On Nov. 20, Delta Air Lines (NYSE:DAL) announced a big order for Airbus' (NASDAQOTH:EADSY) A350-900 and A330-900neo wide-body aircraft. These planes will begin arriving in 2017 and 2019, respectively. They will replace older, less efficient aircraft on long-haul transatlantic and transpacific routes.

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Delta announced an order for 50 Airbus wide-body aircraft last month. Photo: The Motley Fool

The order for 25 of each model represents a nice win for Airbus over Boeing (NYSE:BA). However, Boeing investors shouldn't be too concerned. One of the biggest reasons why Airbus won the day was that Boeing's 787 Dreamliner is so popular that it has a massive order backlog.

Delta needs some new planes
Among the big three U.S. network carriers, Delta Air Lines has the oldest fleet, as it has maintained a disciplined approach to capital investment over the last five years. Delta's wide-body fleet is made up of four different aircraft types: Boeing's 747, 767, and 777 models, and the Airbus A330.

The 777 and A330 fleets both have average ages of less than 10 years, compared to a typical lifespan of 25 years. By contrast, Delta's 747s are nearly 21 years old, on average, while its 767s are about 18 years old, on average. About a third of Delta's 767s are more than 20 years old, meaning that it's time to start thinking about replacement options.

The main competitors
The main contenders for Delta's business were the A330neo and A350 from Airbus and the 787 Dreamliner from Boeing. The A330neo is a new version of the A330 midsized wide body that Airbus first introduced about two decades ago and features new, more fuel-efficient engines. Meanwhile, the A350 and 787 Dreamliner are competing next-generation aircraft designs from Airbus and Boeing.

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Delta became one of the first buyers of the new A330neo. Photo: Airbus Group

Size-wise, the A330neo and 787 are fairly similar, while the A350 is somewhat larger. (The smaller A350 model -- the A350-900 -- is similar in size to the largest Dreamliner variant, the 787-10.) Delta was primarily weighing a combination of the A330-900neo and A350-900 against an order for the 787-9 (the midsized Dreamliner model), according to Airways News.

Comparing Delta's options
The 787-9 seats 280 passengers in a typical configuration, according to Boeing. The A330-900neo is somewhat larger and seats 300 in a standard configuration, according to Airbus. The A350-900 is even bigger, seating 315 in Airbus' standard configuration.

Larger aircraft tend to have lower unit costs, as airlines can spread expenses over more passengers. This gave Airbus an advantage in the competition with Boeing. The A330-900neo's somewhat larger size than the 787-9 and its lower acquisition cost make it very competitive on a unit cost basis despite its lower usage of weight-saving advanced materials.

Meanwhile, the A350 family is fairly similar to the 787 Dreamliner family in terms of its use of advanced materials (and thus, fuel efficiency). However, since the A350-900 is significantly larger than the 787-9, it has lower unit costs, albeit a higher per-trip cost.

Where is the biggest Dreamliner?
Notably absent from the Boeing-Airbus competition was the largest Dreamliner variant, the 787-10. This plane is roughly comparable in size to the A350-900 and a tad larger than the A330-900neo. Like most "stretched" aircraft, it has lower unit costs than its smaller siblings.

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The 787-10 has lower unit costs than the planes that Delta considered. Photo: Boeing

According to aircraft leasing legend Steven Udvar-Hazy, the 787-10 will be more fuel-efficient than the A350-900. The main trade-off is range: the 787-10 has a range of 7,000 nautical miles, compared to a range of 7,750 nautical miles for the A350-900. On the other hand, the A330-900neo's range is 6,200 nautical miles, and it will have even higher unit costs.

The 787-10 will have enough range to cover all of Delta's routes to Europe and all of its existing routes from Seattle to Asia. It wouldn't have enough range for potential new routes like Seattle-Singapore, but Delta could have mixed in a few 787-9s to serve its longest international routes. (It's also not clear that the A350-900 has enough range to serve that route.)

The real problem is that Boeing's 787-10 isn't scheduled to enter into service until 2018. Furthermore, most of the early delivery slots are committed to launch customers like United Continental, which now has orders for 27 787-10 aircraft.

Boeing also had an 850 plane backlog for the 787 series as of October. That's equal to more than seven years of production at today's rate. Even with planned production increases, Boeing doesn't have much flexibility to take big orders for 787 deliveries before 2020.

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Airbus has a shorter backlog for the A350 than Boeing's 787 backlog. Photo: Airbus Group

Delta faced a longer wait to receive the 787-9 in large quantities than for the A350-900 and A330-900neo. Yet the 787-9 would have provided little if any improvement in unit cost performance. The 787-10 offers the lowest unit costs of all, but Delta would have had to wait even longer for that model. Airbus' ability to provide fuel efficient aircraft in the 2017-2019 time frame won the day.

A good problem to have
Ultimately, this big aircraft order will be good for Delta, which will be able to replace its aging Boeing 747s and 767s with significantly more fuel-efficient planes. It also will be good for Airbus, helping it to narrow the gap between its wide-body order backlog and Boeing's.

However, it's not really bad news for Boeing. The biggest reason why it didn't win this round is that the backlog for the 787-9 -- and especially for the larger 787-10 -- stretches for years. Delta wanted planes sooner, so it went with Airbus. As Boeing ramps up 787 production and whittles down its backlog, it should have no problem selling hundreds or even thousands more Dreamliners.

Adam Levine-Weinberg owns shares of The Boeing Company. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.