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When you have a look at the fleets from big airlines like United, Delta, Lufthansa, BA, AF, etc. it seems that they have almost every model Boeing and Airbus released in their fleet. And my question is, why?

What I assume is that such a big variety would increase the costs for maintenance, training, etc.

Thus, for example, why use the A330 as well as the 777? Why operate both the A320 and 737?

For me as a passenger it is comfortable because I like to test and review different aircraft; but for these airlines' Operations divisions it must be horrible!

I know there are probably reasons to do so otherwise they would not have this variety. What are these reasons?

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There are two philosophies for fleet selection:

  1. One type / model or one manufacturer approach
  2. Multi manufacturer divide and conquer approach

First of all, a single model fleet is only possible for airlines that operate on sectors of comparable length. You can't do intercontinental and regional flights with the same aircraft model efficiently.

As you observe, there are obviously costs associated with flying different types of aircraft. A single type fleet like for example Ryanair (B738), Southwest (B73*) and Easyjet (A320 family) operate, means the cost of maintenance, training, etc can be kept low. Adding a few aircraft of another type to the fleet would mean additional tools in stock, diversified training and a separate flight crew group and lots of other cost factors.

Aircraft manufacturers know that too. So if you need another 10 - 20 aircraft, your standard supplier will ask a high price for this fleet extension, knowing that the competition is never able to put in a compelling offer when all additional cost are taken into account. Only if the batch size is large enough the competitor might be able to offset the additional costs with a unit discount.

That is where the mixed fleet operator has an advantage. Since the additional overhead costs of a couple extra aircraft from type A or B are minimal, they have a better position while negotiating with the manufacturers.

Single manufacturer airlines usually order aircraft in large numbers, otherwise they would not get a proper deal; multi type airlines can order aircraft in lower numbers.


And then there are politics. Sometimes a national airline will be subjected to political pressure to buy from a certain manufacturer. This is especially the case for airlines operating in the international / intercontinental market. The extra fleet cost incurred will be (partially) offset by deals in another field.

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  • $\begingroup$ Another advantage to a homogenous fleet is that you can sometimes buy your airframes on the secondary market, especially when you operate a very common type of aircraft. Southwest has always been a big proponent of this, often acquiring used 737s when the fleet needs expanding. $\endgroup$ – voretaq7 Oct 20 '14 at 22:02
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    $\begingroup$ @voretaq7 that secondary market is as accessible to diversified airlines isn't it or am I missing a point? In fact, getting models from the second hand market breaks the single model concept a bit since cockpits are somewhat customized to the first owner. You end up with different types of FMSes etc. $\endgroup$ – DeltaLima Oct 20 '14 at 22:07
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    $\begingroup$ @gsnedders The A318, A319, A320 and A321 are considered to be the same model (A320 family) in the way the -600 , -700, -800 and -900 are all considered 737NG models. Similar / same parts, same type rating, etc. But if you want to make the distinction you should start with Southwest, they operate the classic 737 alongside the 737NG. Only Ryanair adheres strictly to the B738 since a couple of years. $\endgroup$ – DeltaLima Oct 20 '14 at 22:24
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    $\begingroup$ @jp_ Several reasons I can think of: 1. Given the size of the order (31 large aircraft) Airbus had room to reduce total order price into the attractive range. 2) Airbus really wanted to get into the Japanese market 3) Boeing has been sole supplier to Japan for many year, probably complacent. 4) The 787 teething problems were still fresh in mind. Fear to rely on a single vendor 5) The A350 has no real competitor yet, it is available sooner then the B777X. 6) Airbus uses relatively few parts from Japan in their products -> business opportunities for Japan / politics. $\endgroup$ – DeltaLima Oct 20 '14 at 23:34
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    $\begingroup$ @gsnedders After giving it another thought, I've changed model to type in the text. Model is often used in the way I described it earlier, in the ICAO Make/Model/Series taxonomy. However in certification taxonomy, the term type is used. The A318/A319/A320/A321 have a shared type certificate. In that taxonomy the A319-114 is a model which is part of the A319-100 series. $\endgroup$ – DeltaLima Oct 21 '14 at 8:43
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One reason that I have not seen addressed in any of the extant terrific answers is acquisitions. Many times over the past few decades airlines have been sold, merged, or folded. Each time, the airlines' aircraft go to new owners who may or may not already operate acquired aircraft models at the time.

As the airlines' owners change, there is churn in which airlines operate which equipment.

