B737 MAX

The Boeing 737 MAX is a series of narrow-body aircraft developed by Boeing Commercial Airplanes as the fourth generation of the Boeing 737. It succeeds the Boeing 737 Next Generation and incorporates more efficient CFM International LEAP engines, aerodynamic improvements such as split-tip winglets, and structural modifications. The program was announced in August 2011, the first flight took place in January 2016, and the aircraft was certified by the U.S. Federal Aviation Administration (FAA) in March 2017. The first delivery, a MAX 8, was made to Malindo Air in May 2017.

The 737 MAX series includes four main variants—the MAX 7, MAX 8, MAX 9, and MAX 10—with increasing fuselage length and seating capacity. Boeing also developed a high-density version, the MAX 8-200, launched by Ryanair. The aircraft typically seats 138 to 204 passengers in a two-class configuration and has a range of . , Boeing had delivered 1,844 aircraft and held orders for 4,654 more. The MAX 8 is the most widely ordered variant. , the MAX 7 and MAX 10 had not yet received FAA certification, and the agency has not provided a timeline for their approval. Its primary competitor is the Airbus A320neo family, which occupies a similar market segment.

Two fatal accidents, Lion Air Flight 610 in October 2018 and Ethiopian Airlines Flight 302 in March 2019, led to the global grounding of the 737 MAX fleet from March 2019 to November 2020. The crashes were linked to the Maneuvering Characteristics Augmentation System (MCAS), which activated erroneously due to faulty angle of attack sensor data. Investigations revealed that Boeing had not adequately disclosed MCAS to operators and identified shortcomings in the FAA's certification process. The incidents caused significant reputational and financial damage to Boeing, including billions of dollars in legal settlements, fines, and cancelled orders.

Following modifications to the flight control software and revised pilot training protocols, the aircraft was cleared to return to service. By late 2021, most countries had lifted their grounding orders. However, the type came under renewed scrutiny after a January 2024 incident in which a door plug detached mid-flight on Alaska Airlines Flight 1282, causing a rapid decompression. The FAA temporarily grounded affected MAX 9 aircraft, and investigations raised further concerns about production quality and safety practices at Boeing.

Development

Background

In 2006, Boeing began to consider replacing the 737 with a "clean sheet" design that could follow the Boeing 787 Dreamliner. In June 2010, executives postponed the decision. On December 1, 2010, Boeing competitor Airbus launched the Airbus A320neo family, which offered better fuel economy and operating efficiency than the 737 NG, thanks to its engines: the LEAP from CFM International and the PW1000G from Pratt & Whitney.

In February 2011, Boeing CEO Jim McNerney said, "We're going to do a new airplane." The company had been developing a new aircraft to replace the 737 as part of its Yellowstone Project. In March 2011, Boeing CFO James A. Bell told investors that the company might re-engine the 737, but later that month Boeing Commercial Airplanes President James Albaugh said that the company was not sure about that. The Airbus A320neo gathered 667 commitments at the June 2011 Paris Air Show, bringing its order backlog to 1,029 aircraft, an order record for a new commercial airliner.

On July 20, 2011, American Airlines, which had long bought only Boeing jets, announced an order for 460 narrowbody jets including 130 A320ceo (current engine option), 130 A320neo, and 100 737NG. Officials also said they would order 100 re-engined 737s with CFM LEAP if Boeing pursued the project.

Program launch

Faced with the record orders for Airbus and the defection of a long-time customer, on August 30, 2011, Boeing's board of directors approved the launch of the re-engined 737 MAX, which they said would meet or exceed the range of the Airbus A320neo while burning 4% less fuel. Studies for additional drag reduction were performed during 2011, including revised tail cone, natural laminar flow engine nacelle, and hybrid laminar flow vertical stabilizer. To focus on the re-engine project, Boeing abandoned the development of a new design under its Yellowstone Project. Firm configuration for the 737 MAX was scheduled for 2013.

In March 2010, the estimated cost to re-engine the 737, according to Mike Bair, Boeing Commercial Airplanes' vice president of business strategy and marketing, would be , including the CFM engine development. During Boeing's Q2 2011 earnings call, CFO James Bell said the development cost for the airframe only would be 10–15% of the cost of a new program, which was estimated at at the time. Bernstein Research predicted in January 2012, that this cost would be twice that of the A320neo. The MAX development cost could have been well over the internal target of , and closer to . Fuel consumption is reduced by 14% from the 737NG. Southwest Airlines was signed up as the launch customer in 2011.

