Narrative:

At FL280 northeast lit (044R/80 DME) ZME said, 'air carrier 123 descend and maintain FL270 for crossing traffic'. Copilot read back clearance 'leaving FL280 for FL270 air carrier 123'. Some static on frequency (VHF) and sounded like response from center '...123, roger'. Began descent on FMS to FL270, at 27600', center asked our altitude and said we were assigned FL280. Disconnected autoplt and climbed back to FL280 immediately. When workload permitted, copilot ask controller if he had heard our readback. Controller stated that we were probably 'stepped on' by another aircraft on another frequency. That we had acknowledged a clearance for another aircraft with a similar callsign ending with a '23' and that there was no conflict created by our excursion from FL280. The controller mentioned that he was working UHF as well as VHF converging traffic and had a high workload at the time. He also mentioned that our readback was covered up by the transmission from the aircraft with the '23' callsign (on another frequency?). Contributing factors. High controller workload. Numerous aircraft (on different frequencys, VHF and UHF) converging in same airspace sector. Our misinterp of the controller's readback acknowledgement. The good. Controller was alert enough to catch our altitude excursion before it truely became excess, as he said 'you are assigned FL280' almost soon as we left FL280. Supplemental information from acn 143579.the fact that the controller evidently was aware of a 'blocked' transmission but did not ask anyone for clarification also contributed to the incident. Until controllers workload allows close monitoring of readbacks (ie more controllers per sector) this type of incident will continue to occur.

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Original NASA ASRS Text

Title: ALT EXCURSION.

Narrative: AT FL280 NE LIT (044R/80 DME) ZME SAID, 'ACR 123 DSND AND MAINTAIN FL270 FOR XING TFC'. COPLT READ BACK CLRNC 'LEAVING FL280 FOR FL270 ACR 123'. SOME STATIC ON FREQ (VHF) AND SOUNDED LIKE RESPONSE FROM CTR '...123, ROGER'. BEGAN DSNT ON FMS TO FL270, AT 27600', CTR ASKED OUR ALT AND SAID WE WERE ASSIGNED FL280. DISCONNECTED AUTOPLT AND CLBED BACK TO FL280 IMMEDIATELY. WHEN WORKLOAD PERMITTED, COPLT ASK CTLR IF HE HAD HEARD OUR READBACK. CTLR STATED THAT WE WERE PROBABLY 'STEPPED ON' BY ANOTHER ACFT ON ANOTHER FREQ. THAT WE HAD ACKNOWLEDGED A CLRNC FOR ANOTHER ACFT WITH A SIMILAR CALLSIGN ENDING WITH A '23' AND THAT THERE WAS NO CONFLICT CREATED BY OUR EXCURSION FROM FL280. THE CTLR MENTIONED THAT HE WAS WORKING UHF AS WELL AS VHF CONVERGING TFC AND HAD A HIGH WORKLOAD AT THE TIME. HE ALSO MENTIONED THAT OUR READBACK WAS COVERED UP BY THE XMISSION FROM THE ACFT WITH THE '23' CALLSIGN (ON ANOTHER FREQ?). CONTRIBUTING FACTORS. HIGH CTLR WORKLOAD. NUMEROUS ACFT (ON DIFFERENT FREQS, VHF AND UHF) CONVERGING IN SAME AIRSPACE SECTOR. OUR MISINTERP OF THE CTLR'S READBACK ACKNOWLEDGEMENT. THE GOOD. CTLR WAS ALERT ENOUGH TO CATCH OUR ALT EXCURSION BEFORE IT TRUELY BECAME EXCESS, AS HE SAID 'YOU ARE ASSIGNED FL280' ALMOST SOON AS WE LEFT FL280. SUPPLEMENTAL INFO FROM ACN 143579.THE FACT THAT THE CTLR EVIDENTLY WAS AWARE OF A 'BLOCKED' XMISSION BUT DID NOT ASK ANYONE FOR CLARIFICATION ALSO CONTRIBUTED TO THE INCIDENT. UNTIL CTLRS WORKLOAD ALLOWS CLOSE MONITORING OF READBACKS (IE MORE CTLRS PER SECTOR) THIS TYPE OF INCIDENT WILL CONTINUE TO OCCUR.

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.