COPIED CLRNC MEANT FOR ANOTHER ACFT. DEP CTLR DID NOT MONITOR READBACK.

Narrative

FLT ABC; DEN TO INDIANAPOLIS; DEPARTED DEN AT XA34 AM (MDT); 4/88. THE CAPT WAS PERFORMING THE DUTIES OF PNF AND THE F/O WAS THE PF. THE WX WAS CLR; 70 MI VIS. A NORMAL TKOF WAS MADE TO THE N; RWY 35R AT DEN; TO 10000' AND AN 040 DEG HDG. DEN ATC ADVISED ABC TO MAINTAIN 10000'; EXPECT FL200 IN 20-30 MI; TFC AT 10 O'CLOCK. I ACKNOWLEDGED ALL THESE ADVISORIES. THERE WERE MANY OTHER ACFT ON FREQ; INCLUDING AN ACR FLT XBC. WE CONTINUED CRUISING AT 10000' NEBND. APPROX 25 MI NE OF DEN I RECEIVED A CLRNC FOR ABC TO CLB TO AND MAINTAIN FL200. I ACKNOWLEDGED THIS CLRNC AND THE F/O BEGAN TO CLB. APPROX 1 MIN LATER; DEN ATC SAID; 'ABC; MAINTAIN 11000'.' I SAID; 'ROGER; WE'RE AT 11300' NOW AND DSNDING IMMEDIATELY TO 11000'.' THE CTLR SAID WE HAD TFC AT 10 O'CLOCK. WE DID NOT SEE ANY TFC TO OUR LEFT; BUT WE DID SEE AN ACFT AT 2 O'CLOCK; AT LEAST 2000' HIGH AND AT LEAST 5 MI AWAY; HDG SBND. IT WAS TOO FAR AWAY TO IDENT THE TYPE. SHORTLY THEREAFTER ABC WAS AGAIN CLRED TO FL200; TURN E AND INTERCEPT THE ARWY. THE NEXT DAY I WAS INFORMED BY COMPANY THAT I WAS BEING CHARGED WITH AN ALT DEVIATION. IT IS ENTIRELY POSSIBLE THAT I COPIED AND READ BACK A CLRNC INTENDED FOR XBC OR THAT I WAS ISSUED ANOTHER EXPECT CLRNC OR ADVISORY AND MISCONSTRUED THIS AS A PROPER CLRNC. AT THAT TIME; HOWEVER; I WAS SURE THAT I HAD INDEED RECEIVED A PROPER CLRNC TO CLB TO FL200. DURING THIS WHOLE PERIOD OF TIME THERE WERE MANY INSTANCES OF THE FREQ BEING BLOCKED BY SEVERAL ACFT TRANSMITTING AT THE SAME TIME. THE CTLR HAD TO REPEAT INSTRUCTIONS SEVERAL TIMES TO OTHER ACFT. AT NO TIME DID THE CTLR EVEN INDICATE THAT I HAD ANYTHING IMPROPER. SUPPLEMENTAL INFO FROM ACN 85059: CTLR WAS VERY BUSY; TALKING FAST TO MANY ACFT. NO ACKNOWLEDGEMENTS OF ANY PLT READBACKS WERE ISSUED. CAPT AND I BOTH COMMENTED THAT THIS CTLR WAS TOO BUSY.