Original title: Boeing 737 MAX has two consecutive air crashes and grounded all over the world. what is the reason? Source: forward-looking Economist

The U.S. House of Representatives released an investigation report on Boeing Co's 737MAX series air crash on September 16, local time. Earlier, two Boeing Co 737MAX planes belonging to Singapore Lion Airlines and Ethiopian Airlines crashed in October 2018 and March 2019, killing a total of 346 people. Two serious air crashes in succession raised concerns about the safety of Boeing Co's 737MAX aircraft, and countries grounded and investigated.

According to the investigation report released on the 16th, the cause of Boeing Co's 737MAX series air crash is complicated, which is caused by "the wrong technical assumptions of Boeing Co's engineers, Boeing Co's lack of transparency in management, and the serious lack of supervision by the Federal Aviation Administration of the United States."

In fact, the above transparency and regulatory issues generally do not pose a fatal risk to flight safety, and the real causes of these two air crashes are design defects and software problems.

Boeing 737 MAX: high hopes, but turned into sand

The protagonist of the accident, Boeing Co Boeing 737 MAX, is a derivative of the Boeing 737, which first flew in 1967 and has produced more than 10, 000 aircraft so far, making it the best-selling passenger plane in civil aviation history.

To be clear, Boeing Co 737MAX does not refer to a particular model, but includes a range of models, including the Boeing 737 MAX7, the Boeing 737 MAX8, the Boeing 737 MAX9 and the Boeing 737 MAX10. Both of the above-mentioned crashes were Boeing 737 MAX8.

The specific differences of these models are not big, mainly in the carrying capacity, fuselage specifications, voyage and other aspects have some differences. These models are named for their purpose: Boeing Co MAX7, 8, 9 are used to replace the previous generation Boeing Co 737700,800 and 900series respectively. MAX10, on the other hand, is used to snipe rival Airbus A320neo.

The first plane of Boeing Co's 737MAX series was unveiled on December 8, 2015. it completed its maiden flight on January 29, 2016 and was approved by relevant departments in 2017. less than a year later, there was a serious accident. From this we can see that the problems existing in the aircraft itself are indeed very big.

Boeing Co made a lot of money by virtue of the best-selling 737 series airliners, which means "eating old money". In 2010, Boeing Co's European rival Airbus launched a new A320neo model, which is scheduled to be put into use in 2016. Compared to the old A320. A320neo is equipped with a new engine, which effectively improves fuel efficiency and operational efficiency. This improvement received a positive response from a number of airlines, throwing in orders one after another.

Boeing Co felt the pressure and then made the decision to upgrade the 737, using simple and crude methods to increase the size of the engine. As everyone knows, this decision brings serious security risks.

Due to design defects, small wings cannot hold large engines.

In order to cater to the tastes of airlines and improve fuel efficiency, Boeing Co decided to improve the wing design on the basis of the existing 737 series, adopting "advanced technology winglets" and installing a larger engine LEAP-1B, which is the fourth upgrade of these series. However, this upgrade has brought security risks.

Aircraft winglet, also known as "end wing" or run diffuser, is a kind of small wing installed at the wing tip and approximately perpendicular to the wing. As shown below:

To make a behemoth like a plane weigh tens of tons into the sky, it must produce enough lift. Wings are mainly used to generate lift. A pressure difference occurs when air flows through the upper and lower surfaces of the wing. This pressure difference not only produces lift, but also creates a transverse bottom-up, outside-in eddy current at the wingtip. The wingtip vortex drives the air, which contains a lot of energy and is dragged behind the wingtip to form considerable resistance. This resistance is what we call induced resistance.

In order to deal with this situation, wingtip winglet came into being. The airflow on the lower surface of the wing still flows up to the surface, but due to the barrier of the winglet, the air can no longer surround to form an air vortex, thus reducing the induced drag of the aircraft.

Reducing resistance reduces fuel consumption and increases voyage. In order to catch up with his competitors, Boeing Co also brought a new wingtip design to the 737MAX series-double-forked machete winglets.

According to officials, the double-forked machete winglet uses the most advanced technology. In addition to the inward and small forward lift generated by the upper wing, the new lower wing produces outward and small forward lift. The combination of all these factors can make the role of the winglet more balanced and further improve the overall efficiency of the wing.

In addition to the above advantages, Boeing Co's advanced natural laminar flow technology is applied to the winglet surface material of the Boeing 737 MAX. Through the use of detailed design, surface materials and coatings to create laminar flow, the winglet has the characteristics of "natural laminar flow", making the air flow smoother. Thus, the resistance can be further reduced and the fuel efficiency can be improved.

As Boeing Co 737MAX increased passenger capacity and increased take-off weight, Boeing Co needed a stronger engine, so Boeing Co aimed at the LEAP-1B engine.

LEAP-1B belongs to the LEAP series engine, which also includes LEAP-1A and LEAP-1C. This is a large airliner engine developed by CFM International, a joint venture between General Electric Co of the United States and SNECMA (Safran Group) of France, to replace the engine of a single-aisle large airliner.

Of course, these two accidents are not engine pots, and the quality and reputation of this series of engines are guaranteed, of which the LEAP-1C model is the power plant of China's domestic airliner C919. If there is anything wrong with the engine, it is that it is too big. Compared with the pre-improved Boeing 737-800s, the diameter of the Boeing 737 MAX8 is about 1.75m, while the former is about 1.5m.

Don't underestimate the difference of 25 centimeters. You know, Boeing 737-800 engines are only 48 centimeters above the ground. The large diameter engine will undoubtedly further shorten the distance from the ground and increase the risk of engine touching the ground.

