Date: 201507
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 5
Light: Night
Aircraft: A300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Training / Qualification
Second reporter function: Pilot Flying; First Officer
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: Pre-flight
Result: Flight Crew Became Reoriented
Contributing factors: Human Factors; Procedure; Manuals
Primary problem: Ambiguous

Narrative: Our flight plan had us dispatched in Payload. Our dispatcher was trying to give us more fuel for forecast thunderstorms at destination. I understood this because the week before several flights; including this one; had to divert due to insufficient fuel.  My FO and I had not operated in this configuration in a long time; and felt something was not correct. My FO stated that he thought the center tank should be empty. I looked it up in the systems manual; and it read that the center tank could not be fueled; but any fuel in the tank could be burned normally. It did not say the center tank had to be empty. I also checked in the AOM Limitations and Normal chapters; but found nothing about this Mode. We operated the flight with fuel in the center tank and Payload selected. We did not need to be in Payload because the weight was within Range Mode limits so no limitations were exceeded.  When I discussed the situation later with another crew member; he referred me to the AOM supplemental section where it clearly states that the trim tank should be empty. I was still concerned about the situation so I talked to an ACP. I felt the dispatcher should be aware of our mutual mistake so that we could trap the error in the future. I also think the Systems manual should be changed because the way it is written is misleading. Now that the crew and dispatcher are aware of the mistake; it might be helpful to brief maintenance; as they rarely use this Mode.


Second narrative: The flight plan had us leaving with a 98;000 lb. fuel load which is well above what would be required for the approximately four hour flight. The dispatcher gave us the additional fuel because of forecast thunderstorms in the arrival area. The intention was for us to have adequate fuel to hold for several hours; if necessary; to wait for the storms to pass rather than having to divert. However; with the additional fuel and predicted payload we would exceed the maximum landing weight if the thunderstorms were not a factor when we arrived at [destination]. We were therefore dispatched in the 'Payload' mode which allows for a 7;300 lb. increase in maximum landing weight. The captain and I both believed that in 'Payload' the center and trim tanks should be empty; but a 98;000 lb. fuel load would be impossible under these conditions because the remaining fuel tanks; at maximum capacity; can only hold 78;000 lb. We consulted the systems manual which said that the trim and center tanks could not be fueled with the range/payload switch in the payload mode. To us; this made sense since the plane stopped accepting fuel at 78;000 lb. with the inner and outer wing tanks all showing full. However; there was also a statement in the systems manual that said that fuel in the center tank would be burned normally in payload mode which we; mistakenly; took to mean that; so long as the payload mode maximum zero fuel weight was not exceeded; you could indeed have fuel in the center tank and putting the switch in the payload mode just prevented fuel from being pumped into the tanks on the ground and prevented the CGCC from transferring fuel from a wing tank (the inner wings) to the trim tank during flight. We looked in the 'Normal Operations' section of the AOM; but didn't find anything about operating in payload mode; so we put the range/payload switch back to 'range' which allowed the 20;000 lb. of additional fuel to be uploaded in the center tank then placed it back into 'payload'. This was a mistake. The Supplemental section of the AOM does address operations in 'payload' mode and states that the center and trim tanks should both be empty. When we received the load manifest we found that the actual payload was considerably less than the planned payload and; as such; our zero fuel weight was below the maximum for either mode of flight; and our landing weight; based on planned fuel burn; was below 308;600 lb. even with no delay or additional fuel burned during holding. It may be useful to add text to the Aircraft systems manual warning crew members that the sentence stating that fuel in the center tank 'will be burned normally' in payload mode does not imply that; payload permitting; fuel may be carried in that tank during flights in 'payload' mode; or a note could be added to reference AOM supplemental operations for guidance while operating in 'payload.'


Synopsis: A300 flight crew experienced confusion when being dispatched in the 'payload' mode and could not determine if fueling the center tank was allowed. The center tank was fueled; but not permitted by FOM supplemental procedures.