Weight of cabin air at altitude

From:         mezei_jf@Eisner.DECUS.ORG (Jean-Francois Mezei)
Organization: DECUServe
Date:         19 Mar 96 00:44:05 
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On a popular airline web site, there is a mention that on its 747-400s,
when at altitude, the amount of air compressed into the plane weights one

I can understand that this would have an impact since it does make the
plane heavier compared to the adjacent environment (thin air at 10km
altitude) and hence does require more lift than would be needed if the
cabin were not pressurized.

When a plane is designed, is this a major factor taken into consideration
or is that added relative weight when at altitude more than compensated
by the loss of fuel to get up there ?