Delta Air Lines (NYSE: DAL) is one of only two U.S. airlines -- along with United Continental (NYSE: UAL) -- still flying the iconic Boeing (NYSE: BA) 747 jumbo jet. But last year, Delta announced plans to phase out all of its remaining 747s by the end of 2017.

Earlier this month, it accelerated the timeline. After grounding a quarter of its 747 fleet late last year, Delta will now remove another six 747s from service in 2015. That will leave it with just six 747s actively flying.

The rapidly dwindling popularity of what was once its flagship plane might seem like bad news for Boeing. However, since Boeing also makes the best 747 replacement, airlines' eagerness to dump their old jumbo jets could actually be a blessing in disguise.

Departing from its usual strategy
Delta's decision to phase out its 747s by 2017 was somewhat surprising because while some of Delta's 747s date from the late 1980s, others were built as recently as 2002. Delta has distinguished itself from its main competitors in recent years by keeping older planes flying longer -- in some cases, for more than 30 years.

Thus, the early retirement of its 747s represents a clear departure from Delta's normal strategy. (In fact, the company had to take a $397 million charge last year to write down the value of its 747s in connection with this decision.)

However, jumbo jets like the Boeing 747 aren't very cost-effective airplanes to fly. Because they have four engines -- compared to two engines for most airplanes, including relatively large widebodies like the Boeing 777 -- jumbo jets tend to use significantly more fuel. Having two extra engines also drives up maintenance costs.

Meanwhile, because they are so big -- Delta and United Continental both have more than 370 seats on their 747s, and less than 300 on virtually all of their other planes -- it's often necessary to offer lower fares in order to fill a 747. Higher unit costs and lower unit revenue is a recipe for weak profitability.

There are better alternatives available
Delta is replacing its retiring 747s with new Airbus A330s that it is receiving beginning this year, as well as aircraft freed up from international routes where Delta is scaling back.

Beginning in 2017, Delta will also add the new Airbus A350-900 to its fleet. This plane will replace the 747 on some of the longer-haul routes that the A330 can't manage. The A350s are expected to have a 20% lower cost per seat than Delta's Boeing 747s while generating better unit revenue.

For the moment, United Continental plans to keep most of its remaining 747s around until about 2020. But on Thursday, it confirmed it had converted some of its Boeing 787 orders to the larger Boeing 777-300ER. This could be a very attractive 747 replacement, especially if United follows the growing trend of configuring these planes with 10 seats across in economy, rather than the traditional, roomier, nine-across seating plan.

The Boeing 777X is the real game-changer
Some carriers, such as Delta, are eager to dump their 747s in favor of smaller widebodies in order to bolster unit revenue. However, for airlines that don't mind the large size of the last-generation 747-400, Boeing's forthcoming 777X will be an even more appealing aircraft.

The Boeing 777X will be slightly wider, allowing airlines to put 10 seats in each economy row without compromising too much on elbow and shoulder room. Moreover, the 777-9X will be the largest twin-engine plane ever, with space for more than 400 seats in the standard configuration.

In essence, the 777X will be able to do just about everything that the 747-400 does. But thanks to its use of advanced materials and its next-generation engines, it will cost much less to operate. So, it's not surprising that the 777X has already attracted 286 firm orders, even though it won't be available until around 2020.

As 777X development proceeds, airlines become more comfortable with its capabilities and technology, and potential customers figure out their post-2020 fleet plans, the 777X order book will likely rise. Boeing will gladly deliver the 747's final death blow by cannibalizing the jumbo jet market with the 777X -- the latter is likely to become a big cash cow for the aerospace giant a decade from now.

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Adam Levine-Weinberg has the following options: long January 2017 $40 calls on Delta Air Lines, Inc.

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