Air carrier at FL370 with ZJX initiated a descent to FL300 but then experienced a TCAS TA/RA; ATC al...

Attributes
ACN	848521
Time
Date	200908
Local Time Of Day	1201-1800
Place
Locale Reference	ZJX.ARTCC
State Reference	FL
Aircraft 1
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Aircraft 2
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Person 1
Function	Captain Pilot Flying
Experience	Flight Crew Last 90 Days 200 Flight Crew Total 14000 Flight Crew Type 7000
Person 2
Function	First Officer Pilot Not Flying
Experience	Flight Crew Last 90 Days 160 Flight Crew Total 9000 Flight Crew Type 3200
Events
Anomaly	Conflict Airborne Conflict Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance
Miss Distance	Horizontal 12000 Vertical 600

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 right/a. Also at that moment; the jax center 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 right/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.

Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of April 2012 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.