Flight crew vacates assigned altitude during communications misunderstanding.

Attributes
ACN	315540
Time
Date	199509
Day	Sat
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : crg
State Reference	FL
Altitude	agl bound lower : 29000 agl bound upper : 29700
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zjx
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : instrument pilot : commercial pilot : atp
Experience	flight time last 90 days : 150 flight time total : 13500 flight time type : 9000
ASRS Report	315540
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance non adherence : published procedure other anomaly other
Independent Detector	atc equipment other atc equipment : unspecified aircraft equipment other aircraft equipment : unspecified
Resolutory Action	controller : issued new clearance flight crew : returned to intended course or assigned course other
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 60000 vertical : 2000
Supplementary
Air Traffic Incident	other

Narrative:

ZJX transmitted clearance to climb to FL370. Copilot acknowledged 'air carrier X cleared to FL370.' clearance acknowledged with no broken or covered xmissions evident. As we were initiating climb, center called traffic (it was just popping up on TCASII) at FL310, approximately 1 O'clock, 20 mi, direction unknown. At about 29700 ft center called 'air carrier X confirm level 290.' I immediately stopped climb and began descent referencing TCASII display of FL310 traffic while copilot transmitted that we were climbing as cleared to FL370. Jax gave us immediate descent to FL290 and a 045 degree heading. Traffic was acquired visually and, after leveling at FL290, we communicated our understanding of climb clearance and readback of same with acknowledgement. Jax indicated clearance was for air carrier Y. Whether original clearance was misread or misunderstood, our climb could have caused a conflict. Our action and the controller's action, I believe caused separation to remain intact.

Original NASA ASRS Text

Title: FLC VACATES ASSIGNED ALT DURING COMS MISUNDERSTANDING.

Narrative: ZJX XMITTED CLRNC TO CLB TO FL370. COPLT ACKNOWLEDGED 'ACR X CLRED TO FL370.' CLRNC ACKNOWLEDGED WITH NO BROKEN OR COVERED XMISSIONS EVIDENT. AS WE WERE INITIATING CLB, CTR CALLED TFC (IT WAS JUST POPPING UP ON TCASII) AT FL310, APPROX 1 O'CLOCK, 20 MI, DIRECTION UNKNOWN. AT ABOUT 29700 FT CTR CALLED 'ACR X CONFIRM LEVEL 290.' I IMMEDIATELY STOPPED CLB AND BEGAN DSCNT REFING TCASII DISPLAY OF FL310 TFC WHILE COPLT XMITTED THAT WE WERE CLBING AS CLRED TO FL370. JAX GAVE US IMMEDIATE DSCNT TO FL290 AND A 045 DEG HDG. TFC WAS ACQUIRED VISUALLY AND, AFTER LEVELING AT FL290, WE COMMUNICATED OUR UNDERSTANDING OF CLB CLRNC AND READBACK OF SAME WITH ACKNOWLEDGEMENT. JAX INDICATED CLRNC WAS FOR ACR Y. WHETHER ORIGINAL CLRNC WAS MISREAD OR MISUNDERSTOOD, OUR CLB COULD HAVE CAUSED A CONFLICT. OUR ACTION AND THE CTLR'S ACTION, I BELIEVE CAUSED SEPARATION TO REMAIN INTACT.

Data retrieved from NASA's ASRS site as of July 2007 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.