Narrative:

We were directed into position and hold on runway 1L at sfo. Several seconds later we were directed to exit 1L at taxiway microphone and hold short of runway 1R as an air carrier Y flight had to depart first to meet a flow time. The air carrier Y aircraft was directed into position and hold on 1L, and we were directed into position and hold on 1R. Until this point I was making all the radio calls and the captain was taxiing the aircraft per our company's operations specifications. While in position on 1R I heard the tower clear someone for takeoff, however, I wasn't listening closely enough and I made no response to the tower. At this time the captain made a hand signal to me to move the speed levers to the high position as he transmitted 'cleared for takeoff, air carrier X'. The tower did not respond, and a normal takeoff was made. Climbing through about 200 ft the tower said 'air carrier X, cleared for takeoff', I responded by saying 'it's a little late for that now'. Tower's next response was 'that was for air carrier Y' which was a statement neither the captain nor I understood. Upon initial contact with bay departure we were advised that we had taken off without a clearance and given a telephone # to call. The captain did call the # after the flight but at this time I have no knowledge of what was said. Obviously crew coordination fell apart prior to the event, fortunately, no one was hurt. I should have insisted on verification and the takeoff call. That would not have been a problem as communication and rapport in the cockpit had been excellent and the captain had been receptive of my comments. This is what can happen if 1 crew member lets down his guard, even for an instant, at the wrong time.

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Original NASA ASRS Text

Title: UNAUTHORIZED TKOF AS PIC RESPONDS TO CLRNC TKOF REPLY TO WRONG CALL SIGN.

Narrative: WE WERE DIRECTED INTO POS AND HOLD ON RWY 1L AT SFO. SEVERAL SECONDS LATER WE WERE DIRECTED TO EXIT 1L AT TAXIWAY MIKE AND HOLD SHORT OF RWY 1R AS AN ACR Y FLT HAD TO DEPART FIRST TO MEET A FLOW TIME. THE ACR Y ACFT WAS DIRECTED INTO POS AND HOLD ON 1L, AND WE WERE DIRECTED INTO POS AND HOLD ON 1R. UNTIL THIS POINT I WAS MAKING ALL THE RADIO CALLS AND THE CAPT WAS TAXIING THE ACFT PER OUR COMPANY'S OPS SPECS. WHILE IN POS ON 1R I HEARD THE TWR CLR SOMEONE FOR TKOF, HOWEVER, I WASN'T LISTENING CLOSELY ENOUGH AND I MADE NO RESPONSE TO THE TWR. AT THIS TIME THE CAPT MADE A HAND SIGNAL TO ME TO MOVE THE SPD LEVERS TO THE HIGH POS AS HE TRANSMITTED 'CLRED FOR TKOF, ACR X'. THE TWR DID NOT RESPOND, AND A NORMAL TKOF WAS MADE. CLBING THROUGH ABOUT 200 FT THE TWR SAID 'ACR X, CLRED FOR TKOF', I RESPONDED BY SAYING 'IT'S A LITTLE LATE FOR THAT NOW'. TWR'S NEXT RESPONSE WAS 'THAT WAS FOR ACR Y' WHICH WAS A STATEMENT NEITHER THE CAPT NOR I UNDERSTOOD. UPON INITIAL CONTACT WITH BAY DEP WE WERE ADVISED THAT WE HAD TAKEN OFF WITHOUT A CLRNC AND GIVEN A TELEPHONE # TO CALL. THE CAPT DID CALL THE # AFTER THE FLT BUT AT THIS TIME I HAVE NO KNOWLEDGE OF WHAT WAS SAID. OBVIOUSLY CREW COORD FELL APART PRIOR TO THE EVENT, FORTUNATELY, NO ONE WAS HURT. I SHOULD HAVE INSISTED ON VERIFICATION AND THE TKOF CALL. THAT WOULD NOT HAVE BEEN A PROBLEM AS COM AND RAPPORT IN THE COCKPIT HAD BEEN EXCELLENT AND THE CAPT HAD BEEN RECEPTIVE OF MY COMMENTS. THIS IS WHAT CAN HAPPEN IF 1 CREW MEMBER LETS DOWN HIS GUARD, EVEN FOR AN INSTANT, AT THE WRONG TIME.

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.