Boeing 737 Family, from NG to MAX

Boeing 737 is a twin-engine narrow-body aircraft manufactured by Boeing. The 737 continued to evolve into many variants but still remains recognizable as the 737. These are divided into four generations but all are based on the same basic design.

• 737 Original series (first generation) – launched in 1965.
• 737 Classic series (second generation) – launched in 1979.
• 737 NG series (third generation) – launched in 1993.
• 737 MAX series (fourth generation) –  launched in 2011.

Boeing 737 family

• Original 737-100/200: The first generation of 737s, launched in 1965. These aircraft were relatively small and short-range, with a capacity of up to 130 passengers.
• Classic 737-300/400/500: The second generation of 737s, launched in 1979. These aircraft were larger and longer-range than the originals, with a capacity of up to 189 passengers.
• Next-Generation 737-600/700/800/900: The third generation of 737s, launched in 1993. These aircraft were significantly redesigned with new engines, wings, and fuselage, offering improved fuel efficiency, range, and passenger comfort. They have a capacity of up to 215 passengers.
• 737 MAX: The fourth generation of 737s, launched in 2011. These aircraft incorporate further improvements in fuel efficiency, range, and passenger comfort compared to the Next-Generation 737s. They are available in four variants: the 737 MAX 7, 8, 9, and 10, with a capacity of up to 236 passengers.

Boeing 737 NG

The 737NG is an upgraded version of the 737 Classic model. The Boeing 737 Next Generation, abbreviated as 737 Next Gen or 737NG, is the name given to the main models 737-600/700/800/900 series and the extended range -700ER/900ER variants of the Boeing 737 family.

Currently, the 737NG series includes three variants (737-700/800/900) and can seat between 149 and 220 passengers. The Boeing 737NG’s primary competition is with the Airbus A320 family aircraft.

737 NG Technical Specs

The Boeing Next-Generation 737 is powered by two CFM56-7B engines, manufactured by CFM International.

CFM56-7B Engine

The CFM56-7B is the exclusive engine for the Boeing Next-Generation single-aisle airliner. The CFM56-7B engine is a high by-pass, axial flow, dual rotor, two-spool, advanced technology turbofan engine. It is supported by the aircraft’s wing pylon.

Two CFM56-7B engines supply thrust for the airplane and also supply power for the Electric, Hydraulic, and Pneumatic systems. The CFM56-7B is a high bypass ratio, two-spool, turbofan engine. The CFM56-7B operates through a FADEC (Full Authority Digital Engine Control) system. The CFM56-7B uses the “On Condition Maintenance” concept, which means the engine has no periodic overhaul schedules. The engine can remain installed on the aircraft until something important occurs or when the lifetime limits of parts are reached. For this reason, monitoring and maintaining the health of the engine is very important, for different types of tools are available which are:

• Engine performance trend monitoring
• Borescope inspection
• Lubrication particles analysis
• Engine vibration monitoring system

The AlliedSignal 131-9(B) auxiliary power unit (APU) is installed on Boeing 737NG aircraft. APU supplies electrical and pneumatic power to other airplane systems.

Boeing 737 MAX

The Boeing 737 Max is a narrow-body aircraft developed by Boeing as an upgrade to the successful Boeing 737 Next Generation series. The aircraft was designed to compete with the Airbus A320neo family, offering improved fuel efficiency and operating economics.

The 737 MAX (737-8) made its first flight on January 29, 2016, and entered commercial service with launch customer Malindo Air in May 2017.

The Boeing 737 MAX is the fourth generation of the Boeing 737. The MAX comes in four variants: MAX 7, MAX 8, MAX 9, and MAX 10, catering to different range and passenger capacity needs (138-230 seats). MAX 8 is the most popular.

The 737 MAX series has been offered in four variants:

• 737 MAX 7: The smallest variant mainly for regional routes, replacing the 737-700.
• 737 MAX 8: The most popular variant, replacing the 737-800.
• 737 MAX 9: A stretched version of the MAX 8, competing with Airbus A321neo.
• 737 MAX 10: Largest and longest-range MAX variant, competing with Airbus A321neoLR.

