The Aircraft

Aspects of the 737 MAX design, from aerodynamics to new systems designed to help maintenance

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The 737 MAX is similar to the 737NG in many respects but also has some innovations including the LEAP-1B engines and AT Winglets.

When launching a refreshed version of an established product, aircraft manufacturers need to strike a balance. They must introduce new technologies that will make a tangible difference to the aircraft and its economics. Equally, they also must retain the elements of the aircraft that make it popular with customers in the first place.

Striking this balance between innovation and commonality is apparent in the design of the 737 MAX. This is an aircraft that has several differences from the previous 737NG, but also retains a high degree of commonality with its predecessor.

Lower Fuel Burn

The engines are one of the major differences. The MAX has CFM International LEAP-1B turbofans instead of the CFM56- 7Bs on the NG. The new engines are one of the most notable visual changes between the two generations, with the LEAP-1Bs boasting a larger fan diameter of 69.4in (1,762mm) compared to the 61in (1,550mm) fan diameter of the CFM56-7Bs.

Larger engines meant that Boeing had to extend the nose landing gear by 8in (200mm) to ensure sufficient clearance between the LEAP-1Bs and the ground, and this has resulted in the MAX sitting slightly higher on the ground compared to the NG.

Boeing says the integration between the engines and the wing has also been improved to reduce drag and contribute to fuel efficiency. Michael Teal, Chief Engineer on the MAX, told AIR International that the LEAP-1Bs also have an all-new nacelle and secondary chevrons on the thrust reverser to reduce noise.

Boeing says the MAX will burn 14% less fuel than the NG (or 20% less compared to the first NGs produced in the 1990s). The LEAP-1Bs account for 11% of the reduction. Boeing claims that when compared to a fleet of 100 NGs the MAX will emit 310,000 tonnes less carbon dioxide and save 215 million pounds (97,522kg) of fuel per year.

In addition to lower fuel burn and emissions, the MAX also offers increased performance. Maximum take-off weight, maximum zero fuel weight and maximum landing weight for each variant in the MAX family have all increased from the previous generation.

These increases have led to, Teal said, “increased material sizing to support higher weights” and “local changes to the gear doghouse and radar bay to enable [landing] gear growth”.

AT Winglet

The all-new Advanced Technology (AT) Winglets are another important change. AT Winglets differ from the NG’s Blended Winglets by combining upwards-facing and downwards-facing aerofoils, rather than just the upwards-facing aerofoil that the Blended Winglet has.

As with the Blended Winglet, the upwardsfacing aerofoil on the AT Winglet is designed to vector the inward, upward and slightly forward lift components of the airflow around the wing to create lift and reduce lift-induced drag. However, friction caused by the airflow over a winglet itself creates a certain amount of drag. This explains the presence of the AT Winglet’s lower aerofoil. This is designed to generate a further vertical lift component that is then vectored away from the fuselage and slightly forwards.

The combination of this vertical lift component generated by the lower aerofoil and the benefits of the upper aerofoil result in what Boeing claims on its website to be “the perfectly balanced winglet that maximises the overall efficiency of the wing”.

Laminar flow also contributes to a wing’s efficiency; if the flow is laminar, or smooth, there’s less drag. The MAX design team incorporated advanced natural laminar flow technology – some of which was researched using the Boeing ecoDemonstrator Programme – into the surface material specification for the AT Winglet. Boeing says: “The overall combination of advanced design and advanced airflow control means the AT Winglet delivers the greatest contribution to improved fuel efficiency of any winglet.” Boeing says the AT Winglets will contribute 1.8% of the overall 14% fuel efficiency improvement from the NG to the MAX.

There are other seemingly subtle, but significant differences in the MAX’s design from the NG. There is an extended tail cone housing a redesigned auxiliary power unit inlet and a thickening of the tail cross-section above the elevator to improve the steadiness of the airflow around the tail. Boeing says these changes will eliminate the need for vortex generators on the tail and reduce drag by 1%, contributing to the fuel burn improvement.