In November 2014, McNerney said the 737 would be replaced by a new airplane by 2030—probably using composite materials—that would be slightly bigger and have new engines but would retain the 737's general configuration. Boeing talked about developing a clean sheet aircraft to replace the 737. The conceived aircraft was to have a fuselage similar to the 737 though slightly larger, and would make use of the advanced composite technology developed for the 787 Dreamliner. Boeing also considered a parallel development along with the 757 replacement, similar to the development of the 757 and 767 in the 1970s.

Production

On August 13, 2015, the first 737 MAX fuselage completed assembly at Spirit Aerosystems in Wichita, Kansas, for a test aircraft that would eventually be delivered to launch customer Southwest Airlines. On December 8, 2015, the first 737 MAX—a MAX 8 named ''Spirit of Renton''—was rolled out at the Boeing Renton Factory.

Because GKN could not produce the titanium honeycomb inner walls for the thrust reversers quickly enough, Boeing switched to a composite part produced by Spirit to deliver 47 MAXs per month in 2017. Spirit supplies 69% of the 737 airframe, including the fuselage, thrust reverser, engine pylons, nacelles, and wing leading edges.

A new spar-assembly line with robotic drilling machines was expected to increase throughput by 33%. The Electroimpact automated panel assembly line sped up the wing lower-skin assembly by 35%. Boeing planned to increase its 737 MAX monthly production rate from 42 planes in 2017, to 57 planes by 2019. The new spar-assembly line is designed by Electroimpact. Electroimpact has also installed fully automated riveting machines and tooling to fasten stringers to the wing skin.

The rate increase strained the production and by August 2018, over 40 unfinished jets were parked in Renton, awaiting parts or engine installation, as CFM Leap-1B engines and Spirit fuselages were delivered late. After parked airplanes peaked at 53 at the beginning of September, Boeing reduced this by nine the following month, as deliveries rose to 61 from 29 in July and 48 in August.

On September 23, 2015, Boeing announced a collaboration with Comac (Commercial Aircraft Corporation of China) to build a completion and delivery facility for the 737, in Zhoushan, China, the first outside the United States. This facility initially handles interior finishing only, but will subsequently be expanded to include paintwork. The first aircraft was delivered from the facility to Air China on December 15, 2018.

The largest part of the suppliers cost are the aerostructures at (-% of the total), followed by the engines at (-%), systems and interiors at each (-%), then avionics at (-%).

Flight testing and certification

The 737 MAX gained its airworthiness approval based on the 737 legacy series (first approved on December 15, 1967), as a Supplemental type certificate (STC), in lieu of a new design approval. The MAX's first flight took place on January 29, 2016, at Renton Municipal Airport, nearly 49 years after the maiden flight of the original 737-100, on April 9, 1967. The first MAX 8, 1A001, was used for aerodynamic trials: flutter testing, stability and control, and takeoff performance-data verification, before it was modified for an operator and delivered. 1A002 was used for performance and engine testing: climb and landing performance, crosswind, noise, cold weather, high altitude, fuel burn and water-ingestion. Aircraft systems including autoland were tested with 1A003. 1A004, with an airliner layout, flew function-and-reliability certification for 300 hours with a light flight-test instrumentation.

The 737 MAX 8 gained FAA certification on March 8, 2017, and in the same month was approved by the European Union Aviation Safety Agency (EASA) on March 27, 2017. After completing 2,000 test flight hours and 180-minute ETOPS testing requiring 3,000 simulated flight cycles in April 2017, CFM International notified Boeing of a possible manufacturing quality issue with low pressure turbine (LPT) discs in LEAP-1B engines. Boeing suspended 737 MAX flights on May 4, and resumed flights on May 12.

During the certification process, the FAA delegated many evaluations to Boeing, allowing the manufacturer to review their own product. It was widely reported that Boeing pushed to expedite approval of the 737 MAX to compete with the Airbus A320neo, which hit the market nine months ahead of Boeing's model.

Entry into service

The first delivery was a MAX 8 on May 16, 2017, to the then Malindo Air (now Batik Air Malaysia); it entered service on May 22. Norwegian Air International was the second airline to put a 737 MAX into service, when it performed its first transatlantic flight with a MAX 8 named '' Sir Freddie Laker'' on July 15, 2017, between Edinburgh Airport in Scotland and Bradley International Airport in the U.S. state of Connecticut.

