Date: 201707
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: Fuel Storage System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Captain; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Weight And Balance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: Pre-flight; Aircraft In Service At Gate
Result: Flight Crew Overcame Equipment Problem; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: After an aircraft swap due to a grounding MX write-up; we inherited aircraft [for the remainder of the trip]. Upon inspection of the fuel load; we discovered a fuel distribution of 8300 pounds in each of the main tanks and 4200 in the center tank. Due to aircraft fuel load limitations; the Captain inquired why the main tanks were not completely full. The Fueler indicated that he was limited by a 'VTO' figure that an unknown entity came up with and that that limited him to only fuel to 8300 pounds in the main tanks. The Captain; the Agent and I were confused by this. As a Crew we were accustomed to seeing roughly 8600 pounds of fuel in the main tanks (indicating full) with fuel indicated in the center tank. The Agent was also confused as she wasn't sure how to fill out the load manifest paperwork. After much discussion with onsite Maintenance and the Fuel Vendor and in an attempt to bring the aircraft within limitations; the Captain asked the Fueler to fill the main tanks to capacity. The Fueler hooked back up to accommodate the request; but for some reason decided it appropriate to push a force override fueling button on the fuel panel instead of letting the fuel system automatically shut off fuel upon reaching a full tank. As a result; fuel was vented overboard the right wing as the tank overfilled creating an estimated 15 to 20 gallon ramp spill. The Company Ramp Personnel; fuel vendor Personnel and Airport Operations were all quick to respond and got the spill quickly contained and cleaned up. After the fueling and subsequent cleanup was completed we were indicating a fuel load of 8660 in the left tank; 8540 in the right tank (I assume this was lower due to the spill/venting) and 4200 in the center tank. During the next fueling our tanks indicated normal/expected quantities of 8600ish in each main tank while indicating fuel in the center tank.We as a Crew should have made it clear not to force/override any part of the fueling system when adding the additional fuel. The Fueler made no indication that he was unsure of the Captain's instructions. If the tank wouldn't take any more fuel during normal operations; we should have been notified of that. On the training side of the equation; a review of acceptable minimum main tank values (acceptable full) with fuel in the center tank would prove useful.  It is understood that the environment plays a role in the density of fuel; but a 300 pound plus variance didn't seem normal. Great job by Company Ground Operations in executing a swift clean up.


Second narrative: After discussing the incident with the Fueler; airport authority; Company Operations and Company Maintenance; we departed without further incident. On a side note; per the logbook; the aircraft did have maintenance performed on the number 2 Fuel Shut off valve. Additionally; on the subsequent flight; the main tanks of the aircraft were fueled to 8600 pounds without incident.


Synopsis: A Boeing 737 Flight Crew reported that they discovered the fuel distribution for the aircrat was not correct.