Air carrier flight crew climbed above ATC assigned altitude. The controllers referenced the altitude...

Narrative:

I was the pilot flying. We were cleared direct to dag and to FL180. Passing 17;300 ft we set the altimeters to 29.92 in anticipation of leveling at FL180. Shortly after this I heard ATC clear us to FL190. I changed the altitude in the autopilot window and verbalized the altitude assignment. The captain acknowledged FL190. As we climbed through FL180; the captain said we were only cleared to FL180. I disconnected the autopilot and started to level the aircraft passing FL183. ATC; during this time; asked if we were leveling at FL180. The captain told him we had overshot the altitude and were descending. The aircraft topped out at about FL185 and I expedited a descent back to FL180. ATC gave avoidance vectors to another aircraft and turned us right to 090. At no time were any TCAS alerts given. After he verified we were at FL180; he cleared us back to dag. The rest of the flight was uneventful. I obviously misheard ATC instructions. Thinking back; he may have told us of traffic at FL190 and somehow I interpreted it as a climb to FL190. The captain automatically repeated my FL190 call without thinking; as the captain knew we were only cleared to FL180. Attention to detail on both of our parts needs to be emphasized. I give the ATC controller a lot of credit for seeing the situation and quickly vectoring aircraft to avoid a conflict.

Original NASA ASRS Text

Title: Air Carrier flight crew climbed above ATC assigned altitude. The Controllers referenced the altitude of traffic which may have contributed to the misunderstanding.

Narrative: I was the pilot flying. We were cleared direct to DAG and to FL180. Passing 17;300 FT we set the altimeters to 29.92 in anticipation of leveling at FL180. Shortly after this I heard ATC clear us to FL190. I changed the altitude in the autopilot window and verbalized the altitude assignment. The Captain acknowledged FL190. As we climbed through FL180; the Captain said we were only cleared to FL180. I disconnected the autopilot and started to level the aircraft passing FL183. ATC; during this time; asked if we were leveling at FL180. The Captain told him we had overshot the altitude and were descending. The aircraft topped out at about FL185 and I expedited a descent back to FL180. ATC gave avoidance vectors to another aircraft and turned us right to 090. At no time were any TCAS alerts given. After he verified we were at FL180; he cleared us back to DAG. The rest of the flight was uneventful. I obviously misheard ATC instructions. Thinking back; he may have told us of traffic at FL190 and somehow I interpreted it as a climb to FL190. The Captain automatically repeated my FL190 call without thinking; as the Captain knew we were only cleared to FL180. Attention to detail on both of our parts needs to be emphasized. I give the ATC Controller a lot of credit for seeing the situation and quickly vectoring aircraft to avoid a conflict.

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	998589
Time
Date	201203
Local Time Of Day	0601-1200
Place
Locale Reference	ZLA.ARTCC
State Reference	CA
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737-500
Operating Under FAR Part	Part 121
Flight Phase	Climb
Flight Plan	IFR
Person 1
Function	First Officer Pilot Flying
Experience	Flight Crew Last 90 Days 358 Flight Crew Type 5700
Events
Anomaly	Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

Air carrier flight crew climbed above ATC assigned altitude. The controllers referenced the altitude of traffic which may have contributed to the misunderstanding.