Boeing 737 MAX - Causes

Haunted Child of Boeing – The 737 MAX

Tim Takeoff
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7 minutes

On 28th October 2018, a Lion Air Boeing 737 MAX 8 crashed into the sea shortly after taking off from Jakarta. At that time, no-one could foresee the far-reaching consequences this tragic accident would have for the entire industry …

What is the Boeing 737 MAX?

The Boeing 737 MAX is a more advanced version of the Boeing 737, which has been in use since 1967. The 737 is one of the world’s most popular families of passenger aircraft, and the MAX represents its fourth generation. The airframe itself has undergone little to no modification during the redesign, which was launched in 2011 as part of Boeing’s constant competition with Airbus. The competitor launched its “NEO” Generation, a series with numerous modifications and fuel saving engines, back in 2010. As of the end of March 2019, global commissions of the Boeing 737 MAX have reached 5,000.

The MAX design

In order to understand the problems currently plaguing the aircraft, we need to start right at the very beginning, with the 737 MAX’s design. Unlike the previous generation of 737s, the MAX uses new, state-of-the-art LEAP engines, which are more powerful and considerably more fuel efficient. The disadvantage for the 737 is their construction. As these new engines are approximately 20 cm larger in diameter than their predecessors, Boeing was unable to attach them in the same place. Even the 737 NG series’ old engines were extremely low to the ground. In order to use the new LEAP engines, they were mounted on pylons extending considerably further forward, thus raising the entire engine higher. 

Changes in flying behaviour

These extended engine mounts, combined with the generally increased performance, inevitably lead to a radical shift in the plane’s overall flying style. One important element is the effect of the angle of the oncoming air currents on the plane’s tilt level, which is known as the “angle of attack”. Once the angle of attack exceeds 14 degrees, the new LEAP engines cause the plane to generate so much lift that it becomes difficult for the pilot to retain control. The combination of high performance and a high angle of attack can result in a stall.

Computer-assisted flight

Like Airbus, Boeing is now using fly-by-wire technology. Having implemented it in the 777, the 747-8 and the 787, Boeing is now using it in the 737 MAX. This system uses software to precisely “programme” countless aircraft statuses and conditions. These naturally vary according to the aircraft’s fundamental design, as well as the parameters received from the external sensors by the in-flight computer. This computer evaluates all the data and combines it with the entries made by the pilots and the autopilot in order to ensure consistent flying performance.

Highly complex systems – not just a Boeing issue

As well as Boeing, Airbus and many other manufacturers also use fly-by-wire technology. Systems which provide incorrect information can be isolated, either automatically or by the pilot, whose input should always take precedence. The pilot should always understand the automated functions and be able to intervene and correct matters if the aircraft’s performance is unsatisfactory. In all fly-by-wire planes, in-flight computers can also be individually deactivated or switched to secondary mode. The more complicated a system is, the more prone it is to failure. 


Given the design issues with the B737 MAX, Boeing decided to implement a system to automatically counteract the power of the engines. The engineers’ intention was to combat the aircraft’s “pitch-up” moment using trimming, a method also used by pilots to move the entire elevator unit. Boeing named its system, which was intended to automatically reduce the angle of attack in critical situations, the “Maneuvering Characteristics Augmentation System”; “MCAS” for short. The values the MCAS requires to analyse the flight conditions stem primarily from one of two angle-of-attack sensors. These are mounted below the cockpit and measure the oncoming air currents.

The golden rule – redundancy

Strictly speaking, the MCAS contravenes one of the “golden rules” of aviation, which is that any system which greatly influences flight performance must be redundantly set up. The MCAS’ calculations, however, are based primarily on a single sensor.

What happened on Lion Air Flight 610?

Shortly after takeoff on 28th October 2018, the Lion Air plane’s angle-of-attack sensors registered a difference of almost 20 degrees, a situation the aircraft’s computer systems interpreted as a stall. Even though only one sensor was showing a problem, numerous error messages and heavy control yoke vibration communicated this to the pilots. The MCAS gave repeated, unnecessary and dangerous instructions to the elevator unit’s trimming functions to lower the aircraft’s nose. The pilots attempted desperately to counteract these, but the system kept correcting. The forces exerted on the yoke were tremendous. The pilots attempted to hold the yoke, first using the electrically powered trim functions and ultimately with sheer muscle power, but at some point the forces became too great, and the pilots lost control. Just 13 minutes after takeoff, the plane plummeted into the Java Sea.

Problems arose on previous day

On the day before the accident, the same aircraft displayed similar problems with the angle-of-attack sensors. Another fully certified B737 MAX captain, who happened to be travelling on the flight as a passenger, was in the cockpit and brought the issue to the attention of the crew. They managed to deactivate the system and continue the flight manually. When the plane landed in Jakarta, the angle-of-attack sensor was replaced by technicians and the 737 MAX was deemed airworthy. 

What happened on Ethiopian Flight 302?

On 10th March 2019, a similar incident occurred when Ethiopian Airlines Flight 302 took off from Addis Ababa airport. This time, the difference registered between the two sensors was almost 60 degrees. In this case, too, the MCAS intervened multiple times in the aircraft’s trimming processes, causing the plane to drop sharply. Despite following the correct procedures and making multiple manual attempts to trim the aircraft, the pilots were unable to override the system. 157 people lost their lives in the resulting crash.

This video (in English) provides a good explanation of the background to the accident.

The authorities’ reaction

As investigations into both plane crashes are continuing, the final assessment is still pending. On the day immediately following the Ethiopian plane crash, China issued a total ban on the 737 MAX series. The FAA initially confirmed the 737’s continuing airworthiness. It wasn’t until the UK, followed by the EASA and Canada, also imposed bans, that US President Donald Trump issued a global order for all 737 MAX aircraft to be grounded. Planes currently in the air were permitted to continue to their destinations, where they were ordered to remain. Shortly thereafter, the FAA also issued a total ban. Since the ban was announced, all 737 MAX aircraft have remained grounded.

Airworthiness and retraining

As most people can probably imagine, obtaining an airworthiness certification for a new type of plane is extremely complicated and expensive. Manufacturers are, therefore, very keen to ensure that the process runs quickly and smoothly. Airbus and Boeing work closely with the various certification bodies, such as the FAA in the USA and the EASA in Europe. As Boeing was particularly keen to smooth its existing 737 pilots’ learning curve for the MAX, no precision simulator training was initially provided, and the idiosyncrasies of the MCAS system were not initially detailed in the operating manual. Some pilots received just an hour’s training on an iPad before flying the 737 MAX.

What happens next?

All the air transportation authorities are currently reviewing the plane’s airworthiness certification. Boeing is officially working on a soon-to-be-released update to the 737 MAX series. The re-certification process is set to be a very sensitive one for the Illinois-based company. Airbus, too, has had major problems with its angle-of-attack sensors in the past. On 5th November 2014, a Lufthansa Airbus A321 lost almost 4,000 feet in altitude after both sensors froze. The pilots’ exemplary “outside the box” thinking enabled them to deactivate the system. Only after the incident did Airbus modify the process and the manual’s descriptions of the Airbus systems. No-one was injured in the incident. These examples, however, continue to underscore the importance of comprehensive airworthiness assessments and pilot training. As modern planes undergo even more extreme modifications, it becomes all the more important to test all possible flight conditions extensively under controlled conditions.

Main image – Source: VanderWolf Images /

by Tim Takeoff

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