Date: 201406
Local time of day: 1801-2400
Location: ZAB.ARTCC, NM
Flight conditions: VMC
Light: Dusk
Aircraft: B737-500
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZAB
ATC / advisory: Center ZAB
Aircraft component: Fuel Storage System
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Communication Breakdown
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Weight And Balance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Aircraft; Company Policy; Human Factors
Primary problem: Human Factors

Narrative: After a normal departure; we leveled off at FL350 when I noticed something about the fuel load. The center tank was almost empty when I realized we had 8.5 in each wing tank. On 95% of my flights this would be no big deal and is exactly what we want to see. Except that this flight we were in a [different series 737]. Each wing should have had 10.0 with 2.6 in the center for a total of 22.6. This time we had 8.5 in each wing like a -700 and 5.6 in the center for a total of 22.6. So how did we get here? Well; I am guessing they don't see older model aircraft in LAS very often and the agent filled out the fuel slip just like a -700. In fact; the First Officer reminded me that the agent even made the comment; 'we never see [this 737 series] here.'When the fuel is input for the load sheet; only the total is entered so the computer can't trap that error. In fact; the load sheet prints out as if the wings are properly loaded. I made it my habit years ago to add up the three gauges instead of looking at the totalizer to make sure I turn on the center tank pumps if needed. Well; I dutifully turned them on this flight! We had just left a -700 for this older model aircraft and nothing triggered in my brain that the wings should look different on this plane. I added the three gauges up like I had just done a few hours ago in that -700. So how to avoid this in the future? Other than the standard trope of vigilance; pay attention; yada yada yada; how about something as low-tech as a sticker right below the gauges to highlight the fact that full should be 10 and not 8.5? After all; we have a sticker to remind us we have winglets and we can see the end of the wing!! As we fly the -700 more than the older model aircraft by a long shot; this kind of simple gotcha is going to be more common. We can use all the help we can get.How about instead of putting in JUST the TOTAL fuel into the computer for the load sheet; require that the fuel in EACH TANK be input instead. This way the computer can help trap this error.




Synopsis: At cruise altitude the Captain of a second generation B737 noted that; although he had the required fuel on board; it was improperly located with 1;500 pounds less than full 10;000 required in the left and right main tanks and the balance in the center tank.  He shortly realized the fuel distribution would have been correct for the more common B737-700s in their fleet on which the main tanks only hold 8;500 pounds.