If this whole notion of fleet commonality is SO important to be a make or break factor, then we can stop right now with any discussions about AA and US merging because their fleet will be just as complex as DL's became w/ NW and as complex as UA-CO's after their merger.
Obviously, if fleet commonality were so important, AA would not have ordered the 767 and 300 at the same time, nor would they have ordered 320 and 737 family aircraft at the same time, nor would UA have 350s and 787s both on order. WN's all 737 fleet is far from being simple - or one model.
Yes, in a perfect world, you probably would have as few aircraft types as possible. But because airlines have to continually renew their fleets and aren't going to be stuck w/ the same technology or the same manufacturer and because airlines find value in mergers, they accept fleet diversity and less simplification. And then at times you do what WN is doing which is removing the 717s from the fleet while DL finds more value in adding another engine type - power by the hour maintained IIRC - but with some airframe similarities to the rest of its fleet. Yet the 717s are designed to replace CRJ 50 seat flying being done by regional carriers... so DL will reduce its overall costs even tho it is adding another fleet type to its own fleet and will probably shift more revenue as well since passengers prefer larger aircraft and DL's network is large enough that they can and are starting to consolidate RJ flying to 6 hubs into mainline flying to 3-4.
It's not all about costs. If you can generate more revenue even with a higher cost base, you make money. THAT is the name of the game.
Remember, DL and UA are pushing $40B a year in revenue..... carrying an extra $50M in parts is not going to make or break them, although every company is looking for ways to reduce costs. Further, many aircraft parts are part of debt agreements.
UA has no intention of trying to make its fleet uniform. It has two sets of 777ERs with very different capabilities and engines and they have no intention of trying to make them identical in form or function, esp. since right now, they still have a fenced fleet from a labor perspective.
They have two sets of 757s with different engines and capabilities - 767s of different models.
And, once again, DL with its sixteen zillion aircraft configurations is still one of the most profitable airlines in the US. Whatever complexity DL has allowed to creep into its fleet costs is obviously more than offset by a higher revenue generating capability and by the ability to manage costs in other areas - including the refinery.
And, yes, Josh, ALL of DL's 757s are Pratt powered and a pilot on one 757 can fleet any one of them.... so the parts differences comes down to stuff that is not an everyday part of the operation of the airline.
DL's 764s have different cockpits but will have identical cabins to the 763ERs and similar family engines to about half of the 763ER fleet, which DL intentionally equipped with 2 different types of engines and then went out and bought used aircraft with the "newer" engine type.
And DL, just like AA, does isolate certain fleet types to certain parts of its network which reduces costs not only for parts but for a whole lot of other reasons from ramp operations to pilot bases.
And DL's 757 and 767 fleet is well over 200 units....that's alot of commonality from a pilot scheduling standpoint.
Tax deductions only work if you are making money - and the expense of the interest payment is far larger than the profits any airline is generating. If AA generates $2B/year in profits, then fleet simplification or not will not matter. And not all corporate interest IS tax deductible.
All companies, not just airlines, source parts from outside companies.... your point that parts can be obtained from outside vendors for overhauls is just as valid for day to day operations of the airline. Aircraft parts even for line functions are shipped all over the world on a daily basis - and many of those parts are not necessarily held in inventory by an airline. And as you also know, airlines loan parts to each other.
When airlines have 500 plus aircraft in their fleet as AA, DL, and UA all do, the standard arguments don't hold quite as true as they once did.
But once again if fleet simplifcation is so important, then let's call off this AA-US merger nonsense. Yesterday.
