Boeing’s latest 737 Max jets have been grounded in several countries including the United States following the recent crash of Ethiopian Airlines. It was the second hull loss of the 737 Max 8 after the Lion Air crash merely six months ago. This has prompted concerns about a particular safety feature on the aircraft which additional data shows might have been the cause in both incidents, though a concrete link between the two hasn’t been officially established. A new report suggests that Boeing’s safety analysis of this new feature was flawed.

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The suspected link between both crashes is the new MCAS system on this airplane. The system has been developed to push the aircraft’s nose done if the Angle of Attack sensor indicates that the jet is approaching stall (losing aerodynamic lift). Similar systems are used in more commercial aircraft to prevent stall but it’s believed that the MCAS may have been triggered accidentally in both incidents due to faults with the AOA sensors.

According to the Seattle Times, Boeing’s original safety report for the feature mentioned that the MCAS could move horizontal stabilizer a maximum of 0.6 degrees. It only told airlines after the Lion Air crash that the MCAS could actually push the stabilizer by 2.5 degrees. That’s half of the physical maximum. The company reportedly increased the limit because it found in flight tests that a more powerful correction was needed to counter an impending stall.

“The FAA believed the airplane was designed to the 0.6 limit, and that’s what the foreign regulatory authorities thought, too,” an unnamed engineer told the scribe. It’s further claimed that Boeing didn’t account for the fact that the system could reset itself after the pilots manually intervened. This would allow MCAS to keep pushing the nose down which would result in a sudden loss of altitude. Boeing and the FAA also allowed the MCAS system to be activated by a single AOA sensor instead of two.

Black box data from the Lion Air crash showed that the system could have essentially been working against the pilots. The faulty sensor resulted in MCAS initially trying to push the plane’s nose down and even though the pilots tried to regain manual control by press an override switch and pulling back on the controls, the system constantly pushed the nose down with the high 2.5 degree limit. Just two activation cycles could have pushed the nose all the way down resulting in the aircraft crashing into the sea at over 500 miles per hour.

Since an investigation is ongoing, Boeing didn’t respond to the report in the Seattle Times, but did maintain that “there are some significant mischaracterizations.”

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