Spoilers are electronically controlled, and a Honeywell electronic bleed air system provides wing anti-ice protection and cabin pressurisation and environmental controls. On the NG, the spoilers and bleed air system were all hydraulically controlled. Using electronic controls means the number of hydraulic components such as valves and ducting that were required for these systems has been reduced, which translates into a weight saving and a fuel efficiency gain.

Flight Deck

The MAX flight deck has four 15.1in (383mm) multifunction displays (MFDs) rather than the six smaller displays on the NG’s flight deck. Teal said: “The new large displays package brings a modern feel to the 737’s flight deck and provide growth opportunities for future avionics and crew information updates.”

Captain Ed Wilson, the chief pilot for the 737 at Boeing, told AIR International that the larger size of the MFDs has made a noticeable difference: “The primary flight display becomes a larger format display, so you can see it easier [and] make a better judgement about the aircraft attitude; and because we can use an entire screen now, the navigation display just becomes easier to interpret.

“Where we follow what we call our magenta line, where we programme our flight, we can see across the navigation display. The navigation display on the NG only goes down to five miles [8km] range. Now we can go down to half-mile [0.8km] range to see more precisely, and there’s no upper range defined.

“It’s a more enjoyable experience to fly with larger displays. The younger pilots that come along are more used to having larger displays in front of them. I think it’s important that we continue to upgrade the aircraft to more what our pilots will like and use in the future.”

With the 737 MAX 7 variant optimised for hot and high performance, it features what Boeing calls a high-altitude package designed for those environments. This features engines with higher thrust, an alternate forward centre of gravity for improved take-off weight, an auxiliary battery for 60-minute standby power, a gaseous oxygen system with up to 12 cylinders, and advanced avionics for Required Navigation Performance Authorisation Required.

Onboard Network System

One of the most important innovations in the MAX is the onboard network system (ONS), which collects data about the aircraft and makes it available to an airline’s flight, cabin and maintenance crews.

Teal explained: “The heart of the system is the installation of a network file server [NFS] connected to the digital flight data acquisition system [DFDAS]. Legacy NG systems and the new unique MAX systems are connected to NFS to allow management of offboard data communications, software data loading, engine trim balancing and more.”

The NFS is located in the electronics equipment bay. It houses hundreds of gigabytes of data about system despatch status, existing faults, initiated tests, configuration reporting and maintenance page information. It also hosts onboard and offboard data processing functions. The DFDAS increases the availability of the data to be used for onboard functions and offboard analytics, and makes available 100 times more data than the legacy equipment it replaces.

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The four large multifunction displays give the 737 MAX a very different look from the 737NG’s flight deck, but only one panel has been moved. There are no changes to the checklists or flow patterns that pilots use.

The benefit of the ONS is not just the amount of information it generates, but also the accessibility to that information it provides. The data collected during a flight is all centralised by the ONS, and when the aircraft is on the ground after a flight, an airline’s engineering staff can use the flight deck’s built-in test equipment to access it through the cockpit MFDs. This enables engineers to perform maintenance and fault isolation tasks without needing to go physically to the electronics equipment bay to check a system. The idea is to make monitoring and troubleshooting more efficient.

KEY CHANGES FROM NG TO MAX

CFM International LEAP-1B engines

Advanced Technology Winglet

Onboard Network System

Four large-format cockpit multifunction displays

Connectivity has been designed into the system to link the ONS to the crew and an airline’s ground infrastructure. A secure wi-fi network aboard the aircraft is available that can integrate with mobile devices, meaning the ONS can be accessed for paperless operational and maintenance procedures.

Broadband internet protocol (IP) connectivity enables the wireless transfer of aircraft data or software parts between the NFS and the airline’s offices while the MAX is on the ground. The ONS is also capable of integrating with IP-based connectivity systems, such as L-band, Ku-band and Ka-band satellite communications systems, to allow for the secure, high-speed transfer of data to and from the aircraft during flight.

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Boeing is promising the 737 MAX 8 will burn 14% less fuel compared to a 737NG, with the LEAP-1B engines contributing 11% of the reduction.

