Originally Posted by fireman0174
While you are correct that different aircraft size (meaning seats available) permits a carrier to offer the properly sized aircraft in terms of the market requirements for different times of the day, SWA can do that with their various B737 models and save a whole lot of $$$ in the process.
That's a big advantage and credit for that must go to their management.
You've got to choose the right tool for the job. United has used 'wingtip' service to various cities in the past (and present) to make up for the shortage of wider guaged aircraft. The 'wingtip' service would be, for instance, a 737 and an A320 departing within 15 minutes of each other on the exact same route. I used to fly on the LAX-DEN route as a nonrev, and I remember both a 737 and the 757 departing 15 minutes later packed to the gills.
The daily 777 run between ORD and IAD is ALWAYS packed both ways.
It is a matter of choosing the proper tool for the job. You wouldn't use a 777 on a flight from DEN-ABQ and you wouldn't use a 737 on a flight from SFO-NRT. Operations the size of UAL/AMR/DAL/NWA require choosing the proper aircraft guage in order to maximize profits (actually, minimize losses). This is not done by you and I, but is analyzed by computers which weigh a number of factors, some of which you and I would not think of. This isn't throwing a dart at a dartboard and deciding which guage aircraft will fly the BWI-DEN route.
Pilotpip mentioned stocking parts for different aircraft. Those parts are stocked at strategic locations (ie hubs) and moved to out bases when needed. United's biggest bench stock is in SFO.
He also mentioned different towbars. IIRC, it is not different towbars, but different towbar pins that are required for different aircraft. The most expensive towbar pin in United's fleet is the Airbus pin.
The additional costs of carrying multiple fleet types is more than outweighed by the enhanced revenues from being able to choose the proper tool for the job.