In the early 1990's Boeing was faced with a dilemma. While its existing fleet of twin-engine 737 jetliners had a successful service life over the past twenty five years, Boeing executives believed they needed a more capable aircraft in terms of both passenger capacity and range. The choice was either building an updated version of the 737 or designing an entirely new aircraft. The result of that decision, the Boeing 737 Max, is the subject of this blog. We'll trace the development of the Max, its current operating problems, as well as its future.
The 737 Max is the fourth generation of the Boeing 737 series, which first flew in 1967. The original concept of the 737 was a narrow body airliner with a capacity of about one hundred passengers and a range of 1,150 miles. The Max succeeds the 737 Next Generation (NG), which has a range of approximately 3,200 miles and a passenger capacity of one-hundred fifty. The earliest models entered service in 2006 and offered a remodeled interior with reconfigured cabins and other passenger comforts. That same year, Boeing considered a new aircraft design which followed along the lines of the 787 Dreamliner. On December 1, 2010 Boeing's competitor, Airbus, launched the Airbus A320neo family of aircraft to improve both fuel and operating efficiency with new CFM International LEAP and Pratt & Whitney PW 1000G engines. In February of 2011, Boeing executives flatly stated they were going to build a new aircraft. By July of that year, Airbus had gained about 700 commitments from the airlines to buy the A320neo with 460 already on order. Boeing began to feel the pressure from this, and in August 2011 the Boeing board of directors approved the launch of the re-engined 737, with an anticipated fuel burn of 4% lower than the neo. Later that year, Boeing conducted a series of tests resulting in a hybrid laminar flow stabilizer, a revised tail cone along with a natural laminar flow nacelle. The test results influenced Boeing to update the existing 737 design, rather than develop an entirely new aircraft. The updated aircraft was to be designated the 737 Max and Boeing management believed the Max could meet or exceed the range of the Airbus A320neo.
While in mid -2011 Boeing sought to match the A320neo's 15% fuel advantage, though initially the Max was only able to achieve a 10-12% reduction. The 737 Max went through several modifications to make it a more efficient and aerodynamic aircraft. For example, the split tip winglet is designed to maximize lift while increasing fuel efficiency over blended winglet aircraft by nearly two per cent. The engine fans were widened from 61 in. to 69.4 in. by raising the nose gear and placing the engine higher and forward. Electronically controlling the air bleed system along with restructured engine casings with chevrons aid in reducing engine noise. The Max was designed to have a range of 3,800 miles with a passenger capacity of 190, while the Airbus A320neo has a passenger capacity of 180 with a range of 3,400 miles. The 737 Max featured the Boeing Sky Interior with overhead bins and LED lighting based on the 787 Dreamliner configuration. All Max control surfaces were fly by wire controlled and are essentially compatible with the 737NG series planes. However, the automatic stabilizer control system has been enhanced to include the Maneuvering Characteristics Augmentation System (MCAS) in addition to the Speed Trim System (STS). Compared to STS, the MCAS has a greater hierarchy of control and can not be disengaged with the aft and forward column cutout switches. Unlike previous 737 models, the Max wiring system does not allow automatic stabilizer trim control functions to be turned off while maintaining electric stabilizer control.
The Max went through a number of flight tests, with its maiden flight on January 29, 2016, with the plane gaining FAA approval on March 8, 2017. During the certification process, the FAA delegated many evaluations to Boeing, allowing the manufacturer to rate their own product. It was common knowledge at the time that Boeing sought to expedite approval of the 737 Max to compete with the Airbus A320neo, which entered service nine months earlier. A critical element of the Max's flight control system is the Maneuvering Characteristics Augmentation System. MCAS is a flight control parameter utilized on jetliners to improve aircraft handling characteristics and decrease pitch-up tendency at elevated angles of attack. In essence, so the pilot can not make the pitch too high and cause a stall, though Boeing claims the system was not specifically put in place to prevent a stall. A built in danger of MCAS was it may be activated by only one of the two angle of attack sensors on the plane, making the system susceptible to a single point of failure. MCAS could not be disabled by pulling on the control yoke, as with other planes. During certification, Boeing removed a description of MCAS in the Max flight manuals, leaving pilots unaware of the flight system when the plane entered service.
Though the Max flew nearly 500,000 flights without incident during its first two years of service, all of this changed on October 29, 2018 in which a Lion Air 737 Max plunged into the Java Sea about thirteen minutes after takeoff from Soekarno-Hatta International Airport in Jakarta, Indonesia. The flight was a scheduled domestic flight to Pinaokal Pinang, Indonesia with a loss of all 189 passengers and crew. This was the first loss of a 737 Max with the aircraft delivered to Lion Air two months earlier. A subsequent investigation revealed the same plane experienced a similar malfunction a day before the crash with an extra pilot sitting in the cockpit jumpseat diagnosing the problem and instructing the crew how to disable the Maneuvering Characteristics Augmentation System. In October 2019 the Indonesian transportation service released its final report on the crash, determining the cause of the crash due to a faulty angle-of-attack sensor, which caused the plane to dive. Boeing later issued a flight operation manual advising airlines on how to address erroneous cockpit readings. On March 10, 2019 a second Max experienced a crash. An Ethopian Airlines 737 Max crashed approximately six minutes after takeoff from Addis Ababa, Ethiopia, killing all 149 passengers and 8 crew members on board. The aircraft was about four months old at the time with the cause of the crash initially unclear, though its speed after takeoff was reported to be unstable. Later investigations suggested the aircraft was in a diving pattern, similar to the Lion Air 737 Max. The subsequent investigations of both the Lion Air and Ethiopian Air Lines crashes led to Max groundings across the globe with estimates from several aviation investigation firms indicating the Max would not reenter service before 2020. The Max has several problems dogging it in addition to the MCAS problem. Faulty angle-of-attack sensors, which caused the Air Lion crash, need to be addressed. Boeing is planning to have two sensors feed the MCAS system instead of just one. The Max trim wheel has also caused problems. If flight control systems malfunction, Boeing pilots have the option to shut power boosts down and adjust the tail stabilizer manually-a feature unique to the 737 series. Though it's often a hard crank on the wheel, there is no current desire on the part of Boeing to have it removed, but rather have additional training for pilots to better unload forces on the plane. Speaking of training, Boeing now offers full simulator training for all 737 Max pilots. Though previously considered another 737 model, the Max is now treated as a separate aircraft. While the wiring of the Max was not expressly stated during the crash investigations, they're an important part of the plane's operating systems. By having the most functional wiring system, the 737 Max can avert other problems in the event of a forced landing. Software issues not related to MCAS are also being resolved, particularly in the flight display area. Again, as with the wiring issue, the concept of inspecting the software of the Max is a preventative one.
So, what does the future hold for the Max? On June 29, 2020 Boeing completed the first recertification test flight for the 737 Max. In early July 2020, with several certification flights completed, the future of the Max looks brighter. On July 21, 2020 the FAA officially rescinded the grounding order of 2019, pending acceptance by operator airlines and training of their flight crews. While the current global pandemic has greatly restricted air travel, a number of airlines are now more optimistic about the plane as a replacement for older aircraft types with less passenger capacity, range, and passenger comfort. Saving fuel costs at 15 per cent per plane over existing models may be the factor that ushers the Max back in service.