737 MAX Technical Specs

Major Changes in 737 MAX

The following changes were made to the 737MAX model compared to older variants (737NG):

• Redesigned winglets
• Updated CFM56 Leap-1B engines
• Larger engine composite fan blades
• Redesigned engine nacelle and strut
• Redesigned forward equipment compartment
• Relocated PSEU, WXR, and FQPU
• Large format displays
• Longer tailcone
• Strengthened wing, fuselage, and stabalizers
• Aft body vortex generators removed
• Strengthened landing gear
• Electrical landing gear selector
• Nose landing gear 8″ longer
• Longer nose landing gear doors
• Digital bleed air control system
• New APU inlet and door
• APU cooling eductor removed
• Fly-By-Wire flight spoilers
• Direct lift control for spoilers
• 800 NG A/C in all models

Key enhancements in 737 MAX

The Boeing 737 MAX has several key enhancements over its predecessors (737 NG):

Advanced Technology Winglets

The Boeing Advanced Technology (AT) winglet on the 737 MAX is a unique design that combines features from blended, split-scimitar, and raked winglets. It’s the most efficient winglet ever designed for a production airplane, because of the combination of advanced design and manufacturing techniques allowing for natural laminar flow.

Split-tip winglets improved aerodynamic performance and reduced drag. The winglets reduce fuel burn by approximately 2 percent.

Without any winglets, the airflow over the tip of every wing rolls up from the high-pressure area under the wing to the low-pressure area above it. This forms vortices that cause lift-induced drag, lowering the efficiency of the wing.

Efficient Engines

The Boeing 737 MAX is powered by CFM International LEAP (Leading Edge Aviation Propulsion) engines, which are high-bypass ratio turbofan engines. Here’s how they contribute to the efficiency of the Boeing 737 MAX:

• High Propulsive Efficiency: The CFM LEAP engine has high propulsive efficiency, which is the engine’s efficiency in transferring the generated horsepower to effective thrust and propelling the aircraft forward. The LEAP-1B engine has a bypass ratio of 9:1, which makes it capable of moving a large air mass from the inlet to the exit at a much lower speed. This results in greater propulsive efficiency of the engine.
• Large Fan Size: The LEAP-1B has a larger fan size (69 inches) compared to the CFM56-7 (61 inches). A larger fan can move more air, which increases the bypass ratio. A higher bypass ratio generally leads to better fuel efficiency because more air is moved by the fan compared to the amount of air that goes through the engine core. However, the bypass ratio alone doesn’t tell the whole story.
• Fuel Consumption: For the Boeing 737 MAX 8 aircraft, the LEAP engines consume approximately 0.55 lb (0.25 kg) of fuel per generated lbf of thrust an hour. The fuel consumption of a 737 MAX 8 is nearly 15% lower than a typical Boeing 737-800.
• Fan Blade Design: The LEAP’s fan blades are made from 3D woven Carbon Fiber Reinforced Plastic (CFRP) composite, making them lighter and stronger than traditional metallic blades. This weight reduction translates to improved fuel efficiency. Their swept design further optimizes airflow and contributes to reduced noise.

Overall, Boeing claims the 737 MAX provides 15-20% greater fuel efficiency than its predecessors (737 NG). A significant contribution to this fuel efficiency goes to the ultra-efficient LEAP engines.

Advanced Flight Deck

The Advanced Flight Deck in the Boeing 737 MAX is a significant upgrade from previous models, featuring four large 15-inch displays. These displays are common with the 787 Dreamliner and 777X, providing a consistent interface across different Boeing aircraft.

Here are some key features of the Advanced Flight Deck:

• The large display provides pilots with a clear and comprehensive view of critical flight information, including navigation charts, engine data, weather warnings, and system status. This expansive view minimizes head movement and reduces pilot workload.
• Standalone equipment like clocks and certain switches and functions have been incorporated into the displays, freeing up space and reducing complexity.
• More data presented on each screen reduces the need for pilots to toggle between different displays, enhancing situational awareness.
• Intuitive layout and information presentation minimize workload, allowing pilots to focus on critical decision-making.