However, even with such a great improvement, Boeing Co did not redesign the main body of the wing except to change the wingtip. This may be Boeing Co's helpless move under the pressure of Airbus A320neo. After all, there are too many factors to consider and too many tests to be carried out to redesign the wings, and the market does not give Boeing Co so much time.

What should I do? Boeing Co finally adopted the strategy of "tossing and moving". The LEAP-1B engine has a large front section and a slender rear end, which is streamlined. In order to prevent the front part of the engine from hitting the ground, Boeing Co moved it forward as a whole, making the part of the engine protruding from the wing longer.

In fact, there are some problems with Boeing Co's design. The weight of the protruding part of the engine increases the overall weight of the front of the aircraft, which may lead to imbalance during flight. Therefore, because of its stronger engine power, the Boeing 737 MAX is more likely to automatically raise its head up at high angle of attack, causing the plane to stall. There is no official explanation as to why it is easy to look up.

Angle of attack, also known as angle of attack, refers to the angle between the forward direction of the aircraft wing (equivalent to the direction of the airflow) and the wing chord (different from the axis of the fuselage). As shown in the following figure:

The angle of attack is closely related to the aerodynamics of the aircraft. When the angle of attack exceeds the critical value (18 degrees for most aircraft), the lift generated by the wing suddenly decreases and the resistance increases sharply, resulting in a rapid decrease in the flight altitude of the aircraft and stall.

For most aircraft, if there is a stall, the plane will immediately begin to fall, as is the case with Lion Airlines and Egypt Airlines. More dangerous things will happen to a few aircraft. Due to the uncoordinated stall on both wings, the aircraft rotates rapidly around the wing direction where the stall is serious, which is called spin. Spin will not only make passengers dizzy, collision caused casualties, more serious may also tear the aircraft, resulting in disintegration.

Therefore, stall control is not only an important part of pilot training, but also a rigid requirement of flight qualification certification.

It should be noted that in the stall state, the aircraft engine did not stop working, nor did it lose its speed. At this time, the plane was in a state of "force and nowhere to use". Popularly speaking, it is like the weak side in the tug-of-war, which is clearly exerting its strength, but it just can't pull the opponent.

Boeing Co certainly knows this very well, so how does Boeing Co do it?

MCAS system: using risk to solve risk

When the angle of attack exceeds the critical value, it will lead to stall, and the pilot can generally manually lower the head height through the operating lever to avoid risk, which is called trimming. Of course, trimming can also be achieved by adjusting the rudder, the horizontal wing of the tail and so on.

Boeing Co is aware of the design flaws of his own 737MAX, so he designed a software that automatically lowers the head-- the MCAS system, which is called "maneuverability Enhancement system".

The system has two angle of attack sensors, one on each side of the fuselage. Each sensor contains an external blade, which rotates with the air flow and is connected to the internal rotator to detect the angle independently.

The operation logic of the MCAS system is simple: the angle of attack is monitored by the sensor installed in the aircraft's head. if the angle of attack is detected to be close to stall, the system will "bow" the aircraft by adjusting the horizontal wing of the aircraft's tail to avoid stall. According to Boeing Co, the horizontal wing will tilt upward at a rate of 0.27 degrees per second, tilting 2 degrees in 10 seconds.

As can be seen from the above, the MCAS is trimmed by adjusting the horizontal airfoil. And in the information given by Boeing Co, the system will only adjust the horizontal wing.

But is this really the case?

Some domestic pilots simulated Boeing Co's 737MAX flight on two D-class simulators (certified by the Civil Aviation Administration to replace the real plane to train Boeing Co 737MAX pilots), and finally found that when Boeing Co 737MAX was close to stall, the putter might work on its own.

This discovery obviously contradicts the original intention of MCAS. According to reason, MCAS should not operate the joystick.

Inquiring about the patent applied for by Boeing Co in China, we found that a patent design called CN106477055A is similar to the MCAS system. The patent was applied for in 2016, and in the same year, Boeing Co completed Boeing Co's first flight of 737MAX.

According to the design, when the aircraft stalls, the patent can control the flight attitude and speed of the aircraft by adjusting the elevator, the horizontal wing of the tail, spoiler and other parts. It can be seen that the MCAS system should be a subroutine of the patent. According to the pilot's test, we can see that the MCAS system should also be able to control the joystick, spoiler and other parts.

In other words, Boeing Co is not telling the truth.

What is more deadly is that the system is not intelligent. From the point of view of its implementation, as long as the angle is too large (that is, the plane's head is too high), the system will force the nose down. This kind of operation logic will lead to misjudgment. When the aircraft climbs during take-off, the height of the head is generally higher, and forcing the aircraft down may cause the risk of falling.

In addition, Boeing Co stressed that "the leveling system under the MCAS system will not stop working just because the lever is manually operated." In other words, when the pilot is operating the joystick to climb, the MCAS will stop the operation and wrestle with the pilot repeatedly, which unintentionally greatly increases the risk.

This has indeed happened. The day before the Lion Air crash in Indonesia, on a flight from Bali to Jakarta (the model is a Boeing 737 MAX8), the pilot found that the plane had climbed to a certain altitude and the plane suddenly fell. After switching manual mode and pulling up the joystick, the MCAS system starts, the nose is forced to press down, and the fall is more serious. Finally, the crisis was resolved only after the system was shut down manually.

However, the next day, the same thing happened when the plane took off again, this time, the plane crashed six minutes after take-off, resulting in a tragedy.