Narrative:

While in position and hold runway 27R, believed tower said, 'fly heading 255 degrees cleared for takeoff.' read back takeoff clearance and commenced takeoff roll. A couple of thousand ft into takeoff roll tower said, 'fly heading 250 degrees at the MM, cleared for takeoff.' responded, 'we're rolling' and continued takeoff. Cause: previous aircraft to takeoff was a company aircraft who was given 255 degree heading. The tower transmission we believed to be our takeoff clearance was only about half as strong as the tower's normal xmissions. I heard, '255 degree heading...' followed by something indistinguishable. I interpreted it to be, 'cleared for takeoff,' but was probably, '...contact departure.' this misinterp could have been avoided if I had insisted on a loud and clear takeoff clearance or queried the tower prior to beginning takeoff roll. Other considerations: 1) in the past I have experienced and witnessed other aircraft contend with occasional poor reception while 'in position' on runway 27R. Aircraft have had to receive takeoff clearance from aircraft on the taxiway, relaying from the tower, or the aircraft in position has had to move forward to get out of a 'dead zone'. This may have contributed to our reception problem. 2) on our taxi out I noticed the tower controller seemed to have trouble keeping track of aircraft taking off. In one case the tower cleared an aircraft to takeoff when in fact it had already departed and was waiting for permission to contact departure. Although I do not have any other specific examples my general feeling during taxi out was that this controller was not keeping good track of the 'takeoff' aircraft under his control.

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Original NASA ASRS Text

Title: AN ACR B757 FLC STARTED THEIR TKOF ROLL BASED ON AN ATCT LCL CTLR'S XMISSION, BUT NOW REALIZE THAT THE XMISSION MAY HAVE BEEN FOR ANOTHER COMPANY ACFT.

Narrative: WHILE IN POS AND HOLD RWY 27R, BELIEVED TWR SAID, 'FLY HDG 255 DEGS CLRED FOR TKOF.' READ BACK TKOF CLRNC AND COMMENCED TKOF ROLL. A COUPLE OF THOUSAND FT INTO TKOF ROLL TWR SAID, 'FLY HDG 250 DEGS AT THE MM, CLRED FOR TKOF.' RESPONDED, 'WE'RE ROLLING' AND CONTINUED TKOF. CAUSE: PREVIOUS ACFT TO TKOF WAS A COMPANY ACFT WHO WAS GIVEN 255 DEG HDG. THE TWR XMISSION WE BELIEVED TO BE OUR TKOF CLRNC WAS ONLY ABOUT HALF AS STRONG AS THE TWR'S NORMAL XMISSIONS. I HEARD, '255 DEG HDG...' FOLLOWED BY SOMETHING INDISTINGUISHABLE. I INTERPRETED IT TO BE, 'CLRED FOR TKOF,' BUT WAS PROBABLY, '...CONTACT DEP.' THIS MISINTERP COULD HAVE BEEN AVOIDED IF I HAD INSISTED ON A LOUD AND CLR TKOF CLRNC OR QUERIED THE TWR PRIOR TO BEGINNING TKOF ROLL. OTHER CONSIDERATIONS: 1) IN THE PAST I HAVE EXPERIENCED AND WITNESSED OTHER ACFT CONTEND WITH OCCASIONAL POOR RECEPTION WHILE 'IN POS' ON RWY 27R. ACFT HAVE HAD TO RECEIVE TKOF CLRNC FROM ACFT ON THE TXWY, RELAYING FROM THE TWR, OR THE ACFT IN POS HAS HAD TO MOVE FORWARD TO GET OUT OF A 'DEAD ZONE'. THIS MAY HAVE CONTRIBUTED TO OUR RECEPTION PROB. 2) ON OUR TAXI OUT I NOTICED THE TWR CTLR SEEMED TO HAVE TROUBLE KEEPING TRACK OF ACFT TAKING OFF. IN ONE CASE THE TWR CLRED AN ACFT TO TKOF WHEN IN FACT IT HAD ALREADY DEPARTED AND WAS WAITING FOR PERMISSION TO CONTACT DEP. ALTHOUGH I DO NOT HAVE ANY OTHER SPECIFIC EXAMPLES MY GENERAL FEELING DURING TAXI OUT WAS THAT THIS CTLR WAS NOT KEEPING GOOD TRACK OF THE 'TKOF' ACFT UNDER HIS CTL.

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.