Narrative:

We were preparing for the last leg of the day from ZZZ. It was day #2 of a 4-DAY pairing with a captain I had flown with previously. The fueler came up to the cockpit and said the center tank was not taking any fuel. Maintenance was called and arrived within mins. The mechanic came up and said everything was ok and both the wing and cockpit gauges appeared to be working normally. We had arrived with 5700 pounds of fuel and was planned to take off with 23200 pounds. The captain and I both figured our uplift should be 2610 gallons. When the operations agent handed us the load sheet, we noticed the uplift to be 3063 gallons, over 400 gallons off. We then asked the agent to check the fuel slip for a possible error. The math appeared to be correct. The captain then went down to check with the fueler. Both cockpit and wing fuel gauges read that we had the right amount of fuel. The fueler told the captain that he had put 400 gallons in the center tank using the gallons added method, which put our fuel load exactly as planned. With the gauges reading correctly and the fueler's word that he had put only 400 gallons on the center tank, we then pushed for takeoff. Our ATOG was 128900 pounds and we departed with a takeoff weight of 127744 pounds with a bleeds off takeoff. After we had been in cruise for a while, we noticed the center tank gauges reading zero but the low pressure lights had not illuminated. After about what would have been an approximately 2000 pound fuel burn, the lights did illuminate and the pumps turned off. We believe that the fueler had actually put 800 gallons in the center tank, 400 pounds of which did not show on the gauges. That would account for the extra 400 gallons that was on the fuel slip. With the belief that he had only added 400 gallons to the empty tank and the gauges showing correctly, we incorrectly assumed that the error was in the paperwork. When the fueler told the captain he had added only 400 gallons, we made the assumption that it was to an empty tank since the tank had been empty and not used all day. It appears we actually departed approximately 1400 pounds over ATOG in an already heavy, bleeds off takeoff. Maximum taxi and takeoff were never exceeded. The rest of the flight was normal. The captain wrote up the center tank gauge when we arrived. I understand that we are the final check in making sure we are loaded correctly before takeoff and make no excuses for not digging further into the inaccurate uplift prior to takeoff. I made an assumption that I now think was incorrect in that the 400 gallons was added into an empty tank. I believe when they had initially had a problem getting fuel in the center tank they had actually put some fuel in even though the gauge did not show it. This has been a humbling experience and one that I will hopefully never repeat. Thankfully, the takeoff was normal, but had an emergency arose with the added weight, things could have been bad. I hate to think that I put our crew and passenger into a potentially bad situation. I have learned a lot from this humbling experience. In the future, I will make sure the paperwork matches up and not let an assumption put me in a similar situation. Supplemental information from acn 616416: I had the operations agent call the fueler and supervisor back. I repeatedly asked the fueler to explain the process he used to guarantee an accurate amount in the center tank. He assured me he used the 'gallons added' method as required by the fom since dripping the tank isn't an option. I was led to believe that the initial amount that he uploaded that didn't register on the center tank had been idented and the center tank was reading accurately. I told the operations agent that I was satisfied and that he needed to sort out the 400 gallon discrepancy with the fuelers. Earlier in the day, I noted 'err' in small letters on the center fuel gauge. (I've never seen this message before.) no center tank fuel was shown, added, or required up to that point. At cruise, when the center tank showed zero fuel, I reset the fuel used switch and let it run until the first indication of low pressure the center tank. At that point, I checked the fuel used. It was 2400 pounds that never showed up in the center tank. So we had either carried that much, or a portion unknowingly all day, or the fueler (more likely) failed to completely empty the center tank before using the 'gallons added' method of uploading fuel in the center tank. He never actually said he completely emptied the center tank, simply that the fuel was accounted for. He has a future as a lawyer. My fault for not pressing further for the exact verbiage required.

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Original NASA ASRS Text

Title: A B737-300 INACCURATE CTR TANK FUEL GAUGE LEADS TO AN OVERFUELING SIT AND AN OVERWT TKOF FROM ZZZ, US.

