After sluggish takeoff and initial climb; B737 flight crew discovered that takeoff data received over ACARS was for their zero fuel weight and not the actual takeoff weight. Chain of errors between crew and dispatch is discussed.

Narrative

Because of an inoperative performance computer; we sent an ACARS message to Dispatch just before pushback requesting the numbers. The message was formatted like this; providing the Zero Fuel Weight and Takeoff Weight. Performance computer inoperative: 115755; 143654; Runway XX left/right please. We received a message back with this data: Assumed temperature 55 degrees; flaps 5 degrees; V1 121; VFR VR 121; V2 126. The message began with the assumed temperature and finished with the single engine procedure. We entered the numbers; accomplished the pushback and took off. As we were accelerating; the thought crossed my mind that we were not accelerating quite as quickly as normal; but I dismissed it because we were very heavy; so it was not going to exactly jump off the runway. When the First Officer rotated; the airplane flew but again I had that feeling that we were not performing like normal and it was kind of 'mushy' or nose-high. Once we got to altitude; we discussed it and discovered we both had the same thoughts and impressions. I pulled out the FOM to see if I could double-check the numbers and found there are no tables or graphs that would supply a rotation speed or a V2 speed based on weight. Additionally; the FOM specifies what information we need to receive back from Dispatch and we received what is specified in the FOM. However; I also noticed that when we took off from LAX the leg prior; the first part of the message from Dispatch included the input data where the message did not. I am sure you know what we thought next! We must have taken off with data based on the Zero Fuel Weight of 115755 instead of our actual Takeoff Weight of 143654! Could that really be what happened? Yes; that is exactly what happened and a series of messages with Dispatch confirmed that. Here is what I think happened and the human factors involved. My first question to Dispatch was if they need both the Zero Fuel Weight and the Takeoff Weight from us or just the Takeoff Weight. It turns out they only need the Takeoff Weight; even though we supplied both numbers because that is what we need when we run the performance computer. I also asked about how the numbers get from their on-screen 'Performance Computer Surrogate' to the ACARS screen and they have to perform a 'copy and paste;' just like I do in Microsoft Word as I type this up. Dispatch also pointed out that they supply the input data as part of the return message as a crosscheck to avoid any problems or confusion. Let me point out here that the FOM doesn't currently require that information to be included; even though I think it is excellent to have it. So; bottom line; the Dispatcher was probably busy or in a hurry and grabbed the first number from our initial message -- the Zero Fuel Weight of 115755 instead of the Takeoff Weight of 143654 and ran the number with that. Next; during the 'copy and paste' process; the result on screen got highlighted from the 'Assumed Temperature' to the end of the output instead of from the very beginning where it shows the input data. I don't know if that was intentional because it still included the required data or if the Dispatcher intended to highlight the whole thing and it just didn't happen. I would like to think that we would have noticed during the review of the data that the wrong weight was used (can't guarantee it!) but the way it occurred; we had no chance at all. Dispatch later confirmed that the correct numbers should have been: Assumed Temperature 48 degrees; flaps 5 degrees; V1 127; VFR VR 137; V2 140. We rotated 16 KTS slow! Had the rotation been abrupt we could have drug the tail. Worse; had we lost an engine we could have ended up in the water. I later beat myself up for not noticing the numbers just didn't look right. I thought back to the 'old days' when the First Officer did the data out of a book and I had some mental math (long since forgotten!) on what the V2 speed should be as a quick check of his work. I even won a few beers that way; but I no longer do anything like that and pretty much accept what the performance computer says; just like I accepted the numbers Dispatch sent. Thanks for your time and effort! I love reading the information that you publish and I am constantly seeing events that have happened to others and have occasionally changed my habits to avoid the same problem. I hope this helps someone else the same way. This was clearly a case of human factors involving several highly trained and dedicated professionals. So what could keep this from happening again? I have a few thoughts: 1) Automate the output from the Dispatch number-cruncher to go straight to the ACARS message-sender; eliminating the cut-and-paste. 2) Format the return message from Dispatch to put the most important information first and on the first page. That would be: the gross weight; runway and the temperature/V speeds we need for the FMC. The runway and gross weight is what I check when looking at the performance computer so why not here also?