Date: 201306
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
ATC / advisory: Tower ZZZ
Aircraft component: PMC; Performance/Thrust Management Computer
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Human-Machine Interface; Situational Awareness
Second reporter function: Pilot Flying; First Officer
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Weight And Balance
Detector: Person Flight Crew
When detected: Other Takeoff roll
Result: General None Reported / Taken
Contributing factors: Chart Or Publication; Human Factors; Procedure
Primary problem: Ambiguous

Narrative: I noticed on the computer weather was good at destination but reported Minus 57 temperature. During the flight the weather was again good but minus 43 showed on temperature. During the flight we checked two more times and temperature was dropped from weather report. I figured the station realized it and [deleted the] temperature. ATIS on radio said 30 C; and ATIS on the ground said 29 C. The flight plan was handed to me by ops personnel and checked weather on iPhone since coverage was available for Miami. Loaded FMS with flight plan and TPS [Takeoff Performance System] and checked. During taxi out we uploaded numbers and we were below weight and assumed temperature. TPS computed N1 of 84.5 which was a bit low but it was clear with 10;000 foot runway. As we did a rolling takeoff and hit TOGA noticed we didn't get the normal push with thrust but the airplane accelerated normally until about 120. That is when I thought this isn't right and checked again the takeoff weight which was 151;000 and realized this wasn't a normal power setting. I thought of either aborting or taking control and pushing power up manually but we were accelerating and rotated about with 2;000 FT remaining and took off. During the climbout after 10;000 FT I knew we were either well above planned weight or something else was wrong on TPS so I started looking. I looked at planned temperature which we thought said 28 C but we then noticed it was -28 C. I guess with all those earlier wrong temperatures showing up it was loaded into our TPS wrong as well. Thinking outside temperature was 29 C and we were below assumed 30 C we thought at the time it was correct. Obviously we will look at temperatures much closer in the future; and never assume anything. In hindsight either an abort or manually pushing power way up would have been the best options. At the time with length of runway and weather conditions we did neither.


Second narrative: Captain reviewed TPS numbers; [which] showed 22K standard takeoff assumed temperature 34 degrees C. ATIS showed temperature 29 degrees C. Began takeoff roll.  Aircraft seemed sluggish however [it] accelerated and [we were] able to takeoff with 2;000 FT of runway remaining. After cleanup discussed with the Captain the lack of acceleration of the aircraft. Reviewed the TPS and found closeout numbers used a minus 28 degrees C.


Synopsis: B737-800 flight crew experiences a sluggish takeoff from a 10;000 foot runway with an OAT of 29 C and a reduced thrust of an assumed temperature of 34 C. Once safely airborne it is found that the performance calculations were made using an actual OAT of -28 C. The crew believes this error caused the poor performance.