Narrative: WE WERE PREPARING FOR THE LAST LEG OF THE DAY FROM ZZZ. IT WAS DAY #2 OF A 4-DAY PAIRING WITH A CAPT I HAD FLOWN WITH PREVIOUSLY. THE FUELER CAME UP TO THE COCKPIT AND SAID THE CTR TANK WAS NOT TAKING ANY FUEL. MAINT WAS CALLED AND ARRIVED WITHIN MINS. THE MECH CAME UP AND SAID EVERYTHING WAS OK AND BOTH THE WING AND COCKPIT GAUGES APPEARED TO BE WORKING NORMALLY. WE HAD ARRIVED WITH 5700 LBS OF FUEL AND WAS PLANNED TO TAKE OFF WITH 23200 LBS. THE CAPT AND I BOTH FIGURED OUR UPLIFT SHOULD BE 2610 GALLONS. WHEN THE OPS AGENT HANDED US THE LOAD SHEET, WE NOTICED THE UPLIFT TO BE 3063 GALLONS, OVER 400 GALLONS OFF. WE THEN ASKED THE AGENT TO CHK THE FUEL SLIP FOR A POSSIBLE ERROR. THE MATH APPEARED TO BE CORRECT. THE CAPT THEN WENT DOWN TO CHK WITH THE FUELER. BOTH COCKPIT AND WING FUEL GAUGES READ THAT WE HAD THE RIGHT AMOUNT OF FUEL. THE FUELER TOLD THE CAPT THAT HE HAD PUT 400 GALLONS IN THE CTR TANK USING THE GALLONS ADDED METHOD, WHICH PUT OUR FUEL LOAD EXACTLY AS PLANNED. WITH THE GAUGES READING CORRECTLY AND THE FUELER'S WORD THAT HE HAD PUT ONLY 400 GALLONS ON THE CTR TANK, WE THEN PUSHED FOR TKOF. OUR ATOG WAS 128900 LBS AND WE DEPARTED WITH A TKOF WT OF 127744 LBS WITH A BLEEDS OFF TKOF. AFTER WE HAD BEEN IN CRUISE FOR A WHILE, WE NOTICED THE CTR TANK GAUGES READING ZERO BUT THE LOW PRESSURE LIGHTS HAD NOT ILLUMINATED. AFTER ABOUT WHAT WOULD HAVE BEEN AN APPROX 2000 LB FUEL BURN, THE LIGHTS DID ILLUMINATE AND THE PUMPS TURNED OFF. WE BELIEVE THAT THE FUELER HAD ACTUALLY PUT 800 GALLONS IN THE CTR TANK, 400 LBS OF WHICH DID NOT SHOW ON THE GAUGES. THAT WOULD ACCOUNT FOR THE EXTRA 400 GALLONS THAT WAS ON THE FUEL SLIP. WITH THE BELIEF THAT HE HAD ONLY ADDED 400 GALLONS TO THE EMPTY TANK AND THE GAUGES SHOWING CORRECTLY, WE INCORRECTLY ASSUMED THAT THE ERROR WAS IN THE PAPERWORK. WHEN THE FUELER TOLD THE CAPT HE HAD ADDED ONLY 400 GALLONS, WE MADE THE ASSUMPTION THAT IT WAS TO AN EMPTY TANK SINCE THE TANK HAD BEEN EMPTY AND NOT USED ALL DAY. IT APPEARS WE ACTUALLY DEPARTED APPROX 1400 LBS OVER ATOG IN AN ALREADY HVY, BLEEDS OFF TKOF. MAX TAXI AND TKOF WERE NEVER EXCEEDED. THE REST OF THE FLT WAS NORMAL. THE CAPT WROTE UP THE CTR TANK GAUGE WHEN WE ARRIVED. I UNDERSTAND THAT WE ARE THE FINAL CHK IN MAKING SURE WE ARE LOADED CORRECTLY BEFORE TKOF AND MAKE NO EXCUSES FOR NOT DIGGING FURTHER INTO THE INACCURATE UPLIFT PRIOR TO TKOF. I MADE AN ASSUMPTION THAT I NOW THINK WAS INCORRECT IN THAT THE 400 GALLONS WAS ADDED INTO AN EMPTY TANK. I BELIEVE WHEN THEY HAD INITIALLY HAD A PROB GETTING FUEL IN THE CTR TANK THEY HAD ACTUALLY PUT SOME FUEL IN EVEN THOUGH THE GAUGE DID NOT SHOW IT. THIS HAS BEEN A HUMBLING EXPERIENCE AND ONE THAT I WILL HOPEFULLY NEVER REPEAT. THANKFULLY, THE TKOF WAS NORMAL, BUT HAD AN EMER AROSE WITH THE ADDED WT, THINGS COULD HAVE BEEN BAD. I HATE TO THINK THAT I PUT OUR CREW AND PAX INTO A POTENTIALLY BAD SIT. I HAVE LEARNED A LOT FROM THIS HUMBLING EXPERIENCE. IN THE FUTURE, I WILL MAKE SURE THE PAPERWORK MATCHES UP AND NOT LET AN ASSUMPTION PUT ME IN A SIMILAR SIT. SUPPLEMENTAL INFO FROM ACN 616416: I HAD THE OPS AGENT CALL THE FUELER AND SUPVR BACK. I REPEATEDLY ASKED THE FUELER TO EXPLAIN THE PROCESS HE USED TO GUARANTEE AN ACCURATE AMOUNT IN THE CTR TANK. HE ASSURED ME HE USED THE 'GALLONS ADDED' METHOD AS REQUIRED BY THE FOM SINCE DRIPPING THE TANK ISN'T AN OPTION. I WAS LED TO BELIEVE THAT THE INITIAL AMOUNT THAT HE UPLOADED THAT DIDN'T REGISTER ON THE CTR TANK HAD BEEN IDENTED AND THE CTR TANK WAS READING ACCURATELY. I TOLD THE OPS AGENT THAT I WAS SATISFIED AND THAT HE NEEDED TO SORT OUT THE 400 GALLON DISCREPANCY WITH THE FUELERS. EARLIER IN THE DAY, I NOTED 'ERR' IN SMALL LETTERS ON THE CTR FUEL GAUGE. (I'VE NEVER SEEN THIS MESSAGE BEFORE.) NO CTR TANK FUEL WAS SHOWN, ADDED, OR REQUIRED UP TO THAT POINT. AT CRUISE, WHEN THE CTR TANK SHOWED ZERO FUEL, I RESET THE FUEL USED SWITCH AND LET IT RUN UNTIL THE FIRST INDICATION OF LOW PRESSURE THE CTR TANK. AT THAT POINT, I CHKED THE FUEL USED. IT WAS 2400 LBS THAT NEVER SHOWED UP IN THE CTR TANK. SO WE HAD EITHER CARRIED THAT MUCH, OR A PORTION UNKNOWINGLY ALL DAY, OR THE FUELER (MORE LIKELY) FAILED TO COMPLETELY EMPTY THE CTR TANK BEFORE USING THE 'GALLONS ADDED' METHOD OF UPLOADING FUEL IN THE CTR TANK. HE NEVER ACTUALLY SAID HE COMPLETELY EMPTIED THE CTR TANK, SIMPLY THAT THE FUEL WAS ACCOUNTED FOR. HE HAS A FUTURE AS A LAWYER. MY FAULT FOR NOT PRESSING FURTHER FOR THE EXACT VERBIAGE REQUIRED.

Data retrieved from NASA's ASRS site as of July 2007 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.