Air carrier at FL370 with ZJX initiated a descent to FL300 but then experienced a TCAS TA/RA; ATC alleging the aircraft was assigned FL330 not FL300.
Synopsis
Air carrier at FL370 with ZJX initiated a descent to FL300 but then experienced a TCAS TA/RA; ATC alleging the aircraft was assigned FL330 not FL300.
Narrative
We were level at FL370 when I left the cockpit for restroom use. I detected that we had begun a descent soon after leaving. Upon returning; we were passing FL330 on descent; and as I settled into the seat; the First Officer briefed the new altitude assignment of FL300. At about the same moment; we had a TCAS T/A; and I noticed a target below us approx 1000 feet and off to the right about 2 miles; and our altitudes were converging. I turned the autopilot off and initiated a climb just as we received a climb R/A. Also at that moment; the JAX CTR controller asked us to confirm that we were level at FL330. The First Officer replied that no; we were descending to FL300 as cleared. The controller issued an immediate left turn and climb; but I had already climbed back to 330; and the other aircraft reported us in sight. The closest we came was approx 600 ft vertically and approx 2 miles laterally. Upon sitting down upon returning from the cabin; I never had time to confirm the altitude assignment with ATC before we got the T/A and R/A. The First Officer said that he did read back FL300 to ATC; and was not corrected. I have flown with this First Officer before; and he is very regimented in his radio procedures; so I assume that there was some miscommunication between ATC and our aircraft on whether the cleared altitude was 330 or 300.
Second reporter narrative
I thought I read back the correct altitude clearance but obviously the Controller wanted us to go to FL330 originally. However he did not correct the altitude when I read back FL300. I may have misheard the original clearance. We were about to confirm the altitude clearance after the break but the incident occurred just prior to our ability to do that.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.