B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 250; Flight Crew Total 14500; Flight Crew Type 2050
Confusion; Troubleshooting
Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Weight And Balance; Inflight Event / Encounter Fuel Issue
Flight Crew Diverted
Arrived in ZZZ WX room early. Looked at release 1. Had CI 120 (Cross Index) through most of the WSI RPM forecast turbulence. Called dispatch - shift change; new dispatcher than the one who first worked the flight. Asked for CI 75 as I feel the airplanes rides a lot smoother at 75 than 120 and a higher altitude (FL370) if able as that looked better; as well as a little farther north if able over ZZZZZ VOR to avoid most of the turbulence. Dispatcher agreed and asked what REMF (Remaining Fuel) fuel I wanted - I said 1:15 was fine with me as the WX in ZZZ1 was nice. He said no problem.Meet FO (First Officer); walked to gate as R2 (Release #2) was not out yet. Got to airplane; R2 was out; FL370; CI75; REMF 1:16 - as asked for; accepted release. Gate fuel from R2 was 25.2 - fueler put on 25.8 (overfueling by 600lbs). Final weights came up; said we had a drop in TOW CHG of -2588 lbs. Verified T/O speeds/weights. They were off by about 30k lbs. I mentally thought oh - no problem - you/we must have asked for the ZFW (Zero Fuel Weight) for preliminary T/O data by mistake - just order new numbers for both XXR and XXL in ZZZ at our new weight of 150.8. FO did and we were on our way. Single engine taxi out; no line; cleared for immediate T/O.On climb out FMC fuel landing at ZZZ1 shows 5.0. Ok; that's a little low; but we have a high deck angle and the airplane is still climbing out. Double check R2; it shows REMF of 5.0 - ok that checks. [Fuel calculator] shows landing in ZZZ1 with 5.6; so I thought we must have a wind shift somewhere. As we climb out; the FMC forecast landing fuel in ZZZ1 continues to drop; 4.9; 4.8; 4.7; 4.6 - within about 15 minutes both the FO and I see the trend and start looking for where there is a FMC loading issue - possible extended final in ZZZ1; insufficient winds; and incorrect transitions. Cannot find why the fuel is dropping. Check Fuel used function on panel - fuel usage is even (the same) for both engines; and what was forecast to be close to the burn on [Flight Plan] - so no fuel leak.I contact dispatch; say that we are stumped. Everything is loaded correctly; and we left with more than our planned gate fuel and yet we still show landing in ZZZ1 well below [Flight Plan] REMF values. I believe it took dispatch over 30 minutes to get back to us. Over that time we double checked the final weights number(s). Even though it said there was a -2588 TOW CHG from the [Flight Plan] - when compared with the [Flight Plan] ZFW the final weights showed a ZFW of 125.1 where the [Flight Plan] showed a ZFW of 96.1! Almost 30k lbs more than the [Flight Plan] ZFW. Now we had T/O numbers for 150.8 (ZFW plus fuel); so T/O performance wasn't ever at risk; but now we weighed more than planned and our fuel burn was much higher than forecast. Now that we know why our fuel was showing lower than [Flight Plan]. Eventually. The FMC settled into a REMF fuel at ZZZ1 of 3.8; when the center tank was empty - barely FAR 45min. Dispatch finally got back to us and said that they had over three people look at it and couldn't figure why; when the ZFW was entered into [the Flight Plan] - it would kick back and put in a different number.Due to our forecast low fuel state; if we continued to ZZZ1 - in the interest of safety; we elected to divert to ZZZ2 for more fuel. Since we had plenty of fuel to go to ZZZ2 - we did not declare a fuel emergency - because we didn't technically have one if we went to ZZZ2. Stopping for fuel was better than pushing on to a bad outcome. Went to ZZZ2; landed uneventful; went to gate; waited for fueler; departed with same Final weights message - PAX and bag load unchanged (so same final weight numbers were the same; ZFW 125.1) and continued on to ZZZ1 - where we landed uneventfully approximate 45min after schedule. I do believe that both the FO and I caught the discrepancy of our forecast fuel landing in ZZZ1 very early on after takeoff. From then on out; dispatch; the FO; and I were in a CRM exercise on finding out what went wrong; what solutions were the most prudent; and what our next course of action would be. In the end; I believe that diverting to ZZZ2 with sufficient fuel (landed in ZZZ2 with 5.5ish) onboard was the safest course of action given the circumstances as opposed to trying to push it to ZZZ1 and landing on fumes.
B737 Captain reported having to divert for more fuel due to a Zero Fuel Weight (ZFW) flight planning error.