Narrative:

After engine start, cleared to taxi to runway 25. Apparently no other aircraft in pattern or moving on airport as we taxied out. Approaching end of runway we were 'cleared into position and hold.' after sitting on runway with brakes parked for approximately 30 seconds and takeoff checklist completed, tower gave following amended clearance: 'after takeoff, left turn heading 090 degrees, direct pgs, climb unrestricted and maintain 15000'--(cleared for takeoff)'. Copilot read back clearance verbatim, including phrase: 'cleared for takeoff.' takeoff roll was commenced and at about 60 KTS tower called saying, 'you were not cleared for takeoff, I have helicopter traffic off end of runway.' the takeoff was aborted, we turned off runway at first turnoff and were then cleared back to departure end of runway. Tower apologized for 'misunderstanding.' during taxi back and after completing checklists, we both confirmed that we had been 'cleared for takeoff.' the more we thought, we became less sure--but were absolutely sure that during copilot's readback he did say, 'cleared for takeoff,' which the tower operator did not hear nor assimilate. Is this a case of hearing what you expect to hear?? At this point we are not at all positive. At xa am with no other aircraft visible or on the tower frequency, perhaps a bit of routineness slipped in--not being 100% sure of clearance and not receiving acknowledgement of clearance readback led to this situation. Make absolutely sure your readbacks are acknowledged!

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Original NASA ASRS Text

Title: ACR MLG STARTED TKOF WITHOUT CLRNC.

Narrative: AFTER ENG START, CLRED TO TAXI TO RWY 25. APPARENTLY NO OTHER ACFT IN PATTERN OR MOVING ON ARPT AS WE TAXIED OUT. APCHING END OF RWY WE WERE 'CLRED INTO POS AND HOLD.' AFTER SITTING ON RWY WITH BRAKES PARKED FOR APPROX 30 SECS AND TKOF CHKLIST COMPLETED, TWR GAVE FOLLOWING AMENDED CLRNC: 'AFTER TKOF, LEFT TURN HDG 090 DEGS, DIRECT PGS, CLB UNRESTRICTED AND MAINTAIN 15000'--(CLRED FOR TKOF)'. COPLT READ BACK CLRNC VERBATIM, INCLUDING PHRASE: 'CLRED FOR TKOF.' TKOF ROLL WAS COMMENCED AND AT ABOUT 60 KTS TWR CALLED SAYING, 'YOU WERE NOT CLRED FOR TKOF, I HAVE HELI TFC OFF END OF RWY.' THE TKOF WAS ABORTED, WE TURNED OFF RWY AT FIRST TURNOFF AND WERE THEN CLRED BACK TO DEP END OF RWY. TWR APOLOGIZED FOR 'MISUNDERSTANDING.' DURING TAXI BACK AND AFTER COMPLETING CHKLISTS, WE BOTH CONFIRMED THAT WE HAD BEEN 'CLRED FOR TKOF.' THE MORE WE THOUGHT, WE BECAME LESS SURE--BUT WERE ABSOLUTELY SURE THAT DURING COPLT'S READBACK HE DID SAY, 'CLRED FOR TKOF,' WHICH THE TWR OPERATOR DID NOT HEAR NOR ASSIMILATE. IS THIS A CASE OF HEARING WHAT YOU EXPECT TO HEAR?? AT THIS POINT WE ARE NOT AT ALL POSITIVE. AT XA AM WITH NO OTHER ACFT VISIBLE OR ON THE TWR FREQ, PERHAPS A BIT OF ROUTINENESS SLIPPED IN--NOT BEING 100% SURE OF CLRNC AND NOT RECEIVING ACKNOWLEDGEMENT OF CLRNC READBACK LED TO THIS SITUATION. MAKE ABSOLUTELY SURE YOUR READBACKS ARE ACKNOWLEDGED!

Data retrieved from NASA's ASRS site as of August 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.