As interest in higher-capacity models grows in every sector of the air transport business from new turboprops to the largest twinjets, even some of the latest models are clearly losing out. But unlike in earlier cases, demand is weak for the smallest narrowbody variants—years before first deliveries.

The smallest members of the Airbus and Boeing single-aisle series have become the proverbial orphans of the family. Sandwiched between their larger-capacity brethren (top photo) and an emerging generation of purpose-designed small airliners (bottom photo), the A319NEO and the 737 MAX 7 series also are competing for the first time in the same general size category with new products from China and Russia. But the most imminent competition is coming from Bombardier's CSeries.

More than two-and-a-half years after the NEO was launched—with stellar sales including 2,125 firm orders plus almost 800 options and other commitments—a mere 45 A319NEOs have been selected. The situation is even gloomier for the 737-7. Until May 15, it had not booked a single order since the MAX was launched in 2011. Following the decision by Southwest Airlines to convert 30 previously announced orders to the 737-7, the short-bodied model now accounts for only 2% of the 1,315 firm MAX orders as of late May.

Boeing is no stranger to the phenomenon, having seen orders for the 737-800 outstrip those for the 737-700 by more than three-to-one. As a natural replacement for trunk-route equipment such as the Boeing 727-200 and MD-80, the -800 has sold more than 4,220 compared with 1,450 for the -700. The omens also seem favorable for the identically sized 737-8 which has so far accumulated firm orders for 1,121, or 85% of the entire MAX orderbook.

The gradual change in preference for the larger models is illustrated by the fate of the 737-600 which was developed as the Next Generation version of the 737-500. Although the -500 sold moderately well, accumulating sales of 389, the unfortunate 737-600 managed a mere 69 orders and now appears to be no longer offered. The 737-600 was also aimed at the sales-challenged A318 which, at the time of its development, also competed with the MD-95/Boeing 717. With even more options capable of competing directly with the 737-7, Boeing may be wondering if it is about to experience a repeat of the fate of the -600, albeit in a larger size category.

In the current-engine-option A320 family, the A320 has accumulated by far the most orders, roughly two-thirds of the total. Airbus has sold more A319s (1,477) than A321s (1,228) of the current-generation aircraft and with its total orderbook, the A319 has been a significant commercial success in its own right. But Airbus points out that market dynamics have changed dramatically over the past five years. Airlines have placed a much greater emphasis on unit costs and are moving away from the smallest members of the narrowbody families, which have naturally the highest unit cost in that market segment.

As far as Airbus is concerned, production is being adapted accordingly. It has provisions in place, mainly as far as its supply chain is concerned, to move production of the A321 up to 18 a month. That is almost half of the entire A320 output, which is now at 42 aircraft per month, and double the current capacity for the A321. A319 production continues, but its backlog is shrinking and now stands at only 109 aircraft in total (not counting the 45 A319NEOs). The supply chain has to be prepared to build parts for an unprecedented number of A321s even as Airbus prepares to roll over to the A321NEO. The manufacturer also must deal with rapidly declining rates for the A319. The A318, of which only 79 have been built, is not offered in the NEO version.

A similar trend is evident in most other parts of the market as well. Airbus first had serious trouble in marketing the A350-800, the smallest member of the A350 family, and now appears to be trying to shift -800 customers to the larger -900 and -1000. But some airlines believe the -800 is about all they can fill. Airbus had earlier shifted the design optimum to the -900, yet it is contractually obliged to build the aircraft and officially says the family will consist of three types.

The A330 initially experienced stronger sales for the -200, but interest has now almost completely shifted to the larger -300. That trend has been supported by Airbus, which has given the aircraft major increases in its maximum takeoff weight and range.

The same is true for other manufacturers. Boeing 777-300ER (687 orders) has now surpassed the 777-200ER/LR (481) and that gap is expected to widen in the next few years. The 787-8 (535) still leads the 787-9 (355), but the larger variant in the family is not even due to enter service until 2014. And there are some indications that even more airlines are seriously considering the -9. Boeing has also sold five times as many 767-300ERs than -200ERs.

There are two noticeable exceptions: Of the 1,049 orders for the 757, only 55 were for the stretched 757-300. While the aircraft initially looked to be a possible solution for high-demand short-haul routes (and an Airbus A300/A310 replacement) airlines soon found out that turnaround times for the long single-aisle were insufficient. The aircraft is mainly used by charter carriers that, unlike scheduled airliners, can board and de-board through the front and the rear doors. And there were only 38 orders for the 767-400ER, which lacked the range to operate transatlantic routes from places beyond the U.S. East Coast.

Regional manufacturers have witnessed up-gauging trends, too. Embraer has seen customers for its E-Jet family shift from the smaller 170 and 175 models to the larger 190s and 195s. Bombardier has stretched the initial CRJ100/200, a 50-seater, to become the 98-seat CRJ1000. And as far as the CSeries is concerned, airlines have ordered more CS300s (82) than CS100s (63). The CS300 can seat 160 people in a high-density layout; the CS100 is designed for 110 in a standard configuration.

So while the trend can be observed in almost every program, there is a difference between what is happening with the 737-7 and the A319NEO compared with previous cases: Airlines do not even start ordering the types. The discrepancy in demand is much sharper and exists from the very beginning. But the question is whether this is happening because airlines are moving to better alternatives (stretches of smaller aircraft that have been optimized for the size or new designs) or because market fundamentals are shifting more rapidly than in the past.

Many indications point to the latter. If there was a shift from one manufacturer to another, Canada-based Bombardier or newly emerging Comac from China would surely have noticed. But the CSeries is still stuck at a mere 145 firm orders only weeks before its first flight is expected. And a look at its client base shows that Bombardier has, so far, not succeeded in making inroads into the mainline carrier market. Comac has even fewer firm orders, 75, all of which are from Chinese airlines, and its offering appears to be several years away from entry into service.

While both Boeing and Airbus are recording more orders for the larger variants of their narrowbodies, they have not seriously considered stretching the airframes further. Both are invested in fitting more passengers into the 737-900 and the A321, respectively. Airbus recently announced plans to add an optional overwing exit to the A321 and move the third door backward so that the aircraft can accommodate 236 passengers—up from 220 currently—in a high-density layout.

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Orders for New Narrowbodies
Aircraft Firm Orders
A319NEO 45
A320NEO 1,631
A321NEO 449
C919 75
CS100 63
CS300 82
737-7 30
737-8 1,121
737-9 164
Sources: Company reports, Aviation Week Intelligence Network