Improved Cabin Experience

The Boeing 737 MAX features several improvements to the cabin experience compared to its predecessors, aiming to enhance passenger comfort and well-being throughout the flight. Here are some key highlights:

• Boeing Sky Interior: This design philosophy incorporates sculpted sidewalls and window reveals, LED lighting with mood-setting capabilities, and larger overhead bins for improved spaciousness and a more modern feel.
• Larger Windows: Increased window size provides passengers with a better sense of space and improved natural light, enhancing the flying experience. Boeing claims it is 20 percent larger than its competitors.
• Quieter cabin: The use of advanced engines and airframe design contributes to a quieter cabin environment, reducing noise fatigue for passengers and contributing to a more pleasant audio experience in the cabin.

Lowest Maintenance Costs

Boeing claims the 737 MAX has up to 14% lower airframe maintenance costs than the A320neo. Several factors contribute to this advantage:

• Simpler design: The 737 MAX retains a more traditional “mechanical” design similar to earlier 737 models, relying on cables and linkages for control systems. This approach, while considered less advanced, can be easier and cheaper to maintain compared to the A320neo’s fly-by-wire system. In the Boeing 737 MAX, only the spoiler system is controlled by fly-by-wire.
• Commonality across variants: The 737 MAX family shares a high degree of commonality across its variants (MAX 7, 8, 9, 10), allowing airlines to reduce spare parts inventories and streamline maintenance procedures.
• Extensive use of composites: The 737 MAX incorporates more composite materials in its airframe compared to the A320neo. While composites can be lighter and more corrosion-resistant, their repair and maintenance can be more complex and expensive. However, Boeing claims their manufacturing techniques and design choices optimize composite usage for lower maintenance costs.

Note: Boeing itself claims that the 737 MAX uses “more composite materials” compared to the A320neo. This claim is often cited in industry publications and news articles. However, specific details about the extent of this difference are not readily available. Airbus has stated that the A320neo family incorporates “up to 53% composite materials by weight,” and emphasizes the extensive use of composites in its vertical stabilizer, horizontal stabilizer, and wing leading edge (source: Airbus A320neo brochure).

It’s important to remember that the claim of 14% lower airframe maintenance costs is just one element to consider when comparing the 737 MAX and A320neo. Airlines need to evaluate multiple factors based on their specific needs and operating environment to make an informed decision on which aircraft best suits their requirements.

Boeing 737 MAX Groundings

The 737 MAX series faced significant challenges following two major incidents: Lion Air Flight 610 in October 2018 and Ethiopian Airlines Flight 302 in March 2019, resulting in the tragic loss of all passengers and crew on board. Investigations revealed issues with the Maneuvering Characteristics Augmentation System (MCAS), an automated system designed to prevent stalling, which contributed to both accidents.

As a result, the 737 Max was grounded worldwide in March 2019, leading to a comprehensive review of its design, systems, and certification process. After extensive modifications and regulatory approvals, the 737 Max was recertified by aviation authorities and began returning to service with airlines in late 2020 and early 2021.

Boeing implemented modifications to the MCAS system, enhanced pilot training, and improved safety features to address the identified issues.

How did the MCAS cause accidents?

The Maneuvering Characteristics Augmentation System (MCAS) is a flight-stabilizing feature developed by Boeing. It was specifically designed for the Boeing 737 MAX aircraft to improve its handling characteristics and decrease its pitch-up tendency at elevated angles of attack.

During the MAX flight tests, Boeing discovered that the position and larger size of the engines tended to push the nose up during certain maneuvers. To counter this tendency, engineers decided to use MCAS, as a major structural redesign would have been prohibitively expensive and time-consuming.

MCAS was intended to mimic the flight behavior of the previous generation of the series, the Boeing 737 NG. It was supposed to compensate for an excessive nose-up angle by adjusting the horizontal stabilizer before the aircraft would potentially stall. Boeing stressed that it was intended to improve the handling of the aircraft, not as an anti-stall system.

However, MCAS became notorious for its role in two fatal accidents of the 737 MAX, which killed all 346 passengers and crew on both flights. These incidents were caused when MCAS acted on false data from a single angle of attack (AoA) sensor. This led to the worldwide grounding of 737 MAX planes. In 2020, the FAA approved design changes for each MAX aircraft, which would prevent MCAS activation unless both AoA sensors register similar readings, eliminate MCAS’s ability to repeatedly activate, and allow pilots to override the system if necessary.

The 737 MAX is back in service with airlines globally, but public perception and airline confidence are still recovering.

Despite the challenges, the 737 remains a popular and important aircraft for the aviation industry.