B757 crew began takeoff without being certain they had takeoff clearance.

Attributes
ACN	421071
Time
Date	199811
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : sdf
State Reference	KY
Altitude	agl bound lower : 0 agl bound upper : 0
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B757 Undifferentiated or Other Model
Navigation In Use	Other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 100 flight time total : 18000 flight time type : 300
ASRS Report	421071
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 150 flight time total : 9435 flight time type : 500
ASRS Report	421490
Events
Anomaly	non adherence : clearance non adherence other other anomaly other
Independent Detector	other controllera other flight crewa
Consequence	Other
Supplementary
Air Traffic Incident	other

Narrative:

I think we started a takeoff without clearance. We were cleared, I thought for takeoff. There was a problem. Tower said our readback was very weak. We tried 2 other mikes and proceeded with takeoff. On takeoff roll, our flight number was cleared for takeoff. I thought tower was talking to next aircraft in line. After takeoff, we realized we probably started takeoff without clearance. To prevent this, we must continue to be as careful as possible and employ CRM skill as much as possible. Supplemental information from acn 421490: we were cleared to position and hold on runway 17R. Tower reported our radio transmission as weak. After trying 2 other mikes and getting radio checks we thought we heard our takeoff clearance. We began takeoff roll and at about 30 KIAS tower said cleared for takeoff. I am not sure if this was a second takeoff clearance or 'the takeoff clearance.'

Original NASA ASRS Text

Title: B757 CREW BEGAN TKOF WITHOUT BEING CERTAIN THEY HAD TKOF CLRNC.

Narrative: I THINK WE STARTED A TKOF WITHOUT CLRNC. WE WERE CLRED, I THOUGHT FOR TKOF. THERE WAS A PROB. TWR SAID OUR READBACK WAS VERY WEAK. WE TRIED 2 OTHER MIKES AND PROCEEDED WITH TKOF. ON TKOF ROLL, OUR FLT NUMBER WAS CLRED FOR TKOF. I THOUGHT TWR WAS TALKING TO NEXT ACFT IN LINE. AFTER TKOF, WE REALIZED WE PROBABLY STARTED TKOF WITHOUT CLRNC. TO PREVENT THIS, WE MUST CONTINUE TO BE AS CAREFUL AS POSSIBLE AND EMPLOY CRM SKILL AS MUCH AS POSSIBLE. SUPPLEMENTAL INFO FROM ACN 421490: WE WERE CLRED TO POS AND HOLD ON RWY 17R. TWR RPTED OUR RADIO XMISSION AS WEAK. AFTER TRYING 2 OTHER MIKES AND GETTING RADIO CHKS WE THOUGHT WE HEARD OUR TKOF CLRNC. WE BEGAN TKOF ROLL AND AT ABOUT 30 KIAS TWR SAID CLRED FOR TKOF. I AM NOT SURE IF THIS WAS A SECOND TKOF CLRNC OR 'THE TKOF CLRNC.'

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.