An A320 first officer reported that he and the captain misunderstood ZME's instruction to descend.

Attributes
ACN	512910
Time
Date	200105
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : bwg.airport
State Reference	KY
Altitude	msl bound lower : 35000 msl bound upper : 37000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	A320
Flight Phase	descent : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : multi engine pilot : instrument pilot : commercial
Experience	flight time last 90 days : 230 flight time total : 10700 flight time type : 800
ASRS Report	512910
Events
Anomaly	altitude deviation : overshoot non adherence : far non adherence : company policies non adherence : clearance non adherence : published procedure
Resolutory Action	controller : issued new clearance controller : issued advisory
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

At cruise altitude FL390, 30 mi north of bwg, ATC issued clearance to descend to what the captain and I thought was FL340. (Captain was flying.) he set FL340 in the altitude window and began the descent. I read back that we were out of FL390 to FL340. At FL350, ATC came back to us requesting our assigned altitude. We stated cleared to FL340. They said level at FL350 and stated FL370 was the clearance. No conflicts and ATC said 'no problem.' even now we both (captain and I) don't know what the actual assigned altitude was. He either set FL340 in by mistake and I read that back, or I read back FL390 to FL370 and didn't check his FL340 error in the setting. Nothing was distracting us. No heavy ATC (talking) load. Cockpit was quiet at the time.

Original NASA ASRS Text

Title: AN A320 FO RPTED THAT HE AND THE CAPT MISUNDERSTOOD ZME'S INSTRUCTION TO DSND.

Narrative: AT CRUISE ALT FL390, 30 MI N OF BWG, ATC ISSUED CLRNC TO DSND TO WHAT THE CAPT AND I THOUGHT WAS FL340. (CAPT WAS FLYING.) HE SET FL340 IN THE ALT WINDOW AND BEGAN THE DSCNT. I READ BACK THAT WE WERE OUT OF FL390 TO FL340. AT FL350, ATC CAME BACK TO US REQUESTING OUR ASSIGNED ALT. WE STATED CLRED TO FL340. THEY SAID LEVEL AT FL350 AND STATED FL370 WAS THE CLRNC. NO CONFLICTS AND ATC SAID 'NO PROB.' EVEN NOW WE BOTH (CAPT AND I) DON'T KNOW WHAT THE ACTUAL ASSIGNED ALT WAS. HE EITHER SET FL340 IN BY MISTAKE AND I READ THAT BACK, OR I READ BACK FL390 TO FL370 AND DIDN'T CHK HIS FL340 ERROR IN THE SETTING. NOTHING WAS DISTRACTING US. NO HVY ATC (TALKING) LOAD. COCKPIT WAS QUIET AT THE TIME.

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.