Narrative:

We were preparing to leave; the flight was on time and the weather was perfect. Our performance computer was deferred; and this was the second leg in this aircraft. This was the first time that the first officer (first officer) had ever experienced a performance computer deferral; and used the ACARS performance system. All of our preflight procedures were completed; we pushed early and taxied to the runway for departure.I was the pilot monitoring (pm) on this leg; and the takeoff roll was normal. I called V1; rotate; and immediately noticed something wrong. As he began to rotate I told the first officer that we were too slow; and to keep the pitch low. The aircraft was very slow to rotate and lift off; and he did an excellent job smoothly letting the aircraft accelerate and fly away at higher than bugged speeds. Once we cleaned up and climbed away; I realized what must have happened and verified that the ACARS performance data was input incorrectly; and neither of us caught the error. Instead of inputting the takeoff weight (tow); the first officer input the zero fuel weight (ZFW) instead. We had 32;000 pounds of fuel onboard; so our speeds and data were for a 32;000 pound lighter aircraft. Running the numbers showed that the bugged V1 and vr were 21 knots lower than they should have been.from a crew standpoint; I simply should have caught the error. I still can't believe that the common sense aspect of the numbers didn't stand out. I think the major issue that led to the mistake is the different format of the takeoff data on the ACARS versus the performance computer. When using the performance computer; the pilot is accustomed to inputting the ZFW and the tow; and inputting the ZFW into the FMC. The fact that the ACARS input only requires the tow is a minor difference; but probably very easy to screw up. The error here should have been caught in the before push checklist; but again the formatting difference caused a deviation from the normal flow of crosschecking the data. I would suggest that the ACARS input be changed to include both weights; if for no other reason that it is what the pilots are used to putting into the performance computer. From a personal perspective; this was certainly a bit of a wakeup call to be more attentive to crosschecking the data; and to take a bigger picture look to ensure that the numbers make sense.

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Original NASA ASRS Text

Title: The B737 crew reported on a mistake in the performance calculations because of a deferred performance computer and an incorrect entry of Zero Fuel Weight instead of Takeoff gross Weight.

Narrative: We were preparing to leave; the flight was on time and the weather was perfect. Our performance computer was deferred; and this was the second leg in this aircraft. This was the first time that the First Officer (FO) had ever experienced a performance computer deferral; and used the ACARS performance system. All of our preflight procedures were completed; we pushed early and taxied to the runway for departure.I was the Pilot Monitoring (PM) on this leg; and the takeoff roll was normal. I called V1; rotate; and immediately noticed something wrong. As he began to rotate I told the FO that we were too slow; and to keep the pitch low. The aircraft was very slow to rotate and lift off; and he did an excellent job smoothly letting the aircraft accelerate and fly away at higher than bugged speeds. Once we cleaned up and climbed away; I realized what must have happened and verified that the ACARS performance data was input incorrectly; and neither of us caught the error. Instead of inputting the Takeoff Weight (TOW); the FO input the Zero Fuel Weight (ZFW) instead. We had 32;000 pounds of fuel onboard; so our speeds and data were for a 32;000 pound lighter aircraft. Running the numbers showed that the bugged V1 and VR were 21 knots lower than they should have been.From a crew standpoint; I simply should have caught the error. I still can't believe that the common sense aspect of the numbers didn't stand out. I think the major issue that led to the mistake is the different format of the takeoff data on the ACARS versus the performance computer. When using the performance computer; the Pilot is accustomed to inputting the ZFW and the TOW; and inputting the ZFW into the FMC. The fact that the ACARS input only requires the TOW is a minor difference; but probably very easy to screw up. The error here should have been caught in the Before Push Checklist; but again the formatting difference caused a deviation from the normal flow of crosschecking the data. I would suggest that the ACARS input be changed to include both weights; if for no other reason that it is what the Pilots are used to putting into the performance computer. From a personal perspective; this was certainly a bit of a wakeup call to be more attentive to crosschecking the data; and to take a bigger picture look to ensure that the numbers make sense.

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.