FLC RESPONDED TO A TKOF CLRNC THAT WAS GIVEN TO ANOTHER ACFT WITH SIMILAR CALL SIGN.

Narrative

AT THE TIME OF THE INCIDENT MY FLT (XXX) WAS IN POS AND HOLDING ON RWY 6L. MY CREW AND I RECEIVED WHAT WE BELIEVED WAS A CLRNC TO TAKE OFF AND MAINTAIN RWY HDG TO 3000 FT. MY FO READ BACK THE CLRNC AND I MADE THE TKOF. JUST AFTER LIFTOFF; MY FE CALLED OUT TFC AHEAD OF US THAT WAS MOVING R TO L AT OUR ALT. HE TOLD ME THAT HE THOUGHT THAT THE ACFT WAS MUCH CLOSER THAN NORMAL. NEITHER MY FO NOR I SAW THE OTHER ACFT. CONDITIONS AT THE ARPT WERE 500 FT OVCST 3 MI VISIBILITY SO WE WERE SOON IMC. AT THIS TIME WE RECEIVED 'ANOTHER' TKOF CLRNC FROM THE LCL CTLR; FOLLOWED BY A PAUSE AND THEN A HDG CHANGE TO 090 DEGS AND A HDOF TO DEP. THE CTLR DID NOT STATE THAT WE HAD TAKEN OFF WITHOUT A CLRNC. BOTH MEMBERS OF MY CREW STATED THAT THEY DID NOT HEAR THE SECOND CLRNC; BUT WERE PROBABLY DISTRACTED BY THE PRESENCE OF THE OTHER ACFT. MY ANALYSIS OF THE INCIDENT IS AS FOLLOWS: SHORTLY BEFORE RECEIVING OUR 'FIRST' TKOF CLRNC; THE LCL CTLR WAS TALKING TO ANOTHER ACFT WHICH I NOW KNOW WAS WAITING FOR TKOF ON RWY 36. (RWY 36 CROSSES THE EXTENDED CTRLINE OF RWY 6L ABOUT 300 FT BEYOND THE END OF THE RWY.) THE ACFT HAD A CALL SIGN SIMILAR TO OURS (SOMETHING LIKE XYX) AND WAS ACTUALLY GIVEN THE TKOF CLRNC THAT WE THOUGHT WAS OUR OWN. A SUBSEQUENT DISCUSSION WITH THE ATC CTLR INVOLVED REVEALED THAT HE HAD RECEIVED A 'STEPPED-ON' (BY US) READBACK TKOF CLRNC FROM THE GRUMMAN AND THEREFORE DID NOT HEAR OUR ACKNOWLEDGEMENT OF AN INVALID CLRNC. SHORTLY AFTER GIVING THE GRUMMAN CLRNC; HE ISSUED OUR CLRNC AND THEN NOTED THAT WE HAD ALREADY DEPARTED. UNFORTUNATELY; THIS IS A CLASSIC CASE OF HEARING WHAT YOU WANT TO HEAR AND THEN ACTING UPON THAT MISPERCEPTION. I HAVE AMENDED MY TKOF BRIEFING TO INCLUDE THE ADMONITION TO MY CREW THAT 'IF ANYONE HAS A QUESTION CONCERNING THE MEANING OF ANY CLRNC; WE WILL VERIFY THE CLRNC BEFORE ACTING UPON IT.' IN AN INCREASINGLY CONGESTED OPERATING ENVIRONMENT; THE USE OF 'POS AND HOLD' CLRNCS WHEN DEPS ARE BEING MADE IN IMC IS QUESTIONABLE AT BEST. GIVEN THE PREVALENCE OF SIMILAR SOUNDING CALL SIGNS; I BELIEVE THAT THE OPS ARE INHERENTLY DANGEROUS NO MATTER WHICH PERSON OR PARTY MAY BE DEEMED AT FAULT. SUPPLEMENTAL INFO FROM ACN 486869: FATIGUE WAS ALSO A FACTOR IN THIS SIT. WE TOOK OFF FROM OAKLAND; CA; WITH ABOUT 4 HRS SLEEP PRIOR. I WAS UNABLE TO SLEEP IN BEFORE THE FLT. IN THE 2 WKS PRECEDING THIS FLT; WE HAD BEEN FLIP-FLOPPING FROM DAY TO NIGHT FLYING.