In 2017, Boeing discovered an error with the angle of attack (AOA) Disagree alert, which notifies pilots of discrepancies with the sensors that track the lift of a plane's nose. Those that did not purchase the optional indicator were without the safety feature.
By becoming optional, the alert had been treated in the same way as a separate indicator showing raw AOA data, which is seldom used by commercial pilots and had been an add-on for years.
Boeing engineers knew in 2017 that the 737 Max display system software did not correctly meet the AOA disagree alert requirements but concluded "the existing functionality was acceptable until the alert and the indicator could be de-linked in the next planned display system software update".
Boeing said Sunday that the planes could be flown safely without the alert, but said it will be included in the 737 Max before the planes are flown again.
When the discrepancy was found, Boeing conducted a review and determined that the existing function was acceptable and chose to delink the alert and the indicator in the next planned update for the display system software.
A Federal Aviation Administration spokesperson confirmed that Boeing alerted them of the non-functioning warning light in November, shortly after the Indonesia Lion Air crash, which occurred the previous month.
A spokesperson for Southwest Airlines, the largest operator of the 737 Max, told The Associated Press that Boeing had informed it of the indicator issue in November, following the Lion Air crash. The Trump administration grounded all 737 Max jets worldwide, creating financial and logistical problems for three major U.S. airlines, while Boeing continues working to fix the problem.
"Boeing's timely or earlier communication with the operators would have helped to reduce or eliminate possible confusion", the FAA said.
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Erroneous data from a sensor responsible for measuring the AOA or the angle at which the wing slices through the air is suspected of triggering a flawed piece of software that pushed the plane downward in two recent crashes. "Until after Lion Air, our manuals said that worked", Weaks said. That allowed the airline to activate the sensor-disagree warning lights on its 34 Max jets earlier this year, she said.
Senior FAA and airline officials increasingly are raising questions about how transparent the Chicago aerospace giant has been regarding problems with the cockpit warnings, according to people familiar with their thinking.
The worldwide fleet of 737 Max planes totalled 387 aircraft at the time of the grounding.
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.
The 737 Max has been grounded around the world for nearly eight weeks.
But the new jets were already being delivered, and USA regulators were not immediately told as Boeing did not believe the apparent mistake posed any risk. Boeing said the problem stemmed from software delivered to the company.
Boeing also did not flight test what would happen to the MCAS system if the single AOA sensor failed, CNN previously reported.