MD88 FLC; WITH FO FLYING; TOOK ANOTHER ACFT'S DSCNT CLRNC. CTLR CLBED ACFT BACK TO ASSIGNED ALT. PIC; EXPECTING DSCNT CLRNC; AND DISCUSSING PROPOSED ATC CTL INSTRUCTIONS WITH FLC; ASSUMED SIMILAR CALL SIGN WAS THEIRS AND INSTRUCTED THE FO TO BEGIN DSCNT.

Narrative

THIS FLT SEGMENT WAS THE THIRD LEG OF AN OPERATING EXPERIENCE ROTATION FOR A FO THAT HAD RECENTLY COMPLETED SIMULATOR TRAINING. IT WAS HIS FIRST DAY IN THE ACFT ITSELF. THE FLT WAS AT FL230 AND CLRED TO DSND TO FL190. AT FL190 THE CTLR INFORMED US THAT WE COULD EXPECT CLRNC TO 11000 FT IN A FEW MINS; WE HAD TFC IN FRONT OF US. THIS SAME LEG HAD BEEN FLOWN EARLIER IN THE DAY AND THE CREW WAS DISCUSSING SOME OF THE ISSUES PREVIOUSLY ENCOUNTERED. ON THAT LEG WE HAD BEEN HELD AT ALT AND A REASONABLY QUICK DSCNT FOLLOWED BY A RWY ASSIGNMENT OPPOSITE THE ATIS INFO WAS BEING REVIEWED. DURING THIS DISCUSSION CTR ISSUED A CLRNC I UNDERSTOOD AS OUR CALL SIGN TO DSND FROM FL190 TO 11000 FT. THE CALL SIGN WAS FAMILIAR; THE CURRENT (FL190) ALT AND LOWER (11000 FT) ALT MATCHED BOTH OUR CURRENT AND ANTICIPATED ALT ASSIGNMENT. I ACKNOWLEDGED THE CLRNC AND DSCNT WAS INITIATED. SHORTLY AFTER BEGINNING DSCNT THE TCASII ALERTED US TO TFC AND IT WAS VISUALLY SIGHTED. CONCURRENT WITH THAT WE OVERHEARD A SIMILAR CALL SIGN TO OUR OWN RPT TO CTR THEY BELIEVED SOMEONE HAD TAKEN THEIR CLRNC. THE CTLR THEN ASKED US TO VERIFY OUR ALT AS FL190 AND OUR RESPONSE WAS NEGATIVE; WE WERE DSNDING TO 11000 FT. CTR IMMEDIATELY INFORMED US WE WERE NOT CLRED TO DSND AND TO MAINTAIN FL190. AT THIS POINT WE TERMINATED THE DSCNT AND CLBED BACK TO FL190 FROM APPROX 17800 FT. I HAD NOT QUESTIONED THE CLRNC AS IT WAS ISSUED. WITH REGARD TO HUMAN FACTORS; THE POSSIBILITY OF MISTAKING A CALL SIGN IS ALWAYS VERY REAL. IN THIS PARTICULAR CASE; SEVERAL OTHER ASPECTS ARE PERTINENT. THE PROBABILITY OF A COMBINATION OF SIMILAR CALL SIGNS; IDENTICAL CURRENT ALTS; IDENTICAL ANTICIPATED ALTS; COMS ON THE SAME FREQ; IN THE SAME SECTOR; AT THE SAME TIME IS REASONABLY REMOTE. COCKPIT DISCUSSIONS CAN DIMINISH THE ABILITY TO HEAR CLEARLY AND IN THIS INSTANCE I FEEL IT HAD A DEFINITE AND SIGNIFICANT IMPACT. SUPPLEMENTAL INFO FROM ACN 395036: THERE WAS NO IMMINENT CONFLICT. WE WERE EBOUND; THE TFC WAS WBOUND OFF OUR R SIDE AND CLBING.