After deliveries of the 737 Max commenced in 2017, Boeing discovered a software issue prevented the angle-of-attack (AOA) Disagree alert from working if customers had not chosen the optional AOA indicator.
“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,” says the company in a statement issued on 5 May.
“Several months” after 737 Max deliveries started in 2017, Boeing engineers discovered that “the 737 Max display system software did not correctly meet the AOA Disagree alert requirements.”
“The software delivered to Boeing linked the AOA Disagree alert to the AOA indicator, which is an optional feature on the Max and the NG. Accordingly, the software activated the AOA Disagree alert only if an airline opted for the AOA indicator.”
Boeing contends that neither system is necessary for safe operation of the aircraft, but provide “supplemental information” to pilots.
Following the discovery of the issue the manufacturer followed a standard resolution process.
“That review, which involved multiple company subject matter experts, determined that the absence of the AOA Disagree alert did not adversely impact airplane safety or operation. Accordingly, the review concluded, the existing functionality was acceptable until the alert and the indicator could be delinked in the next planned display system software update.”
Company leadership, however, was not aware of the issue. It was only brought to their attention following the 29 October 2018 crash of Lion Air flight JT610, a Boeing 737 Max 8, that killed 189 passengers and crew.
Erroneous data from an AOA sensor, and its influence on the 737 Max’s Manoeuvring Characteristics Augmentation System (MCAS), was implicated in the Lion crash. MCAS, and faulty AOA data, have also been implicated in the 10 March crash of Ethiopian Airlines ET302, which killed 149 passengers and crew.
At the time of the crashes, MCAS relied on data from only one AOA sensor at a time, though it alternated between left and right sensors. MCAS forces the aircraft’s nose down if it registers an unusually high AOA.
“Approximately a week after the Lion Air accident, on November 6, 2018, Boeing issued an Operations Manual Bulletin (OMB), which was followed a day later by the FAA’s issuance of an Airworthiness Directive (AD),” it says.
“In identifying the AOA Disagree alert as one among a number of indications that could result from erroneous AOA, both the OMB and the AD described the AOA Disagree alert feature as available only if the AOA indicator option is installed.”
In 2017, Boeing says a Safety Review Board it had convened confirmed the company’s view that that the absence of a 737 AOA disagree alerts did not present a safety issue, and this was shared with the FAA.
“Boeing is issuing a display system software update, to implement the AOA Disagree alert as a standard, standalone feature before the Max returns to service,” it says.
“When the Max returns to service, all Max production aircraft will have an activated and operable AOA Disagree alert and an optional angle of attack indicator. All customers with previously delivered Max airplanes will have the ability to activate the AOA Disagree alert.”
The world’s 737 Max fleet remains grounded pending certification of the software update.