B737-700 crew had an altitude deviation in zjc class a airspace.

Attributes
ACN	520190
Time
Date	200108
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : sav.vortac
State Reference	GA
Altitude	msl bound lower : 34000 msl bound upper : 37000
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B737-700
Flight Phase	cruise : enroute altitude change
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 250 flight time total : 10080 flight time type : 6060
ASRS Report	520190
Events
Anomaly	altitude deviation : excursion from assigned altitude other anomaly other
Resolutory Action	controller : issued alert controller : issued new clearance none taken : detected after the fact
Consequence	other
Supplementary
Problem Areas	ATC Human Performance Flight Crew Human Performance
Primary Problem	Ambiguous

Narrative:

While en route from isp to jax, we were given vectors off course and told to slow as much as possible for spacing into jax. We were level at FL370. We heard ZJX give us a descent to FL250. The first officer read back 'descend to FL250' and set it in the altitude window on the MCP. I repeated what I heard, 'descend to FL250' and verified that it was set in the altitude window and commenced a 1000 FPM rate of descent. While passing through FL345, the controller told us to climb back to FL350. I verified that altitude and climbed back up. Nothing else was said of this incident and we proceeded on to jax. This must have been my mistake, but all the procedures were complied with. I just need to listen more carefully.

Original NASA ASRS Text

Title: B737-700 CREW HAD AN ALTDEV IN ZJC CLASS A AIRSPACE.

Narrative: WHILE ENRTE FROM ISP TO JAX, WE WERE GIVEN VECTORS OFF COURSE AND TOLD TO SLOW AS MUCH AS POSSIBLE FOR SPACING INTO JAX. WE WERE LEVEL AT FL370. WE HEARD ZJX GIVE US A DSCNT TO FL250. THE FO READ BACK 'DSND TO FL250' AND SET IT IN THE ALT WINDOW ON THE MCP. I REPEATED WHAT I HEARD, 'DSND TO FL250' AND VERIFIED THAT IT WAS SET IN THE ALT WINDOW AND COMMENCED A 1000 FPM RATE OF DSCNT. WHILE PASSING THROUGH FL345, THE CTLR TOLD US TO CLB BACK TO FL350. I VERIFIED THAT ALT AND CLBED BACK UP. NOTHING ELSE WAS SAID OF THIS INCIDENT AND WE PROCEEDED ON TO JAX. THIS MUST HAVE BEEN MY MISTAKE, BUT ALL THE PROCS WERE COMPLIED WITH. I JUST NEED TO LISTEN MORE CAREFULLY.

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.