A B737 crew became distracted while discussing the windshear takeoff performance calculations at the same time ATC issued a line up and wait clearance and so took off without clearance.
Synopsis
A B737 crew became distracted while discussing the windshear takeoff performance calculations at the same time ATC issued a line up and wait clearance and so took off without clearance.
Narrative
Prior to pushback from the gate; I advised the flight attendants to remain seated due to weather on our departure. Just prior to taxi; Ground Control changed our departure altitude from 5;000 FT to 3;000 FT. After that change; we were cleared to taxi via Charlie to 13R and cross Runway 18. When the First Officer switched over to the Tower frequency; we heard the Tower give a windshear report of minus 20 KTS. I had the First Officer recalculate our takeoff data using the Windshear button on the performance computer; while I stopped the aircraft and set the brake. When the First Officer gave me the performance date; I began to brief a precautionary takeoff and repositioned the flaps from our original flaps one setting; to flaps five. I was just completing the briefing of the new V speeds and flap setting when the Tower cleared us to 'line up and wait.' The tapes revealed that the First Officer properly read back the clearance. After the Tower cleared us to 'line up and wait;' there was a slight delay because we still had to complete the Before Takeoff Checklist. During this short lapse of time to perform the checklist; our understanding of the clearance somehow failed. We incorrectly took off when our clearance was to 'line up and wait'. Although we had some last minute weather and tasks to deal with; I thought they were manageable. I am perplexed as to how the breakdown happened. We both heard the clearance; yet we misinterpreted the clearance after the six seconds it took to complete the checklist. The change I would make in the future is to be more cognizant of being saturated. Although we completed all the last minute tasks; we didn't fully absorb the communication from the Tower.
Second reporter narrative
I believe that the reason behind the misunderstanding was that I was too concerned with the weather/windshear/precautionary takeoff. Another contributing factor was the fact that Tower called us while we were preparing for the precautionary takeoff. Since there was no other traffic (landing or takeoff); I was expecting a takeoff clearance. I rushed to respond to the Tower and heard what I wanted to hear instead of what Tower actually said. This is a big lesson for me. I can't believe that I let us take off without a takeoff clearance. But I did! The big lessons are: I need to slow down and be more deliberate; especially when I encounter adverse weather and/or something different.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.