Narrative:

Flight abc, den to indianapolis, departed den at XA34 am (mdt), 4/88. The captain was performing the duties of PNF and the first officer was the PF. The WX was clear, 70 mi visibility. A normal takeoff was made to the north, runway 35R at den, to 10000' and an 040 degree heading. Den ATC advised abc to maintain 10000', expect FL200 in 20-30 mi, traffic at 10 O'clock. I acknowledged all these advisories. There were many other aircraft on frequency, including an air carrier flight xbc. We continued cruising at 10000' nebnd. Approximately 25 mi northeast of den I received a clearance for abc to climb to and maintain FL200. I acknowledged this clearance and the first officer began to climb. Approximately 1 min later, den ATC said, 'abc, maintain 11000'.' I said, 'roger, we're at 11300' now and descending immediately to 11000'.' the controller said we had traffic at 10 O'clock. We did not see any traffic to our left, but we did see an aircraft at 2 O'clock, at least 2000' high and at least 5 mi away, heading southbound. It was too far away to identify the type. Shortly thereafter abc was again cleared to FL200, turn east and intercept the arwy. The next day I was informed by company that I was being charged with an altitude deviation. It is entirely possible that I copied and read back a clearance intended for xbc or that I was issued another expect clearance or advisory and misconstrued this as a proper clearance. At that time, however, I was sure that I had indeed received a proper clearance to climb to FL200. During this whole period of time there were many instances of the frequency being blocked by several aircraft transmitting at the same time. The controller had to repeat instructions several times to other aircraft. At no time did the controller even indicate that I had anything improper. Supplemental information from acn 85059: controller was very busy, talking fast to many aircraft. No acknowledgements of any pilot readbacks were issued. Captain and I both commented that this controller was too busy.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of August 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.