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Airlines usually try to avoid having more than one type for each size exactly because it makes maintenance more complicated and expensive. Low-cost carriers go even further and operate only one type altogether.

Airlines that have diverse fleet usually have acquired them by merging with other companies that had different fleets. Note that all the companies you mention each swallowed several other airlines recently enough to still have those aircraft in active service.

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Some amount of diversity is good for business.

From time to time, an airworthiness directive might force all planes of a particular model to be grounded. This has happened for the 787, for example, due to concerns over battery safety. Even with more established models, urgent safety issues may surface. A high-profile example would be the Airbus A330 and A340 pitot tubes, after the Air France Flight 447 accident. Grounding the entire airline's fleet would be an unacceptable business risk.

Having a fleet of planes from more than one manufacturer could result in better deals during purchase negotiations.

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  • $\begingroup$ The A380 would be another example of an aircraft that at least some airlines grounded for a while in the aftermath of the uncontained engine failure in QF32. $\endgroup$ – reirab Dec 30 '14 at 21:44
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Most of the primary reasons are already covered here, but another reason can simply be availability. This is part of the reason cited for Delta's recent choice to order A350s and A330neos rather than more 787s. While Delta already has 18 787-8s on order, the lack of availability of additional order slots in the time frame that Delta needed to replace its aging 747s and 767s was, according to Boeing, part of the reason that Delta chose to order A350s and A330neos rather than adding to its existing 787 orders. Additionally, the 777X was also not going to be ready to deliver in the time frame that Delta needed.

“This was a long and highly competitive campaign,” said Doug Alder, a Boeing spokesman. “Boeing competed for the order with the 787-9, but we did not have enough 787 positions available in the time frame that met Delta’s requirement.”

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Consider sometimes that an airline doesn't own these aircraft - they may be leasing them for a short term.

Aircraft leasing comes in two flavors - ACMI "wet lease" where all the airline is doing is slapping their logo on the aircraft, everything else Aircraft, Crew, Maintenance Insurance is included; and then a "dry lease" where only the aircraft is leased.

This is normally done during busy periods (for example during the annual Hajj season) many airlines will operate a type different than their normal operations. The 747 is a favorite for leasing during this period.

So although you may be seeing the aircraft being flown by your favorite airline, the leasing cost may offset the profit for that particular route.

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And then there's airlines that have fleets of aircraft inherited from other airlines they took over/merged with.
And there's the airlines in the process of replacing their fleet with something new.
Airlines that are expanding and decide to buy new aircraft of a more modern type and keep the old ones for now because it makes no economic sense to scrap or sell them.

The reasons are plenty, even if the airline could in theory use one single type for its entire route network and use it efficiently (which use pattern would seriously limit the scope of an airline's operations, you'd not be able to have for example flights within Europe and flights from Europe to the USA and Japan).

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  • $\begingroup$ Your last paragraph is irrelevant to the question. The question doesn't ask why airlines generally don't fly a single plane type (e.g., only B737s and nothing else). It asks why there are many airlines that have directly competing planes in their fleets, e.g., the same airline owning both A320s and B737s. $\endgroup$ – David Richerby Oct 21 '14 at 13:13
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    $\begingroup$ @DavidRicherby Judging by your comments, you seem to be quite knowledgeable on the subject. Care to share an answer? $\endgroup$ – Selali Adobor Oct 21 '14 at 17:09
  • $\begingroup$ @AssortedTrailmix Thanks but the existing answers already cover everything I know. $\endgroup$ – David Richerby Oct 21 '14 at 17:18
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An airliner (a large one as you mentioned) tend to get airplanes of different manufacturers for several reasons. Some are:

Destinations

An airline's airplanes fly to destinations of varying distances and traffic loads from its hub. Operating different types of aircraft according to the needs of that particular route would drive the need of different types of airplanes.

Deals and Incentives

Airplane manufacturers may offer different deals to an airline at different times. Such business decisions are always made to a company's best interests.

Hiring and Training

Airlines tend to hire flight crew with different ratings so that will have a wider experience and proficiency.

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    $\begingroup$ This mostly doesn't answer the question. Sure, airlines need planes with different ranges and capacities, so it makes perfect sense for an airline to operate, say, Boeing 777s and Airbus A320s. But the question is asking about why an airline would fly directly competing planes, e.g., Boeing 737 and Airbus A320. And why is it an advantage for an airline to have both Boeing- and Airbus-trained pilots? That's a consequence of having a mixed fleet but it doesn't seem to have any direct advantages on its own. $\endgroup$ – David Richerby Oct 21 '14 at 13:10

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