The feature at issue is known as the Angle of Attack (AOA) Disagree alert and was created to let pilots know when two different sensors were reporting conflicting data. After media leaks pointed to Boeing's dodgy behaviour after the Lion Air disaster last October, new reports have suggested that the aviation giant was not forthcoming about its new plane's faulty alert system with airlines.
"Investigators of the Lion Air plane crash. say a faulty sensor fed the system erroneous data, and the system forced the nose of the plane down repeatedly".
Boeing did not tell customers or the United States aviation regulator that an alert mechanism in the cockpit was optional on the 737 Max, rather than standard as in previous models, until after a fatal crash.
Boeing said Sunday that because in-house experts decided that the non-working light didn't affect safety, the company made a decision to fix the problem by disconnecting the alert from the optional indicators at the next planned update of cockpit display software.
"Accordingly", continued Boeing, "the software activated the AOA Disagree alert only if an airline opted for the AOA indicator".
Boeing then conducted an internal review, which "determined that the absence of the AOA Disagree alert did not adversely impact airplane safety or operation".
Senior company leadership was not involved in the review and were first made aware of the issue in the aftermath of the Lion Air accident, Boeing said.
The sensors malfunctioned during an October flight in Indonesia and another in March in Ethiopia, causing software on the planes to push their noses down.
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The FAA put the bulk of blame on Boeing's shoulders, saying, "Boeing's timely or earlier communication with the operators would have helped to reduce or eliminate possible confusion".
Neither the Lion Air aircraft nor the Ethiopian Airlines jet had the feature.
Boeing revisited the issue in December 2018 by convening a Safety Review Board (SRB) to determine for the second time whether the absence of the AOA Disagree alert would amount to a safety issue, according to the company. That allowed the airline to activate the sensor-disagree warning lights on its 34 Max jets earlier this year, she said.
Boeing didn't tell airlines or the FAA about this decision.
The panel determined the issue to be "low risk", and said Boeing would have to fix it as part of an overall package of enhancements to the Max in response to the Lion Air accident.
It is also being reported Boeing did not do any trial of flight test to know what could happen to the MCAS system if the single AOA sensor fails.
"The Boeing design requirements for the 737 MAX included the AOA Disagree alert as a standard, standalone feature, in keeping with Boeing's fundamental design philosophy of retaining commonality with the 737NG".
That update was never done before the MAX fleet was grounded in March following the second crash.
Boeing is also developing a software upgrade and changes to pilot training that must be approved by global regulators before the jets can fly again.