Narrative:

After all checklists for pushback had been completed; we lost ground power on the aircraft prior to having the APU started. So the aircraft went to battery only stand-by power for about 30 seconds. Ground power was reconnected and the APU was started on put on the busses. After regaining electrical power; we re-verified that all the data was still in the FMC. We rechecked the route and that all the takeoff data was still in the FMC. There was a CDU fault that took about two minutes to clear and then everything was back up 100 percent. We received pushback clearance and taxied to 1L at mci. V1 and vr were the same (130 knots) and V2 was 135 knots. I pushed the power up; hit the autothrottles; and asked for takeoff thrust. My first officer (first officer) called takeoff thrust set and I re-established my hand on the thrust levers. My first officer then called 80 knots and then V1 rotate. I thought the call came rather fast in the takeoff sequence; but at the moment I was focused on the engine instruments. I removed my hand from the thrust levers and began my rotation. It was at that moment that I realized we were only at about 100 knots. The nose of the aircraft was off the ground; but we were well below the briefed speeds. I had started my rotation based on my first officer call. I kept the nose off the ground but stopped my rotation and pushed the thrust up until the aircraft felt as if it wanted to fly. We left the ground at about 120 knots; still 15 knots below V2. I let the aircraft accelerate to V2 and then contained a normal climb. At this point we realized what had happened. Due to the power loss; the V2 bug on the MCP had reset its self to 100 knots. My first officer said that when he saw it on the speed tape that he thought that the speeds i.e. V1; vr; and V2 were all the same and called rotate.from now on; anytime I have an electric power loss on the aircraft; I am going to run the full before start originating; before push; and before taxi checklist to make sure none of these critical items have been missed or reset due to the electrical power loss.

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Original NASA ASRS Text

Title: A B737 flight crew reported they rotated prematurely when the FMC performance data was lost following an electrical power failure during pushback.

Narrative: After all checklists for pushback had been completed; we lost ground power on the aircraft prior to having the APU started. So the aircraft went to battery only stand-by power for about 30 seconds. Ground power was reconnected and the APU was started on put on the busses. After regaining electrical power; we re-verified that all the data was still in the FMC. We rechecked the route and that all the takeoff data was still in the FMC. There was a CDU fault that took about two minutes to clear and then everything was back up 100 percent. We received pushback clearance and taxied to 1L at MCI. V1 and Vr were the same (130 knots) and V2 was 135 knots. I pushed the power up; hit the autothrottles; and asked for takeoff thrust. My First Officer (FO) called takeoff thrust set and I re-established my hand on the thrust levers. My FO then called 80 knots and then V1 rotate. I thought the call came rather fast in the takeoff sequence; but at the moment I was focused on the engine instruments. I removed my hand from the thrust levers and began my rotation. It was at that moment that I realized we were only at about 100 knots. The nose of the aircraft was off the ground; but we were well below the briefed speeds. I had started my rotation based on my FO call. I kept the nose off the ground but stopped my rotation and pushed the thrust up until the aircraft felt as if it wanted to fly. We left the ground at about 120 knots; still 15 knots below V2. I let the aircraft accelerate to V2 and then contained a normal climb. At this point we realized what had happened. Due to the power loss; the V2 bug on the MCP had reset its self to 100 knots. My FO said that when he saw it on the speed tape that he thought that the speeds I.E. V1; Vr; and V2 were all the same and called rotate.From now on; anytime I have an electric power loss on the aircraft; I am going to run the full Before Start Originating; Before Push; and Before Taxi Checklist to make sure none of these critical items have been missed or reset due to the electrical power loss.

Data retrieved from NASA's ASRS site 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.