Boeing aimed for 737 MAX to match the 99.7% dispatch reliability of the 737 Next Generation (NG). Southwest Airlines, the launch customer, took delivery of its first on August 29, 2017. Boeing planned to deliver at least to aircraft in 2017, 10–15% of the more than five hundred 737s to be delivered in the year.

Grounding and recertification

The 737 MAX was grounded after two fatal crashes, Lion Air Flight 610 on October 29, 2018, and Ethiopian Airlines Flight 302 on March 10, 2019, in which a total of 346 people died. The day following the second crash, China became the first air authority to ground the aircraft, followed the next day by Australia, the European Union, India, Malaysia, Singapore, South Korea, and Turkey. The United States Federal Aviation Administration was one of the last to ground the aircraft, defending against groundings by issuing a Continued Airworthiness Notice to operators on March 11, garnering criticism before finally grounding it on March 13, 2019.

Contributing to the accidents was the Maneuvering Characteristics Augmentation System (MCAS), which activated unexpectedly due to erroneous angle of attack data, and inadequate pilot training. Investigations found Boeing did not fully inform operators about MCAS and found shortcomings in the FAA's certification process for the aircraft.

In the twenty months the aircraft was grounded, Boeing redesigned the computer architecture that supported MCAS. As initially designed, data from just one of the aircraft's two angle-of-attack (AoA) sensors was fed into MCAS. When erroneous data from that sensor was fed into flight computers, it caused repeated uncommanded activation of MCAS, which applied nose-down trim to the horizontal stabilizer. The accident investigations revealed that the AoA sensor on Lion Air Flight 610 was miscalibrated, and the Ethiopian Airlines Flight 302 sensor was likely damaged by a bird strike during takeoff. Boeing was criticized for using data from just one of the two sensors, representing a single point of failure on a flight control system.

Before the crash of Lion Air Flight 610, pilots were not informed by Boeing of the existence of MCAS and were not required to undergo simulator training on the difference between the 737 MAX and earlier 737 versions. Boeing and the FAA would later require simulator training to demonstrate an MCAS activation to pilots.

The final report by the National Transportation Safety Committee of Indonesia into the Lion Air crash criticized Boeing's design and the FAA's certification process for the MCAS flight-control system and said the issues were compounded by maintenance issues and lapses by Lion Air's maintenance crews and its pilots, as well as Xtra Aerospace, a US-based company that supplied Lion Air with a replacement AoA sensor that was likely miscalibrated.

In the crash of Ethiopian Airlines Flight 302, the U.S. National Transportation Safety Board and France's Bureau of Enquiry and Analysis for Civil Aviation Safety identified pilot error and inadequate training by Ethiopian Airlines as critical contributing factors to the crash.

Boeing faced legal and financial consequences, as no deliveries of the MAX could be made while the aircraft was grounded, and airlines canceled more orders than Boeing produced during this period. Boeing found foreign object debris in the fuel tanks of 35 of 50 grounded 737 MAX aircraft that were inspected and had to check the remainder of the 400 undelivered planes. The FAA curtailed Boeing's delegated authority and invited global aviation stakeholders to comment on pending changes to the aircraft and to pilot training. The FAA lifted its grounding order in 2020; all aircraft must be repaired to comply with various airworthiness directives.

After being charged with fraud in connection of both crashes of the 737 MAX, Boeing settled by paying over in penalties and compensation: a criminal monetary penalty of $243.6 million, $1.77 billion in damages to airline customers, and $500 million to a fund for the families of crash victims.

Production slowdown and suspension

From mid-April 2019, Boeing announced that it was temporarily cutting production of the 737 aircraft from 52 per month to 42 amid the 737 MAX groundings. Production of the LEAP-1B engine continued at an unchanged rate, enabling CFM to catch up its backlog within a few weeks.

As the 737 MAX recertification moved into 2020, Boeing suspended production from January to conserve funds and prioritize stored aircraft delivery. The 737 MAX program was the company's largest source of profit. Around 80% of the 737 production costs involve payments to parts suppliers, which may be as low as per plane. After the announcement, Moody's cut Boeing's debt ratings in December, citing the rising costs due to the grounding and the production halt including financial support to suppliers and compensation to airlines and lessors which could lower the program's margins and cash generation for years. The rating agency also warned that the production halt would have wide and harmful impact to the whole aerospace and defense supply chain and the ramp-up would be slower than previously anticipated. CFM International reduced production of the LEAP-1B for the 737 MAX, in favor of the LEAP-1A for the Airbus A320neo, but was prepared to meet demand for both aircraft.

