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Wednesday, November 29, 2023

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A programming error in the flight control system is the culprit behind two Boeing 737 Max plane crashes. (Video: Melissa Porter/SDSU)

What Caused Boeing’s 737 Max Crashes

Flaws in the flight control system are linked to malfunctioning ‘angle of attack’ sensors.
By Padma Nagappan

“Those two accidents are not due to poor design, it’s more because of a judgment call in programming, a small error that made such a big impact.”

Two fatal crashes of the redesigned 737 Max 8 aircraft within five months put Boeing squarely in the center of worldwide scrutiny, with victims’ families, regulators and the public questioning the safety of the jets’ flight control system. 

When the October 2018 Lion Air crash in Indonesia was followed by the Ethiopian Airlines crash in March 2019, regulators grounded the aircraft pending investigation and fixes to the system. Boeing expected the aircraft to be back in the air this fall, but its return has been delayed by new concerns and continued advocacy from victims’ families for recertification of the plane.

At the crux of the problem is the “angle of attack” sensor, said San Diego State University aeronautical engineering professor and researcher Joseph Katz, who studied this very problem a few years ago for an aerospace client. He is not involved in the ongoing investigations with Boeing.

“Angle of attack” refers to the relative angle between airflow and the airplane’s orientation. When the aircraft is in flight mode, the angle at which it lifts increases, and so the lift force increases as well. Since the plane’s weight remains constant, the pilot increases the angle of attack as the airplane slows down. But if the airplane continues to slow down as the angle of attack increases, it will stall and could lead to a crash, Katz said.

The glitch with sensors 

A couple sensors are located on either side of the nose and cockpit that alert the pilot to the current angle of attack at any given time. Pilots need accurate measurements of the angle, which the sensors provide, much like the gauges on the dashboard of a car indicate speed and revolutions per minute of the engine’s crankshaft. 

For the newer airplanes, like the 737 Max 8, a safety measure is built into the automated flight control system that reduces the angle of attack by pitching the nose down when it senses the angle is very high and approaching stall conditions. 

“In the two recent Boeing 737 Max crashes, it appears that the sensor sent wrong information to the flight control system indicating the angle of attack is too large and approaching stall, so the system steered the aircraft by pitching the nose down to correct the angle of attack,” Katz explained. “The pilots realized this was a wrong command and they tried to override the system, but it appears they could not figure out how to override it. So the planes went into a dive and eventually crashed.”

He came to this conclusion based on his review of investigation reports that appeared in technical industry publications. The final ruling will take a while to be published, but the information released so far indicates this was the scenario. 
Prior Research
An aerodynamics expert, Katz has spent more than 30 years studying airplane crash reports and black box data, and he said analysis often shows some degree of pilot error in crashes. 

Katz explained the angle of attack can be miscalculated when pilots are under pressure because modern airplanes are operated by complex computer systems, so pilots have less control when malfunctions happen and therefore, may not be adequately prepared to overcome certain problems. “When I teach our students, I emphasize that aerodynamics and fluid mechanics are a complicated science and it is not always intuitive or logical,” he said. 

He has worked with about two dozen clients around the world on different technical issues with aircraft, and he and his students are currently involved in a joint project with an Israeli university to design certain aspects of surveillance drones.  

Katz became familiar with the issues involving the angle of attack sensor after consulting for the aerospace client which needed to convert an Airbus 320 airplane into a cargo plane. To do this, the location of cabin doors needed to be shifted, and there was concern about how this would affect the functioning of the sensors.

In his study, he experimented with moving the sensors’ location to different parts of the aircraft and studying how they reacted to the aircraft’s condition. Katz gave recommendations on the effect the location of the sensors would have on the flight control system. He presented these findings in 2015 at an American Institute of Aeronautics and Astronautics conference. 
Next steps 

“The 737 Max is a wonderful aircraft. There’s nothing wrong with it, but it’s the flight control system, which was programmed so the pilot cannot easily do a manual override of the automated system which can get thrown off by signals from the angle of attack sensor,” Katz said. “Boeing says pilots were given training on how to bypass the code, but somehow it could not be done on those fatal flights. There is reference to a bird hitting the sensor and throwing it off track.”

Boeing cites instances when the system wrongly steered the aircraft and trained pilots were able to successfully override it, but they were unable to do so in the two crashes that happened, he pointed out. 

The company has issued fixes to the flight control system and is in the process of getting them approved by the Federal Aviation Administration (FAA), after which pilots will have to be retrained, so “it’s a long, complicated process and this is why they can’t fly these airplanes, and the reason why they’ve been grounded by airlines,” Katz said.

“We have the technology but it’s not mature yet, or fully trustworthy. We are not at the phase where pilots should be left out of the cockpit. We think pilots still should fly the airplanes. Those two accidents are not due to poor design, it’s more because of a judgment call in programming, a small error that made such a big impact.”