When Boeing executives talk about sticking to the basics in their 737 MAX reengining program, they are not kidding.

Besides the improvements they expect from the new aircraft's CFM Leap-1B engines, Boeing is keeping a tight rein on the technology risks of the biggest upgrade to the 737 since the Next Generation series was launched nearly 20 years ago. To make the MAX work, Boeing needs to distinguish it from rival Airbus's A320NEO while raising the benchmark of what its customers can expect in performance and reliability.

The MAX must be sufficiently advanced to achieve double-digit percentage point improvements in fuel burn and operating efficiency over the NG. Engines and aerodynamic improvement carry most of that burden, but there are numerous technology swap-outs that the company might include in MAX to make the airplane more attractive. However, all of them come at a price, not the least of which is their potential disruption of a finely honed manufacturing process at Boeing's Renton single-aisle jet factory south of Seattle, which is midway through the biggest increase in 737 production rates in history.

It is essential that Boeing get right this next phase in the four-decade 737 story. The family is a priceless asset and must remain so. Boeing expects 70% of all aircraft sales in the next two decades to be single-aisle transports.

Since deciding last August against its New Small Airplane (NSA) project in favor of the MAX, Boeing has been cautiously working out just how far it needs to go with technology advances in order to compete with the NEO. The company's designers felt they had a game-changing prospect in the NSA that would trump a basic engine upgrade to the A320. At first, so did Boeing's customers. But as fuel costs rose, they began asking for relief sooner than the NSA would be available. The overwhelmingly positive response Airbus gained from the NEO—it quickly shot past the 1,000-order mark—pushed Boeing to shelve the NSA.

With the MAX, Boeing wants to avoid the temptation of adding cost and complexity to the 737 program; it needs to stay focused on what airlines value most, says the MAX's chief project engineer, Michael Teal. “Customers are looking for improved economics,” he says.

Teal comes to the MAX from the Boeing 747-8 where, as chief engineer, he witnessed firsthand what can happen when unexpected issues turn a fast-tracked derivative into an over-budget development marathon. Those harsh lessons are keeping the MAX team focused on its development schedule.

Boeing Commercial Airplanes President and CEO Jim Albaugh has hinted that a service entry for the first MAX might be brought forward from the official goal of the fourth quarter of 2017. But Teal says that is only “if we can.” There will be no overconfident promises like those made early in the 787 and 747-8 programs that erupted into embarrassing schedule lapses. “I was on those phone calls in 2008, and I didn't like” them, he says.

Boeing will spend the rest of this year “getting the final concept done” before marching on to a firm configuration in mid-2013, Teal says. Design will take place in 2014, assembly in 2015 and first flight in 2016. The 737-800-sized MAX 8 is to be produced first.

This plan reflects the evolution of airline orders for single-aisle jets. In the early days of the 737NG program, the 126-149-seat 737-700 was easily the best-seller, in no small part because of its popularity with launch customer Southwest Airlines. Like many others, Southwest—also the MAX launch customer—is now ordering bigger single aisles, having moved up to the 162-189-seat 737-800. This up-gauging trend is widespread. As of May, there were 1,415 orders for the 737-700, which entered service in December 1997; for the -800, which came on the scene in April 1998, there were 4,053—more than all 737-100s, -200s, -300s, -400s, -500s and 600s combined.

There is a bigger backlog for the 737-900ER, the closest thing Boeing has to a 757 replacement, than for the -700, even though it entered service a half-decade later. Consequently, the MAX 9 will be the next to enter service, in 2018, and the MAX 7 will follow in 2019.

After considering a huge range of design options—including split trailing edges and hybrid laminar flow—Boeing's choices for the MAX underscore how it is restricting itself to a strict diet to assure as smooth a production transition as possible to the MAX from the NG.

The CFM Leap-1B engine is the principal reason Boeing anticipates a 13% reduction in fuel burn compared to the 2012-standard Next Generation 737. The engine's fan is expected to slightly exceed 69 in. in diameter, so the Leap is larger and heavier than the CFM56-7B. But that weight is more than offset by the Leap's larger 8.5:1 bypass ratio, which will contribute an anticipated 11% fuel burn benefit (see p. 61). Lower drag in the aft fuselage and introduction of novel “dual-feather” winglets account for the rest.