Boeing did not publicly say how long the suspension would last. The last pre-suspension fuselages entered final assembly in early January 2020. Boeing was reported to internally expect production to be halted for at least 60 days. Industry observers began to question if Boeing's projection of record production rate of 57 per month would ever be reached. In early April, the COVID-19 pandemic led Boeing to shut down its other airliner production lines and further delayed recertification of the MAX.

Recertification and return to service

In early January 2020, an issue was discovered in the MAX software update, which impacted its recertification effort. As of mid-January, Boeing expected the MAX to return to service by mid-2020. In late April, following the COVID-19 pandemic, Boeing then hoped to win regulatory approval by August 2020. Between June 29 and July 1, the FAA and Boeing conducted a series of recertification test flights. Transport Canada and EASA each concluded their own independent recertification flights in late August and early September. On November 18, the FAA announced that the MAX had been cleared to return to service. Before individual aircraft could resume service, repairs were required as set out in an airworthiness directive from the FAA. Airline training programs also required approval.

On December 3, American Airlines made a demonstration flight for journalists to explain the FAA-required modifications, to regain public trust. The first airline to resume regular passenger service was Brazilian low-cost Gol on December 9. The first in the United States was American Airlines on December 29.

Transport Canada and EASA both cleared the MAX in late January 2021, subject to additional requirements. Other regulators worldwide progressively ungrounded the aircraft, including those in the UAE, Australia, Kenya, and Brazil. The Indian Directorate General of Civil Aviation (DGCA) rescinded its ban on MAX airplanes in late August on the condition that they meet the requirements set by the FAA and EASA. China's civil aviation regulator (CAAC) cleared the 94 jets stored by 11 carriers in China to fly again in December 2021. Deliveries of Chinese airplanes stored by Boeing is expected to resume in 4Q 2023. However, EASA forbade airlines from performing RNP AR approaches with the MAX. In response to the recertification, some booking sites introduced tools allowing travelers to filter results to avoid flying on the type.

Production ramp-up and recovery

In late January 2020, production was expected to restart in April and take a year and a half to clear the inventory of 400 airplanes, ramping up slowly and building over time: Boeing might have delivered 180 stored jets by year-end and produce an equal number. Boeing did not disclose any possible effect on deliveries caused by the FAA's withdrawal of Boeing's delegated authority to certify the airworthiness of each aircraft. MAX supplier Spirit AeroSystems said it does not expect to return production rate to 52 per month until late 2022. On May 27, Boeing resumed 737 MAX production at a low production rate, with the rate planned to increase towards 31 per month in 2021.

On August 19, Boeing announced that it had received new orders for the 737 MAX for the first time in 2020. Per a statement from the company, Poland's Enter Air SA entered into an agreement to buy up to four 737s. On October 28, Boeing indicated that it expected to deliver about half of the 450 stockpiled aircraft in 2021, and the majority of the remainder in 2022, noting that some of these aircraft will need to be re-marketed and potentially reconfigured. The delivery rate will also condition the production rate for new aircraft, to avoid compounding the problem. In November, Boeing saw more than 1,000 order cancelations since the grounding in March 2019. Some of these already-built aircraft have seen their order canceled and Boeing is working to find new customers to take delivery.

In late January 2022, Boeing's Chief Financial Officer said the 737 program was producing at a rate of 27 aircraft a month and was on track to ramp up the production. On March 4, Boeing reportedly had preliminary plans to ramp up production of the 737 MAX aircraft to about 47 a month by the end of 2023 as the company looked to extend its recovery from successive crises. On July 12, the company said it had met its goal of increasing 737 production to 31 per month when it reported its June order and delivery tally.
In September, however, the company noted that it was regularly having to pause production due to component shortages and other supply chain problems.

In late January 2023, Boeing announced that a fourth production line for the 737 MAX would open at the Boeing Everett Factory in Everett, Washington. The line will replace the discontinued Boeing 787 line at the factory. However, after the January 2024 Alaska Airlines Flight 1282 accident in which a door plug became detached (after not being bolted in place by Boeing) and resulted in an uncontrolled decompression of the aircraft, the FAA announced it would not grant any production expansion of the 737 MAX until it is satisfied that more stringent quality control measures have been enacted.

