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Bloomberg posted an article that says, among other things, the A350 ULR variant increases range by utilizing "additional space already available in the existing tanks to carry an extra 6,340 gallons of fuel."

What is the deeper technical explanation, or why not use this space in the regular A359 variant?

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Bloomberg says minor changes to "piping and venting" made the capacity available. It came from previously unused space in the center fuel tank.

  • The a359 center tank filling point is not at the highest point in the tank, so it needs a vent in order to use that space. Without the vent, the tank airlocks and no more fuel will flow in.
  • Piping refers to changes to that tank's inerting system, designed to prevent explosion of fuel vapor in the tank. Inerting has been required for wingbox center tanks ever since TWA800. See Leeham for more detail, search the page for Update2.

As to why this is not part of every a359, it is operating cost. The regional variant has a 260t MTOW and therefore lower landing and navigation fees. The 280t MTOW needed for the ULR will be available from 2020 on the standard model, so there is very little difference otherwise. If you order the ULR you can paper change it to a regional, though I don't know anyone who would.

Leeham also has an interesting article on how seating (passenger weight) has to be limited to get maximum range from the ULR.

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  • $\begingroup$ Looks like it ate into the payload too according to the Leeham link. $\endgroup$
    – user14897
    Commented Jun 2, 2018 at 3:21
  • $\begingroup$ All fuel increases eat into payload. plane designs have a "useful load" and that useful load is divided between fuel load and payload. I don't know of any commercial planes that can get both full tanks(and full range) and their maxium rated payload at the same time. Also I would guess that maximum landing weight did not change with that increase from 260t to 280t mtow. $\endgroup$
    – Max Power
    Commented Feb 19, 2020 at 22:54

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