The all-important engine installation is an evolution for the 737 and builds on the mounting design used for the 787. The installation moves the engine “a little forward and up,” says Teal. By cantilevering the engine out ahead of the wing, Boeing is avoiding the need for a dry bay above the engine, thereby preserving fuel volume. “It's not new technology, but we wanted a little more room under the nacelle,” says Teal. The bottom of the MAX nacelle will be 17 in. off the tarmac, 1 in. less than an NG's.

The nose-wheel landing gear is 8 in. longer than the 737's and prompted Boeing to move the front bulkhead of the nose undercarriage bay—the “doghouse”—and an associated inspection hatch forward about 8 in. The leg extension also means that an aerodynamic fairing is required to accommodate the bulge of the nose wheel. “We're trying to minimize this, and we're still trying to make it smaller,” says Teal. Nonetheless, he says the aerodynamic impact is negligible.

Building on the 787 program's application of a natural laminar flow (NLF) nacelle, Boeing is “looking at opportunities as to how we can keep the NLF attached” in the MAX, Teal says. But the option of a hybrid laminar flow control system (HLFC) for the vertical fin will not be undertaken. It was originally studied as part of the interim “737NG Plus” upgrade that was to be a gap-filler between the 737NG and NSA. “There is the complexity [in the design] and the build of it, as well as [its] questionable value on short flights,” Teal explains. The HLFC system has been developed as a drag-saving device for the stretched 787-9 and, pending full development, is expected to be offered as a 787-8 performance upgrade.

The recently announced dual-feather winglet is the most distinguished external feature of the MAX. The baseline blended winglet is credited with providing a 3-4% fuel-burn improvement over a 737 without winglets. Boeing expects the MAX's feathered design to save up to 5.5% in fuel burn, or the equivalent of an additional 1-1.5% above the 737NG standard, says Teal.

The feathered winglet integrates a downward-tilted version of Boeing's raked-tip configuration with a more conventional winglet. Its ground clearance of 10 ft. 2 in. will be 2 in. greater than the NG's winglet.

The new design has undergone low-speed wind tunnel tests at Qinetiq's U.K. facility and Boeing's transonic tunnel. “The data showed it performed as expected,” says Teal. The winglet design team will perform further work on the concept as the fuselage's design details and build plans firm up in 2013. The dihedral of the winglet, combined with anhedral of the modified raked tip, assures that the MAX's wing span is within the “Code C” gate size of the 737NG family.

The fuselage's aerodynamic cleanup is focused on the very aft section just behind the auxiliary power unit (APU). This marks the first tail cone taper revision since the 737's original short, stubby fuselage design emerged in the 1960s. The redesign eliminates the need for the vortex generators that current models use to “help calm down the airflow,” says Teal. Extending the cone in a 787-style allows the flow to “clean up nicely,” according to computational fluid dynamics analysis. A new low-drag APU inlet also will be integrated into the tail, while a horizontal root fillet fairing, or “strakelet,” will be added to reduce drag around the empennage.

Although APU upgrades were considered as part of the original 737NG Plus package, Teal says none are planned for the MAX. Instead, system changes will focus on the adoption of fly-by-wire actuated wing spoilers and a digitally controlled engine bleed system for the environmental control system. “The air conditioning packs are not changing, but the control for getting bleed air is going digital,” he says. The new spoiler system will save weight and installation costs.

Since the spoilers also will be connected directly to the flight control system, they can be used for maneuver load alleviation (MLA). By symmetrically deflecting the spoilers under certain conditions, wing-bending loads are reduced. This allows use of a slightly lighter wingbox. Other wing changes were considered, including an improved trailing edge for better low-speed handling. While aspects of these studies, such as a mini-split flap, are expected to be tested as part of an upcoming EcoDemonstrator program, Teal says the MAX will not use them. “I don't think we need them,” he says.

