; we had descended another 150 feet before stopping the descent.
Air carrier Captain reported an NMAC on arrival to BED with a VFR departure out of the same airport.