A B747 flight crew reported a NMAC with a Cessna at 4500 feet after they were issued a descent clearance from ATC for 6000 feet to 4000 feet.
Synopsis
A B747 flight crew reported a NMAC with a Cessna at 4500 feet after they were issued a descent clearance from ATC for 6000 feet to 4000 feet.
Narrative
Descending into ZZZ on ZZZZZ arrival between points ZZZZZ1 and ZZZZZ2. Speed 190 assigned by ATC. Flaps 5. IMC conditions with intermittent broken and obscured clouds. Cleared to descend 6000 to 4000 ft. While passing through approximately 5000 ATC advises us of Cessna VFR traffic climbing to 4500; at our 10 O'clock; about 5 miles away. While changing descent modes to decrease descent rate with vertical speed; received a TCAS RA. Executed the AOM TCAS RA escape maneuver (initially stopping descent; and then a climbing maneuver. Advised ATC of TCAS RA. ATC eventually stopped the climb of the Cessna (which we think was above the 4500) and vectored it out of our path. Estimating we passed close to 300 of one another. Relief Pilot briefly had visual contact with the Cessna but was unable to maintain due to the clouds. Unsure why we were cleared to descend to 4000 with another aircraft so close to our flight path and climbing to 4500.Unsure why VFR traffic was flying in IMC conditions. After the TCAS RA; vectored and landed Runway XXL. Suggestions: Hire more air traffic controllers. Do not clear a 747 jet aircraft to descend through another aircrafts altitude when closing on one another. Do not allow VFR traffic into IMC conditions without the proper equipment (i.e. TCAS on the Cessna).
Second reporter narrative
Arrival ZZZZZ to ILSXXL ZZZ Approach gave us a descent from 6000-4000. We were in IMC conditions. Controller radioed another aircraft to level off at 4500 and gave them our position; heavy aircraft; descending to 4000. That aircraft; a Cessna; reported us in sight. Controller radioed us to reported the traffic at our 12 oclock. We were IMC; in and out of clouds. We received a Traffic TCAS. At that moment we popped out of clouds to see the Cessna at our 11 to 12. I radioed to the controller that we were in and out of clouds and conditions were marginal VFR. At that moment we received TCAS RA to level off. Captain disconnected the Autopilot and Auto throttles and leveled off. It appeared that the Cessna was turning towards us and climbing. Controller radioed the Cessna to level off and maintain visual separation. I reported the TCAS RA. TCAS advisory then changed from level off to climb. Captain advanced the thrust and began to climb. TCAS RA showed that the Cessna was within 500 feet of our aircraft. We continued to climb clear of conflict; reestablished automation and landed in ZZZ. It appeared that the Cessna was not maintaining VMC conditions and following the controllers instructions. The Cessna reported us in sight; which may have lead the controller to believe we were in VMC. That expectation bias probably contributed to the lack of separation given by ATC and causing us to have the TCAS RAs. Secondarily it's important for a controller to understand the work load that is imposed upon the crew during a TCAS RA event and give reasonable time for the crew to recover and get automation reestablished and back on original altitude and heading. We also noted that the TCAS alert volume was very weak but the instrument visual cues were operating normally.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.