Certification of the MAX 7 and MAX 10

Following the recertification of the MAX 8 and MAX 9, Boeing resumed work to certify the MAX 7 and MAX 10. In March 2022, there were rumors that Boeing would request an exemption from the U.S. Aircraft Safety and Certification Reform Act of 2020, a safety reform law passed in response to the MAX crashes. The act requires airliners to be fitted with an engine-indicating and crew-alerting system (EICAS) if type certificated after December 31, 2022. Adding this feature would make the MAX 7 and MAX 10 different from other MAX variants, necessitating additional training for pilots.

In November 2022, Boeing announced expected delays to the certification of the MAX 7 and MAX 10, then expected in early 2023 and early 2024 respectively. In December, two proposals to exempt the MAX 7 and MAX 10 from the new EICAS requirements were considered for inclusion in a U.S. defense spending bill—one a simple two-year extension to the deadline, the second an exemption for aircraft whose certification applications were submitted before the law was enacted, combined with some equipment changes—but neither proposal was included in the final spending bill.

The U.S. Congress agreed in December 2022 on a bill allowing Boeing to certify the MAX 7 and MAX 10 without EICAS but required that the company must install a third angle-of-attack sensor in all 737 MAX types as previously demanded by European and Canadian regulators. The company also must install a switch to disable the stick shaker, which distracted pilots during the MAX crashes. Boeing would have to retrofit these design changes to all 737 MAXs already delivered in Canada, Europe, and the U.S. within three years of MAX 10 certification.

Boeing requested an additional exemption for the MAX 7 in December 2023. The exemption was related to a problem with the engine anti-ice system Boeing had announced in August 2023 that affected all MAX variants. Boeing had found that if pilots left the engine anti-ice system running after icing was no longer an issue, the system could heat the carbon composite inlet at the front end of the pod surrounding the engine (known as a nacelle) to break and fall off, potentially damaging the engine or fuselage. Boeing said that it was working on a fix for all MAX variants and requested that it be exempted from correcting the MAX 7 before it was allowed to enter service. Boeing withdrew its exemption request in January 2024 after being asked to do so in meetings with the U.S. Congress held after the Alaska Airlines Flight 1282 accident. , Boeing estimated that the development, testing and validation of the fix to the anti-ice system would take an additional nine to 18 months.

, the MAX 7 and MAX 10 have not been certified, with the FAA declining to put any timetable on approval. The delays have set back the fleet plans of major carriers including Southwest Airlines and United Airlines, the biggest customers for the MAX 7 and MAX 10 respectively. United also gave a rare, public rebuke of Boeing saying it was "disappointed" with the company and would no longer include the MAX 10 in its fleet planning, and had a meeting with Airbus to discuss securing more favorable production slots to enable the airline to introduce A321neos more rapidly to cover the delayed MAX 10s.

In January 2025, Boeing requested another time-limited exemption for the MAX 7 and MAX 10's stall management yaw damper (SMYD) system incorporating the required angle-of-attack enhancements, to allow time for certification of the system to a higher design assurance level in line with "increased regulatory expectations".

Design

In mid-2011, one design objective was matching fuel burn of the 737 MAX to that of the Airbus A320neo's 15% fuel-burn advantage. The initial 737 MAX reduction was 10–12%; it was later enhanced to 14.5%. The fan was widened from to by raising the nose gear and placing the engine higher on the wing and further forward. The split tip winglet added 1–1.5% fuel burn reduction and a re-lofted tail cone another 1%. Electronically controlling the bleed air system improved efficiency. The new engine nacelle included chevrons, similar to those of the Boeing 787, which also helped to reduce engine noise.

Aerodynamic changes

The 737 MAX uses a split-tip winglet, designed to reduce vortex drag, which improves fuel efficiency by maximizing lift, while staying in the same ICAO aerodrome reference code letter C gates as current Boeing 737s. It resembles a three-way hybrid of a blended winglet, wingtip fence, and raked wingtip. A split-tip wingtip was first proposed for the McDonnell Douglas MD-12, a 1990s twin-deck aircraft concept. A MAX 8 with 162 passengers on a flight is projected to have a 1.8% lower fuel burn than a blended winglet-equipped aircraft (like many 737NG aircraft) and 1% lower over at Mach 0.79. The new winglet has a total height of .

Other improvements include a re-contoured tail cone, revised auxiliary power unit inlet and exhaust, aft body vortex generator

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