A flight crew accepted a clearance and descended below assigned altitude.

Attributes
ACN	440638
Time
Date	199906
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : fak.vortac
State Reference	VA
Altitude	msl bound lower : 13000 msl bound upper : 25000
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zdc.artcc
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	other vortac
Flight Phase	cruise : enroute altitude change
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : atp
Experience	flight time last 90 days : 120 flight time total : 5300 flight time type : 120
ASRS Report	440638
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

On descent into the washington, dc, area, 25 mi south of fak VOR, ZDC issued a clearance from FL250 to 13000 ft. I set 13000 ft in the altitude alerter and the captain and I used our airline's established altitude awareness procedures. We began our descent. Descending through FL210, ATC asked our altitude. We replied out of FL210 for 13000 ft. ATC then gave us a clearance to 16000 ft and advised us we were only cleared to FL220, not 13000 ft. Later we were told by ZDC that there was a flight XXXX also on the frequency. We were flight YYYY. In our opinion these were not very similar call signs since ATC was using 'call sign XXXX' as opposed to 'call sign YYYY.' we read back the 13000 ft clearance and was never given a negative response.

Original NASA ASRS Text

Title: A FLC ACCEPTED A CLRNC AND DSNDED BELOW ASSIGNED ALT.

Narrative: ON DSCNT INTO THE WASHINGTON, DC, AREA, 25 MI S OF FAK VOR, ZDC ISSUED A CLRNC FROM FL250 TO 13000 FT. I SET 13000 FT IN THE ALT ALERTER AND THE CAPT AND I USED OUR AIRLINE'S ESTABLISHED ALT AWARENESS PROCS. WE BEGAN OUR DSCNT. DSNDING THROUGH FL210, ATC ASKED OUR ALT. WE REPLIED OUT OF FL210 FOR 13000 FT. ATC THEN GAVE US A CLRNC TO 16000 FT AND ADVISED US WE WERE ONLY CLRED TO FL220, NOT 13000 FT. LATER WE WERE TOLD BY ZDC THAT THERE WAS A FLT XXXX ALSO ON THE FREQ. WE WERE FLT YYYY. IN OUR OPINION THESE WERE NOT VERY SIMILAR CALL SIGNS SINCE ATC WAS USING 'CALL SIGN XXXX' AS OPPOSED TO 'CALL SIGN YYYY.' WE READ BACK THE 13000 FT CLRNC AND WAS NEVER GIVEN A NEGATIVE RESPONSE.

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.