Of course CASM and RASM are not necessarily correlated.... but you don't need an aircraft with the capabilities that a 772ER has in order to fly 9-12 hour flights which is what the 333 does.
And just because the 772ER can carry more cargo doesn't mean you can profitably use that capacity or you offset the higher costs of the heavier aircraft.
Case in point... according to DOT data, DL's 764s operating between ATL and GRU carry more cargo per flight than AA's 772ERs operating between MIA and GRU, DL's configuration of the 764 has more seats than AA's 772s, and the 764 empty weight is almost 100K pounds less than the 772ER.
AA COULD carry alot more cargo but with multiple flights, the incremental capacity is not exploited. Many of AA's 777s are used in markets to/from LHR, GRU, and EZE where the incremental value of additional flights offsets the additional carrying capacity of the aircraft.
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The 333 can indeed fly the Pacific... DL uses it regularly on SEA-NRT (which is shorter than DFW-EZE and GRU), Japan to/from HNL, and in the summer on LAX-NRT where DL has the largest share of the LAX-Japan market. DL is also the largest carrier in the Hawaii-Japan market.
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Having a low CASM aircraft goes a very long way to providing a competitive edge... and of course that is what AA is banking on with its new aircraft orders.
The multi-billion dollar question is how AA is going to use all those new promised aircraft and grow the airline which is what they must do in order to get the benefit from their restructuring. Given that all of their cornerstone markets are highly competitive markets (with DFW facing new competition as the Wright Amendment falls and MIA having to compete w/ FLL domestically), it is not a given that AA can grow the airline as they say they will.