Flight Crew Last 90 Days 213
Situational Awareness
Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Weight And Balance
General None Reported / Taken
Human Factors
Showed up to originator with MEL for performance computer inoperative. MEL directs the first method to obtain performance data is through the ACARS takeoff and landing performance module. On our aircraft this was functional so we used it. We loaded the ATIS information which was Runway 16L; winds 180/14G20; temp 16/14; wet-good; 29.62. From the loadsheet; we meant to load TOW 121.5. The takeoff data output page gave us 53C/16C Reduced; 116;116;122 which seemed light but we only had 80 people onboard and not a lot of fuel (16.9) so seemed plausible. When we ran the Before Push Checklist using the ACARS data; we failed to notice that the only weight on that page is the Take off weight (TOW) so when the First Officer (FO)called 'Zero Fuel Weight;' I read and responded; ' 104.7 set.' I saw it on the top right line of the ACARS page so it seemed correct. What I did not notice was that this line shows and is labeled TOW! We had accidentally entered the Zero Fuel Weight where we should have entered the TOW. The FO is used to entering Zero Fuel Weight and TOW in the performance computer. And then all he entered in the FMC is the Zero Fuel Weight and the plane calculated the TOW by adding the fuel to it. When you use the ACARS data; you only enter the TOW and never the Zero Fuel Weight so it is completely out of normal practice.  When he called out 'Takeoff Weight' on the Before Push Checklist I'm not sure what happened because I clearly did not verify that specific weight. I believe it was because this was the first time using this method and I couldn't find the number readily and subsequently just paid lip service to the verification thinking it must be right. The end result was that our speeds were off by a good bit. The number when we re-ran them on the performance computer (which was actually functioning fine but was unusable due to the MEL) should have been 127; 127; and 131 for a TOW 121.5 not 104.7. I noticed and commented to the FO that the plane was sluggish on rotation to about 200 feet and; as a result; I did not place any demands on the plane. The speed built rapidly due to strong winds and quickly returned to normal flight feel and characteristics. It was fortunate that we were not very heavy. If we had been going on a long flight and had a large fuel load; the results could have been much worse.  Of course; there is no excuse for not verifying the TOW when prompted on the checklist. My only defense (which is admittedly weak) is that it was an unfamiliar alternative and I was unaware of the pitfalls. I think some things could be done to help Crews. First; in order to comply with the Before Takeoff Checklist items; using this alternative method of data; requires the Crew to return to the PERF INIT page to read the Zero Fuel Weight and then go back to the ACARS module to read the TOW and continue the remainder of the checklist items.  It would be safer and more convenient if the ACARS output page listed the Zero Fuel Weight above the TOW. I know it in the format we have now because for takeoff the physics involved do not care about Zero Fuel Weight for takeoff speed calculations but the checklist demands a crosscheck. It might prevent this situation in the future. Another possible consideration might be removing or moving the Zero Fuel Weight prompt from the Before Push Checklist to the Before Start Checklist or even just move it into the preflight briefing process. The problem is the time we get the Zero Fuel Weight does not permit it this early in our new briefing preflight sequence. That is why I think the best solution is my first recommendation.  Put an entry for the FO to enter Zero Fuel Weight into the ACARS takeoff data input page and then just reproduce it on the output page. At the least; Crews should be made aware that the ACARS output page will not give you an ability to verify Zero Fuel Weight.
B737-700 Captain reports using the zero fuel weight instead of the takeoff weight in the weight and balance computations; a 17;000 pound error; resulting in low V speeds and sluggish takeoff performance.