A B757-200 experienced an apparently inappropriate TCAS RA when they encountered an aircraft cleared to climb through their altitude; based on visual separation criteria.
Synopsis
A B757-200 experienced an apparently inappropriate TCAS RA when they encountered an aircraft cleared to climb through their altitude; based on visual separation criteria.
Narrative
We were passing through 12;400' when our descent clearance was revised to 11;000' for 'traffic at 2-3 o'clock; 10;000.' The target; an airliner; announced they had us in sight. ATC then cleared them to climb to our altitude; 11;000; and maintain visual separation. We then gained visual contact with the traffic. It was clear he would cross in front of us; fairly close by. (His range was decreasing; but as he was not on a constant bearing from us; it was clear we would not actually hit.) ATC queried the traffic; something like; 'How's it looking? You gonna be able to maintain separation' The pilot wasn't sure; and said as much. They were almost at our 12 o'clock; and we would pass just behind them. So; ATC gave them a descent to 10;000' which they started immediately. Just after the descent clearance; we could see the aircraft nose over; his altitude read 10;900' with a downward trend arrow. Just then our TCAS gave an RA; 'Descend; crossing; descend' The Captain said something to the effect of; 'I'm not doing that. He's descending; we'll descend right into him' and did not follow the TCAS RA. Then; inexplicably; the TCAS RA intensified; still commanding a descent. Well; by then; the jet (clearly in sight; and clearly below us; and clearly descending) was no longer a factor. The Captain clicked off the autopilot; initiating a response (even though we knew the TCAS guidance was wrong.) Very quickly; 'Clear of conflict' sounded. We reported our TCAS RA. I was conflicted about writing this report; but I DID want to draw attention to TCAS. The way I see it; had we been in IMC and; therefore; forced to follow TCAS direction; this event would have been alarming! We would have lost what little separation we had; in my opinion. Descending into an airplane that is clearly descending? TCAS software clearly did not give appropriate guidance; nor did it self correct when the initial guidance was so clearly wrong. I lost a bit of faith in TCAS today.
Second reporter narrative
After a descent of approx 200 feet I decided to abandon the RA and climb or level off instead. I did this because it was CLEAR that if I continued as directed by the TCAS I felt it would put us in a possible mid air collision. The weather was VFR and I saw the aircraft at that moment. It was clear that the aircraft we were to avoid was below us! I believe the TCAS solution was wrong. Even after it was obvious that we should not descend the TCAS gave us a second descend RA. I also suspect the other flight crew did not respond to their RA if they got one; which I believe they should have. There was also a comment on the radio from the traffic about turning 'away' from traffic. That came over radio AFTER we advised ATC that we were executing a TCAS RA. I think there are two primary reasons for this incident. The first being the mis-handling of aircraft by ATC. There was plenty of airspace for them to keep us separated. Secondly; I don't believe the TCAS worked properly and give us bad information.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.