Narrative:

2 aircraft with similar call signs were on the frequency--air carrier a xyz and air carrier B xyz. I had just relieved another controller and was briefed that the 2 flts were aware of each other already. Air carrier a xyz was at FL370 and air carrier B xyz was also at FL370. I issued a descent clearance to air carrier Z xyz to FL350, which he acknowledged. Several seconds later, after obtaining permission from another sector, I cleared air carrier Z xyz to FL330. The readback was totally smeared by someone else (probably B xyz), so I went back to air carrier Z xyz and asked him to confirm he received his clearance to FL330. He said he had. In the meantime air carrier B xyz had been handed off to ZBW. I switched air carrier B xyz to ZBW frequency. At this time he was still level at FL370. Several seconds after switching air carrier B xyz, I noticed his mode C read 363. After a second or two of wondering where he was going, I realized he probably took the clearance for air carrier Z xyz. The smeared readback was no doubt air carrier B xyz reading back the clearance at the same time as air carrier Z xyz. I called the ZBW sector and alerted them to stop him at FL330, and that he had apparently taken someone else's clearance. There were no other aircraft beneath him and no sector boundaries were violated. No conflicts occurred. I failed to repeat the company name when I issued the clearance to air carrier Z xyz; however, both flts had been told about the other. The smeared readbacks really caused the problem, because neither one was understandable, so I had no way of knowing air carrier B xyz took the clearance. One obvious factor here is the similarity of call signs. We have seen a great increase in the # of call signs that are similar sounding. Another factor is that the single pilot in the cockpit first officer air carrier B xyz apparently wasnot listening as closely as he could have. The copilot of air carrier B xyz needed to be more attentive. Supplemental information from acn 144754: I left the cockpit to use the blue room and left the copilot in charge. ZOB gave air carrier Z xyz a descent to FL330. My copilot thought the clearance was meant for us. He responded and descended.

Related reports:

Google
 

Original NASA ASRS Text

Title: ACR ACCEPTED DESCENT INTENDED FOR ANOTHER ACFT WITH SIMILAR SOUNDING CALL SIGN.

Narrative: 2 ACFT WITH SIMILAR CALL SIGNS WERE ON THE FREQ--ACR A XYZ AND ACR B XYZ. I HAD JUST RELIEVED ANOTHER CTLR AND WAS BRIEFED THAT THE 2 FLTS WERE AWARE OF EACH OTHER ALREADY. ACR A XYZ WAS AT FL370 AND ACR B XYZ WAS ALSO AT FL370. I ISSUED A DSNT CLRNC TO ACR Z XYZ TO FL350, WHICH HE ACKNOWLEDGED. SEVERAL SECS LATER, AFTER OBTAINING PERMISSION FROM ANOTHER SECTOR, I CLRED ACR Z XYZ TO FL330. THE READBACK WAS TOTALLY SMEARED BY SOMEONE ELSE (PROBABLY B XYZ), SO I WENT BACK TO ACR Z XYZ AND ASKED HIM TO CONFIRM HE RECEIVED HIS CLRNC TO FL330. HE SAID HE HAD. IN THE MEANTIME ACR B XYZ HAD BEEN HANDED OFF TO ZBW. I SWITCHED ACR B XYZ TO ZBW FREQ. AT THIS TIME HE WAS STILL LEVEL AT FL370. SEVERAL SECS AFTER SWITCHING ACR B XYZ, I NOTICED HIS MODE C READ 363. AFTER A SECOND OR TWO OF WONDERING WHERE HE WAS GOING, I REALIZED HE PROBABLY TOOK THE CLRNC FOR ACR Z XYZ. THE SMEARED READBACK WAS NO DOUBT ACR B XYZ READING BACK THE CLRNC AT THE SAME TIME AS ACR Z XYZ. I CALLED THE ZBW SECTOR AND ALERTED THEM TO STOP HIM AT FL330, AND THAT HE HAD APPARENTLY TAKEN SOMEONE ELSE'S CLRNC. THERE WERE NO OTHER ACFT BENEATH HIM AND NO SECTOR BOUNDARIES WERE VIOLATED. NO CONFLICTS OCCURRED. I FAILED TO REPEAT THE COMPANY NAME WHEN I ISSUED THE CLRNC TO ACR Z XYZ; HOWEVER, BOTH FLTS HAD BEEN TOLD ABOUT THE OTHER. THE SMEARED READBACKS REALLY CAUSED THE PROB, BECAUSE NEITHER ONE WAS UNDERSTANDABLE, SO I HAD NO WAY OF KNOWING ACR B XYZ TOOK THE CLRNC. ONE OBVIOUS FACTOR HERE IS THE SIMILARITY OF CALL SIGNS. WE HAVE SEEN A GREAT INCREASE IN THE # OF CALL SIGNS THAT ARE SIMILAR SOUNDING. ANOTHER FACTOR IS THAT THE SINGLE PLT IN THE COCKPIT FO ACR B XYZ APPARENTLY WASNOT LISTENING AS CLOSELY AS HE COULD HAVE. THE COPLT OF ACR B XYZ NEEDED TO BE MORE ATTENTIVE. SUPPLEMENTAL INFO FROM ACN 144754: I LEFT THE COCKPIT TO USE THE BLUE ROOM AND LEFT THE COPLT IN CHARGE. ZOB GAVE ACR Z XYZ A DSNT TO FL330. MY COPLT THOUGHT THE CLRNC WAS MEANT FOR US. HE RESPONDED AND DSNDED.

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.