Narrative:

I was preparing for takeoff at the hold line of runway 23, holding behind a passenger plane. After his departure, I told tower that I was ready for takeoff. He instructed me to hold short. I did. Shortly thereafter, I heard what I thought was my number (bxxx Q) cleared for takeoff. I did. After crossing a crossing runway, I saw an airplane pass underneath me going from my right to left. I thought at the time that it was sort of unusual to be that close on takeoff. Then the tower controller asked me if I took off, when I was not cleared to do so. I said that I thought I was. He was a bit upset to say the least. At that point, I realized that I was not cleared for takeoff, but an airplane that had a similar call sign. The other airplane had the letter Q as a last letter. When I heard Q in the call sign I took off, not realizing the difference. After I arrived at my first destination I did call the tower, and did talk to a supervisor, I explained the situation to him and apologized for the error. He said the conflict was not as serious as it could have been, and that they were not going to pursue it any further. I simply did not understand the controller when he said cleared for takeoff. I misunderstood the numbers because of their similarities.

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Original NASA ASRS Text

Title: SMA PLT HEARS TKOF CLRNC FOR SIMILAR CALL SIGN AND DEPARTS OVER TOP OF ACFT TAKING OFF ON CROSS RWY.

Narrative: I WAS PREPARING FOR TKOF AT THE HOLD LINE OF RWY 23, HOLDING BEHIND A PAX PLANE. AFTER HIS DEP, I TOLD TWR THAT I WAS READY FOR TKOF. HE INSTRUCTED ME TO HOLD SHORT. I DID. SHORTLY THEREAFTER, I HEARD WHAT I THOUGHT WAS MY NUMBER (BXXX Q) CLRED FOR TKOF. I DID. AFTER XING A XING RWY, I SAW AN AIRPLANE PASS UNDERNEATH ME GOING FROM MY RIGHT TO LEFT. I THOUGHT AT THE TIME THAT IT WAS SORT OF UNUSUAL TO BE THAT CLOSE ON TKOF. THEN THE TWR CTLR ASKED ME IF I TOOK OFF, WHEN I WAS NOT CLRED TO DO SO. I SAID THAT I THOUGHT I WAS. HE WAS A BIT UPSET TO SAY THE LEAST. AT THAT POINT, I REALIZED THAT I WAS NOT CLRED FOR TKOF, BUT AN AIRPLANE THAT HAD A SIMILAR CALL SIGN. THE OTHER AIRPLANE HAD THE LETTER Q AS A LAST LETTER. WHEN I HEARD Q IN THE CALL SIGN I TOOK OFF, NOT REALIZING THE DIFFERENCE. AFTER I ARRIVED AT MY FIRST DEST I DID CALL THE TWR, AND DID TALK TO A SUPVR, I EXPLAINED THE SITUATION TO HIM AND APOLOGIZED FOR THE ERROR. HE SAID THE CONFLICT WAS NOT AS SERIOUS AS IT COULD HAVE BEEN, AND THAT THEY WERE NOT GOING TO PURSUE IT ANY FURTHER. I SIMPLY DID NOT UNDERSTAND THE CTLR WHEN HE SAID CLRED FOR TKOF. I MISUNDERSTOOD THE NUMBERS BECAUSE OF THEIR SIMILARITIES.

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.