Date: 201508
Local time of day: 0001-0600
Location: ORD.Airport, IL
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Tower ORD
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Situational Awareness
Second reporter function: Captain; Pilot Not Flying
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Weight And Balance
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: I arrived at the gate and put in numbers from the flight plan. The fueler was still putting fuel on the airplane. I did not get the planned final weights at that time since the fuel was not loaded. I did load the zero fuel weight and numbers but I did not obtain a planned final weights. I did the walk around and came back; the fuel was still not loaded. When fuel was loaded the fuel slip was received and the gallons were added to the ACARS initialization page and from there a planned takeoff weight was obtained. We taxied out and went through our new procedures; starting an engine and taxiing in Chicago. We managed to get everything done in sequence. The final weights were loaded and were executed; then an accept prompt appeared without v speeds. This was executed and we obtained v speeds and put those into to v-speed page.In flight when trying to retrieve landing data we got a Standardized Computerized Aircraft Performance (SCAP) error. I thought it was an issue with the fuel slip but after we reviewed our paperwork we realized the takeoff weight was not accurate. We calculated our actual weight and used that for our landing speeds.


Second narrative: First Officer requested planned take off performance data prior to push back. We reviewed the data and agreed that it was reasonable. During taxi out a final weights message was received and that data was used for requested actual take off performance data. Neither of us observed that there was a minus 13;456 pound difference from planned data. Performance Initialization was up linked and when takeoff data was up linked an accept prompt was available but no V speeds were available. Take off clearance was imminent so the First Officer manually entered V speeds from the takeoff data message and we departed.Upon requesting landing data prior to descent a Standard Computerized Aircraft Performance (SCAP) error message was received and upon reviewing the paperwork it appeared that there was an approximate minus 13;456 pound error in the Zero Fuel Weight (ZFW) in the FMC. A correction was entered and we used that data for landing data and REF speeds.


Synopsis: B737 flight crew reports receiving a final weights message that is 13;456 pounds less than planned which goes undetected. V speeds are calculated and entered and the flight departs. Upon requesting landing data prior to descent a Standard Computerized Aircraft Performance (SCAP) error message was received and the landing weight is corrected in the FMC.