To handle higher loads associated with the MAX's heavier operating weight, the airframe will be locally strengthened with regauging of skins, spars and structures in the fuselage, empennage, wing and landing gear. “If you have heavier engines, this increases the torsion loads into the body and these are reacted through pickle forks,” Teal says, referring to structures in the wing-fuselage join area. The existing design will be retained but “just gauged up” for the MAX, he adds.

Also under consideration is the replacement of the longitudinal beam—called a crease beam—which, in the dual-lobe configuration of the 737 fuselage, works with the floor beams to smooth out-of-plane loads at the intersection of the two lobes. “As we work through the certification basis, if the decompression analysis works out, there might be an opportunity to go to a one-piece truss,” Teal says.

By the time the MAX enters production in Renton, Boeing expects the factory to be producing 42 airplanes per month from the plant's two final assembly lines. Changes needed to accommodate the new airplane are still being considered, but the general goal is for MAX fuselages to flow seamlessly down the line with the NG's. Early planning includes the possibility of shifting an engine buildup area off Line 1 in Renton's Building 4-82 elsewhere to make room for a proving line for early MAX production.

Spirit AeroSystems provides the 737's fuselages from Wichita and is still in the early planning stages for what accommodations will be necessary for the MAX. But Vice President Forrest Urban, who leads MAX integration as head of advanced projects, says only minor tooling changes are anticipated. The company wants to avoid significant changes to the assembly process in its big Plant 2.

Fuselage alterations, such as in Section 48 or at the doghouse, will be accommodated offline and brought to the plant's final assembly, Urban says. This same approach is used for Boeing's P-8 Poseidon maritime patrol aircraft, which is based on the 737NG fuselage. Urban expects changes for the MAX to be less extensive than those for the P-8.

“We think the NG is the most efficient, highest-quality production process anywhere,” he says. To keep it that way, the company will turn to its Spirit Exact design-build software process to smooth the MAX's transition into the 737 line.

As of May, Boeing had recorded 451 MAX orders. The new program is leading the 737's charge past the 10,000-total-order mark. As of last week, Boeing was within 221 orders of that milestone, which no other commercial jet has reached. The company does not expect the head start Airbus achieved with the NEO to affect the sales balance between the A320 and 737 over the long run. To European reporters, Vice President Randy Tinseth, Boeing's head of marketing, said the MAX will build on the 737's “higher lease rates, higher 'fair market' values and higher residual values” to attract orders.

With the MAX order count growing, the marketing heat is on for both manufacturers as they head toward next month's Farnborough air show.

Footnotes: *All 737 Next Generation model data are with winglets. 737-900ER range is with two auxiliary belly fuel tanks. MAX estimates are based on a nominal 2% gain from aerodynamic and engine improvements. **With two auxiliary tanks. ***737-900ER's capacity decreases as auxiliary tanks are added to gain range. With no tanks, 1,824 cu. ft.; with 1 tank, 1,674 cu. ft.; with 2 tanks, 1,585 cu. ft. MAX 9 is expected to be the same.
By the Numbers
737NG 737 MAX
Maximum range:
737-700/MAX 7 3,440 nm ~3,500 nm*
737-800/MAX 8 3,115 nm ~3,177 nm*
737-900ER/MAX 9 3,235 nm ~3,300 nm*
Fuel capacity:
737-700/MAX 7 6,875 gal. same
737-800/MAX 8 6,875 gal. same
737-900ER/MAX 9 7,837 gal. same**
Maximum takeoff weight:
737-700/MAX 7 154,500 lb. ~157,590 lb.*
737-800/MAX 8 174,200 lb. ~177,685 lb.*
737-900ER/MAX 9 187,700 lb. ~191,454 lb.*
737-700/MAX 7 1-class 149 149
2-class 126 126
737-800/Max 8 1-class 189 189
2-class 162 162
737-900ER/MAX 9 1-class 220 215
2-class 180 180
737-700/MAX 7 966 cu. ft. same
737-800/MAX 8 1,555 cu ft. same
737-900ER/MAX 9 1,585-1,824 cu. ft. same***
Cruise speed:
737-700/MAX 7 Mach 0.785 Mach 0.79
737-800/MAX 8 Mach 0.785 Mach 0.79
737-900ER/MAX Mach 0.791 Mach 0.79
Source: Boeing