Narrative:

The performance computer recommended a flaps 10 take off. I checked the difference between flaps 5 (approximately 1000 foot stopping margin) and flaps 10 ( approximately 1600 foot stopping margin) and concurred that flaps 10 was better. This was the first flaps 10 takeoff that I have done in literally several years. It's always flaps 5. After start I called to set flaps 5 like always and the first officer complied just like every flight. We ran the checklist and all was just as normal (except we had the wrong flap setting). Taxi out was normal but a little busy with the airport construction going on combined with night operations. We were cleared for takeoff and; just about the time I hit the toga button; my first officer said; 'south***; flaps 10' and instantly slapped the lever to 10. That startled me and delayed what should have been an abort. After one or two seconds (while my brain was trying to process what just happened); the takeoff warning horn beeped about two or three times. I looked up saw the flaps at 10 and the speed going past 80 knots (spring loaded to only abort for fire or failure) and I elected to continue the takeoff. The takeoff and clean-up were otherwise normal. This whole event occurred in about three seconds. By the time I reacted; the airspeed was past 80 knots and going up and I made the snap decision that the safest action at that point was to continue rather than abort. I do not feel the threat/problem to be complacency so much as habit. 99.9% of my takeoffs have been flaps 5 so I was totally spring loaded into that routine. Kudos to my first officer for catching the mistake.

Related reports:

Google
 

Original NASA ASRS Text

Title: After planning a 10 degree flap takeoff due to their aircraft's planned weight and a short runway; the B737-700 flight crew; out of habit; set five degrees instead. Shortly after the start of the takeoff roll the First Officer realized the error and reset the flaps to 10 degrees and the takeoff was continued.

Narrative: The performance computer recommended a Flaps 10 take off. I checked the difference between Flaps 5 (approximately 1000 foot stopping margin) and Flaps 10 ( approximately 1600 foot stopping margin) and concurred that Flaps 10 was better. This was the first Flaps 10 takeoff that I have done in literally several years. It's always Flaps 5. After start I called to set Flaps 5 like always and the First Officer complied just like every flight. We ran the checklist and all was just as normal (except we had the wrong flap setting). Taxi out was normal but a little busy with the airport construction going on combined with night operations. We were cleared for takeoff and; just about the time I hit the TOGA button; my First Officer said; 'S***; Flaps 10' and instantly slapped the lever to 10. That startled me and delayed what should have been an abort. After one or two seconds (while my brain was trying to process what just happened); the Takeoff Warning horn beeped about two or three times. I looked up saw the flaps at 10 and the speed going past 80 knots (spring loaded to only abort for fire or failure) and I elected to continue the takeoff. The takeoff and clean-up were otherwise normal. This whole event occurred in about three seconds. By the time I reacted; the airspeed was past 80 knots and going up and I made the snap decision that the safest action at that point was to continue rather than abort. I do not feel the threat/problem to be complacency so much as habit. 99.9% of my takeoffs have been Flaps 5 so I was totally spring loaded into that routine. Kudos to my First Officer for catching the mistake.

Data retrieved from NASA's ASRS site as of April 2012 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.