Narrative:

While flying at 25;000 ft; I received what I thought was 'air carrier wxyz climb and maintain 3-4-0'. I read back the instruction and the captain; who also heard and understood the transmission to be as above; began the climb. Reaching FL270; ATC contacted us to confirm our altitude. I told ATC we were climbing as instructed to FL340. At that point ATC advised that we were not the aircraft assigned the climb and to remain at our current altitude. We leveled off at our new altitude at FL270 (the one ATC directed us to upon the discovery of the deviation) and were immediately issued a new climb to FL340. Upon arrival; the captain contacted ATC to inquire whether or not they needed more information. ATC advised the captain that no separation issue occurred; no further paperwork would be needed and the investigation was closed on their end. Cause; several factors led to this mistake. The first was a similar sounding call sign. The aircraft that received the climb clearance was another carrier wxyx. Three of the four digits in this number are the same as wxyz and the last and only digit different; are both similar sounding one syllable numbers. At cruise power; with wind noise; cooling fans; and 'scratchy' headsets; 'five' and 'nine' sound the same. Other possible factors were fatigue on my part; I was not at 100% though I felt ok; if I were in a better condition; I may have been able to discern the difference. Because of a downgrade and the pay change that accompanies it; I have been flying maximum hours to offset the costs. This event happened on the last of a 5 day stretch of flying. Another factor is anticipation of a further clearance. We were in the level off for a climb to a further sector of ATC. I was expecting a climb to our filed cruise altitude shortly and perhaps my expectation of a clearance led me to believe we had been cleared. It is hard to give a suggestion for this mistake remedy. Perhaps the only one I can offer is a headset upgrade in our jet aircraft. Both pilots in the above event were on headsets at the time of the event. Although new to the aircraft I have found that most pilots either won't use the provided headset; opting for their own higher quality headset; or choosing to go to the speakers so the can hear better. ATC using 'niner' versus 'nine' could help; but I think a different set of flight numbers that are unique to our airline would solve a lot of these problems. Our w-x and w-y numbers are repeated a lot. Flight levels; runways; and other ATC call signs all use this combination.

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Original NASA ASRS Text

Title: An air carrier reporter initiated a climb issued to another same company aircraft with a similar sounding four digit flight number. The reporter suggests improved headsets; strict adherence to phraseology standards and company flight numbering changes as possible solutions to the problem.

Narrative: While flying at 25;000 FT; I received what I thought was 'ACR WXYZ climb and maintain 3-4-0'. I read back the instruction and the Captain; who also heard and understood the transmission to be as above; began the climb. Reaching FL270; ATC contacted us to confirm our altitude. I told ATC we were climbing as instructed to FL340. At that point ATC advised that we were not the aircraft assigned the climb and to remain at our current altitude. We leveled off at our new altitude at FL270 (The one ATC directed us to upon the discovery of the deviation) and were immediately issued a new climb to FL340. Upon arrival; the Captain contacted ATC to inquire whether or not they needed more information. ATC advised the Captain that no separation issue occurred; no further paperwork would be needed and the investigation was closed on their end. Cause; several factors led to this mistake. The first was a similar sounding call sign. The aircraft that received the climb clearance was another carrier WXYX. Three of the four digits in this number are the same as WXYZ and the last and only digit different; are both similar sounding one syllable numbers. At cruise power; with wind noise; cooling fans; and 'scratchy' headsets; 'Five' and 'Nine' sound the same. Other possible factors were fatigue on my part; I was not at 100% though I felt OK; if I were in a better condition; I may have been able to discern the difference. Because of a downgrade and the pay change that accompanies it; I have been flying maximum hours to offset the costs. This event happened on the last of a 5 day stretch of flying. Another factor is anticipation of a further clearance. We were in the level off for a climb to a further sector of ATC. I was expecting a climb to our filed cruise altitude shortly and perhaps my expectation of a clearance led me to believe we had been cleared. It is hard to give a suggestion for this mistake remedy. Perhaps the only one I can offer is a headset upgrade in our jet aircraft. Both pilots in the above event were on headsets at the time of the event. Although new to the aircraft I have found that most pilots either won't use the provided headset; opting for their own higher quality headset; or choosing to go to the speakers so the can hear better. ATC using 'niner' versus 'nine' could help; but I think a different set of flight numbers that are unique to our airline would solve a lot of these problems. Our W-X and W-Y numbers are repeated a lot. Flight levels; runways; and other ATC call signs all use this combination.

Data retrieved from NASA's ASRS site as of April 2012 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.