The US planemaker has been trying for weeks to dispel suggestions that it made airlines pay for safety features after it emerged that an alert created to show discrepancies in Angle of Attack (AOA) readings from two sensors was optional on the 737 Max.
But only 20% of customers had purchased the optional feature, and neither Lion Air nor Ethiopian Airlines had functioning angle of attack disagree indicators on their 737 Max fleets, The New York Times reports.
The AOA alert will tell pilots if a sensor is under stress while "disagree alert" will tell whether sensors are acting contradictory to each other.
Boeing did not tell customers or the U.S. 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.
Immediately following the company's reiteration that it did not merit action in 2017, Boeing confirmed that it was brought up again in 2018 and a Safety Review Board (SRB) was convened to "consider again whether the absence of the [safety] alert from certain 737 Max flight displays presented a safety issue". A subsequent review "determined that the absence of the AOA Disagree alert did not adversely impact airplane safety or operation", according to the company.
While it didn't know about the issue until after the Indonesian Lion Air crash, the FAA said that when it did find out, it had "determined the issue to be "low risk", and decided that it would be sufficient to fix the issue in the next software update.
Airlines around the world grounded their 737 Max planes in March this year after the Ethiopian Airlines disaster, in which 157 people died.
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Boeing's latest disclosure raises new questions about the 737 Max's development and testing - and the company's lack of transparency.
In both flights, for example, investigators found that faulty "angle of attack" sensors activated software that forced the planes into nosedives.
The FAA statement then adds: "However, Boeing's timely or earlier communication with the operators would have have helped to reduce or eliminate possible confusion". "Until after Lion Air, our manuals said that worked", Weaks said.
Boeing blamed the error on software provided to the company by an outside source, though did not give further details. Boeing went on to reiterate that when it made the discovery of the faulty software in 2017, it did not warrant any recall after a thorough review.
Disagree alerts would notify pilots whether a sensor is malfunctioning or not.
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.
Boeing also did not flight test what would happen to the MCAS system if the single AOA sensor failed, CNN previously reported. Engineers initially believed the alert was standard in all 737 Max aircraft.
The 737 Max has been grounded around the world for nearly eight weeks.