Date: 201201
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: Fuel Quantity-Pressure Indication
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer
Human factors: Confusion
Second reporter function: Captain
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Weight And Balance; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight; Pre-flight
Result: General Maintenance Action; General Flight Cancelled / Delayed
Contributing factors: Procedure; Human Factors; Aircraft
Primary problem: Aircraft

Narrative: As we were completing our preflight duties for the flight; the Agent brought in the load schedule.  As I was entering the info; the Captain completed the fuel audit and realized there was a discrepancy with the fuel numbers.  The fuel the aircraft had come in with the night before and the fuel recently added were not correctly adding up to the total fuel on board.  Captain informed the Agent.  Apparently; the Agent found the discrepancy with the fuel being entered incorrectly from the previous flight.  The Agent brought down a new load schedule and the fuel audit was successfully completed.  After we took off; I believe we were in cruise when the Captain realized that the center fuel tank was indicating more fuel than what we took off with and the total fuel on board was more than what we took off with.  When we landed; it was decided that we would remove all the fuel from the center tank at a remote pad; which we did.  The fuel removed from the tank equaled the amount the gauge had read beforehand.  We returned to the gate; refueled; did our preflight duties again.  The fuel audit was correct; the gauge was reading accurate so it was not written up for discrepancies.


Second narrative: The flight was scheduled to tanker fuel.  The Release called for a total fuel load of 23.8.  The logbook showed the aircraft terminated the prior night with 8.5 on board.  The digital fuel gauges read that the wing tanks were full (10.0) and the center tank had 3.8 for a grand total of 23.8.  The fuel tank gauges tested normal during pr-flight and the FMC showed the same fuel total of 23.8.  I calculated a fuel audit using 23.8 minus 8.5 for a total of 15.3 pounds/2;300 gallons added in anticipation of what I should see on the load sheet.When the load sheet arrived; the gallons added were 3;163 gallons (21;192 pounds).  Since this was a large error; I had the Agent bring the jetway back and went out to talk to him.  He said that Maintenance was called out during the fueling (before we arrived) but he didn't think there was any problem.  I asked the Agent to call for the Fueler and Maintenance.  During that time; I reviewed the fuel slip for this flight.  It reflected the correct starting and ending fuels in each tank (agreeing with what I was seeing on the gauges and the totalizer) but the math for the fuel meter clearly added up to the 3;163 gallons; reflected on the load sheet.  Had that many gallons been added; the total fuel on board should have showed 29.6.Maintenance arrived and said that there really was no problem that he saw during the refueling.  Only something about the Center Tank gauge had blanked for a short time but came back and all indications were normal.  I inquired as to dripping the center tank and the Mechanic confirmed with a phone call that you could not drip the center on the Classic.  I asked if there could have been any errors with reading and recording the meter? (there had been a large storm and heavy rain all morning and the ramp was very wet).  The Agent then phoned to have a Fuel Supervisor meet us.  I explained to Maintenance and Operations that until this was resolved; we were not going to be able to go anywhere.The Fuel Supervisor then met us and was holding a piece of paper.  Supervisor said that there was an error on the fuel meter log.  He said the gallons added was actually 2;263 (15.1 pounds); which fell very close to the original fuel audit numbers I had expected.  After conferring with Maintenance and Operations; the First Officer and I felt the situation was resolved and we pushed the aircraft and were on our way.After leveling off at 22;000 feet; I performed a systems check.  I noted that the center tank now had approximately 6.9 in it.  I pointed this out to the First Officer and we started checking things.  The FMC now reflected what the three tanks were showing.  The center tank pumps had been on throughout the flight and the wing tanks were within 200 pounds of full.  Since we were now descending; I verified that we did not have any gross weight landing issues and we then calculated our approach speeds based on this new weight.  Approach and landing were uneventful.  As we taxied in; we notified Operations to not fuel the aircraft until I spoke to the Agent and Maintenance Control.We shut down with 25.9 indicating on the gauges and the FMC.  With a calculated taxi and enroute burn of 3.1 plus the 25.9 on board; this meant I left with approximately 29.0 on the aircraft.  This is very close to what the original gallons pumped should have shown.  I explained this to Maintenance Control and told them that I felt the only way to know for sure how much gas was in the center tank was to defuel it.  They agreed.  We then returned to the gate and refueled the aircraft to the new Dispatch fuel.  There were no discrepancies in the defuel audit or the refuel audit and the gauges worked properly and tested correctly.  We continued our next leg with no issues.There are two items that I am concerned about.  The first was we apparently took off using performance data that was approximately 6;000 pounds too light.  We used a takeoff weight of 100.8.  There were no noticeable indications during the takeoff/departure that the jet was not performing normally.  Secondly; I did not make a logbook writeup after conferring with Maintenance Control about any center tank fuel gauge problem.  While on the ground; everyone felt there was no problem because the Fueler said the fuel slip was wrong.  On looking back; the original number provided by the Fueler was accurate and there must have been more to the Maintenance callout during refueling than anyone knew.  The fact that the gauge was off by exactly the amount of the discrepancy of the fuel slips is quite a coincidence.  I now believe the Center Tank gauge is in fact working correctly; so there must have been a problem in ZZZ.  I defueled the tank; audit is good; then refueled; and the audit is good.  I am sure how much fuel is on board; so again; there is no malfunction that I am aware of; therefore; I did not make a Logbook entry.


Synopsis: B737-300 Captain discovers during preflight that the fuel added plus the fuel on board did not add up to the present fuel on board.  It is decided the fueler made an error and the flight departs.  Enroute; center tank fuel increases indicating that the original fuel sheet was correct and that the aircraft departed with 6000 pounds more fuel than thought.  Maintenance can find no discrepancies with the center tank gauge and no logbook entry is made.