Commonality

The ONS, the aerodynamic changes, the larger cockpit displays and the engines are innovations in the MAX that constitute a major refresh from the 737s of the past, but Boeing has also sought to maximise commonality with the NG.

Teal noted: “Structural architecture remains common with the NG. By keeping the similar structural architecture, we were able to use the existing production system and supply chain.”

Keeping the airframe common, and therefore retaining the existing support arrangements, matters because it means that the established NG operators that have ordered the MAX in big numbers – examples include Southwest Airlines and Ryanair – should be able to integrate the new 737 into their fleets efficiently.

The flight deck provides a good example of the benefits of this commonality. Despite the MAX having those four large MFDs instead of the smaller displays, Capt Wilson told AIR International that just one panel – which houses the switches that control the autobrakes – has been moved in the cockpit, to the lower aisle stand in front of the thrust levers. Everything else, including the overhead panel, is in the same position as on the NG.

This matters to an operator, because it means there are no changes to the checklists or flow patterns that pilots use to operate the aircraft – and that is especially important for customers that will operate both the NG and the MAX in terms of training and then rostering their line pilots on to either model.

Capt Wilson explained: “At this point [December 2016], we have provisional approval from the Federal Aviation Administration to only use as little as twoand-a-half hours’ computer-based training to transition from an NG to a MAX. There’s no flight time required.

“We fully anticipate that airlines will fly both types of aircraft. Some of our biggest customers, say, Southwest and Ryanair, have ordered a considerable number of airplanes and they’ll fly a considerable number of NGs still, so their pilots should be very capable of moving from aircraft to aircraft without any trouble at all.”

Another aspect of commonality between the NG and MAX flight decks is provision for Boeing’s integrated approach navigation functionality. This is designed to provide instrument landing system-like procedures, display features and autopilot control laws for non-precision approaches. It is an option on the NG and remains so on the MAX for customers who want it.

Capt Wilson said there is also a high level of commonality with the NG in how the MAX handles: “It flies almost identically to an NG. I flew with a pilot who had never flown the aircraft and is currently on our tanker testing. He’s qualified on the 737, but he’s been flying the KC-46. He came down from the flight and said he couldn’t tell the difference between the two aircraft. He did not notice any difference in handling qualities. That’s a big pat on the back for the engineers in how they were able to design the aircraft to keep everything similar to the NG.”

Boeing Sky Interior

The similarities between the MAX and its predecessor continue in the passenger cabin. The MAX has as standard the Boeing Sky Interior, which was introduced as an option on the NG in 2011. The Sky Interior introduced sculpted sidewalls, larger windows and bigger overhead stowage bins, the idea being to create a lighter, airier feel to the cabin.

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The upwards-facing aerofoil on the AT Winglet vectors the inward, upward and slightly forward lift components of the airflow around the wing and the lower aerofoil generates a further vertical lift component that is vectored away from the fuselage and slightly forwards.

Here again, though, alongside the commonality there are touches of innovation. Although the MAX’s cabin will not feature the lower pressure altitude of the 787 Dreamliner, the MAX’s electronic bleed air system is designed to provide a more constant flow of air to the cabin compared to that in an NG.

The Sky Interior in the MAX will also have what Boeing calls Space Bins. These are larger overhead stowage bins that will carry six standard-sized bags, two more than the bins in the Sky Interior on the NG. Boeing says: “Airlines can expect quicker boarding and turnaround processes at the gate. With a lower bin lip height, Space Bins provide increased visibility into the back of the bins and make bag loading even easier. They’re also as easy to close as the current pivot bins, yet require no bin assist mechanism to facilitate closure.”

This combination of improving the established Sky Interior sums up Boeing’s overall approach with the MAX – take what are considered the best parts of the NG and then make certain modernisations to give the latest aircraft an edge on its predecessor. In other words, trying to find that balance between innovation and commonality.

Mark Broadbent

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The 69.4in fan diameter on the LEAP- 1B engines meant that Boeing had to extend the nose landing gear by 8in to ensure sufficient ground clearance.