Narrative:

We had just departed lga and the second controller gave us a turn to 240 and a climb to 12000 ft. I was the pilot flying. At about 11300 feet we received a TA from an aircraft to the north that was descending. I reduced our climb rate to mitigate the closure rates but that did not help. When we reached approximately 11800 feet we received a descending RA. It was a very aggressive RA. We descended below 11300 feet when we got the 'clear of conflict' alert. The TCAS said the other aircraft was +900. The first officer had immediately alerted ATC that we were deviating to comply with the RA as we were solid IMC. The controller told us the other aircraft was supposed to be level at 13000 feet. This was the first alert we were given about any traffic. Clearly they were not. We received a new heading and clearance to climb back to 12000 feet and a phone number to call. Once we were stabilized again I transferred controls to the first officer so I could check on the flight attendants. Because we had just left sterile cockpit there was a good chance the flight attendants were out of their seats. The descent was abrupt and I wanted to check that they were ok. They were fine and so were the passengers. The rest of the flight continued without incident. On the ground at our destination I called the number we were given and spoke with a supervisor. He was just asking for more details so he could look into it. Later; an FAA supervisor called and told me that he had spoken to the crew that was not at the altitude they should have been. They were indeed not at their assigned altitude but were somewhat behind us when it happened. Because we were solid IMC there really was nothing we could have done differently as we were not able to visually identify the intruder aircraft. Nothing we could have done differently in this situation with the weather and no traffic alert given.

Related reports:

Google
 

Original NASA ASRS Text

Title: EMB175 flight crew reported receiving a TCAS RA for opposite direction traffic they had not been advised of.

Narrative: We had just departed LGA and the second controller gave us a turn to 240 and a climb to 12000 ft. I was the pilot flying. At about 11300 feet we received a TA from an aircraft to the north that was descending. I reduced our climb rate to mitigate the closure rates but that did not help. When we reached approximately 11800 feet we received a descending RA. It was a very aggressive RA. We descended below 11300 feet when we got the 'clear of conflict' alert. The TCAS said the other aircraft was +900. The First Officer had immediately alerted ATC that we were deviating to comply with the RA as we were solid IMC. The controller told us the other aircraft was supposed to be level at 13000 feet. This was the first alert we were given about any traffic. Clearly they were not. We received a new heading and clearance to climb back to 12000 feet and a phone number to call. Once we were stabilized again I transferred controls to the First Officer so I could check on the flight attendants. Because we had just left sterile cockpit there was a good chance the flight attendants were out of their seats. The descent was abrupt and I wanted to check that they were ok. They were fine and so were the passengers. The rest of the flight continued without incident. On the ground at our destination I called the number we were given and spoke with a Supervisor. He was just asking for more details so he could look into it. Later; an FAA supervisor called and told me that he had spoken to the crew that was not at the altitude they should have been. They were indeed not at their assigned altitude but were somewhat behind us when it happened. Because we were solid IMC there really was nothing we could have done differently as we were not able to visually identify the intruder aircraft. Nothing we could have done differently in this situation with the weather and no traffic alert given.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.