Whether DL or UA would have chosen to have the number of fleet types in their combined fleet that they now have is far less signfiicant than the fact that both carriers are using their resources to generate far better financial results than AA, who we might remember CHOSE to buy both A300s and 763s at the same time and is now CHOOSING to buy the A320 and 737 families - where there is even less performance difference.
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DL and UA/CO are carrying HIGHER revenues using the same or smaller/lighter aircraft on routes where they compete with AA - and that is why AA is in the financial shape it is in.
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And DL and UA have developed their route systems enough that they can match their diverse fleet to the routes they are operating.
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When AA achieves revenue on par with its peers and costs on the same metric, then the details will be something that matters.
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In the meantime, UA will carry far more revenue per flight ORD/LAX-Asia while DL's 764s generate as much revenue per flight ATL-Brazil and JFK-LHR as AA carries on its own flights JFKLHR or MIA-Brazil using a much heavier 777.
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BTW, DOT data (reported in Aviation Daily - hardly a secret source) for the year ended June 10 (I need to get fresh data but the point remains the same) shows that DL's 333s had a DOT reported aircraft CASM of 4.1 which was the lowest reported CASM for any US carrier widebody aircraft. The average stage length for DL's 333s was 3930, well within the capabilities of that aircraft type. As of that date, DL is the only carrier that reported data for the 333.
For the same period, 4 carriers - AA, UA, CO, and DL all reported data for the 777. AA and UA each reported average stage lengths for their 777s of 4400 and 3700 (rounded to nearest hundred) while CO and DL's 777s were both very close to 6000 miles.
What is noteworthy is that the average CASM for the US carrier 777 fleet was 6.7 with CO, DL, and UA all at 6.2 or 6.3. AA, OTOH, was at 7.9 with the notable difference from the other carriers being maintenance costs and 18 fewer seats than the US industry average for that fleet type.
In fact, the 764 fleet operated by CO and DL had an average seat count of 242 seats, a CASM of 5.6, and an average stage length of 3900 miles.
In JFKLHR last summer, DL carried the same amount of revenue per flight (almost to the dollar) as AA yet DL used 764s while AA used 777s with a substantial CASM penalty.
In LAXNRT last summer DL used a 333 while AA used a 777... .DL carried almost twice as much revenue per flight as AA but DL used an aircraft with a CASM substantially below AA's...suppose that market helped contribute to DL employees' profit sharing? I think so.
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SO, CO and DL are using an aircraft that could operate the vast majority of the flights AA uses ts 777s for with 98% of the number of seats and doing so with a CASM that is just 71% of AA's CASM for its 777 fleet.
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Having the right aircraft for the right mission makes all the difference in the world when it comes to the end of the quarter and costs and revenues are added up.
And arguments about the flexibility that comes w/ extra capability when it might be needed rings hollow when you consider that AA by choice, not once but twice, has ordered two similar models from Airbus and Boeing.