Boeing has completed the critical design review of an enhanced angle-of-attack (AOA) system for the 737 MAX and will flight-test the improvements on the stretched 737-10 variant in 2022.
Faulty AOA data was a factor in two fatal 737-8 accidents which led to the grounding of the MAX family in March 2019. The aircraft was cleared to return to service in late 2020 pending design changes that included modifications to the existing AOA system and the longer-term development of further enhancements.
Both accidents were linked to problems with the maneuvering characteristics augmentation system (MCAS) software added to the 737 MAX. The MCAS, which automatically adjusts the horizontal stabilizer based on AOA data, was added to the 737’s speed-trim system to ensure the MAX handled like its predecessor, the 737 Next Generation, during certain flight profiles.
In the two fatal accidents involving Lion Air Flight 610 in October 2018 and Ethiopian Airlines Flight 302 in March 2019, the MCAS activated based on faulty AOA data that told the aircraft's flight-control software its nose was too high just after takeoff. Neither crew reacted as Boeing's hazard assessment anticipated, and both aircraft crashed after uncontrollable dives triggered by repeated MCAS activations.
Among FAA-mandated changes Boeing made to the MCAS was ensuring the two sensors compare AOA readings and are within 5.5 deg. before triggering nose-down stabilizer inputs. Although EASA originally pushed for the addition of a third sensor to independently calculate a reading to supplement the two existing ones, the European safety agency subsequently agreed in January 2021 to allow the MAX to return to service following a commitment from Boeing to develop a “synthetic” sensor pulling AOA data from different sources.
“We're putting more monitors onto the airplane which would catch any kind of erroneous AOA indication that you would have and then we're also adding a switch,” says Mike Fleming, senior vice president of the 737 MAX return to service, Commercial Customer Support and Commercial Derivative Programs. “Even after all these monitors that we added into the airplane, if—for whatever reason—you have an erroneous AOA indication and you got the stick shaker which tells you you're approaching a stall, the pilots could shut that off.”
Fleming says the enhanced system will monitor five different parameters “that will help us determine whether we have an erroneous signal or not. And then if we determine that we have an erroneous signal, then we'll suppress that and you won't get the issue.”
The revised system will be flight-tested on the 737-10 in 2022 and is likely to be rolled in as a retrofit to the MAX following certification of the stretched derivative which is planned for 2023. Flight tests of the baseline 737-10, which began with the first flight on June 18, are going well, says Fleming. He adds that a third test aircraft, focused on cabin and interior systems evaluations, is eventually expected to join the two now in the program.
Boeing has also completed flight tests of the shorter fuselage 737-7 and has “finalized all of our paperwork associated with the certification with the FAA on that airplane,” says Fleming. “We are getting very close from our perspective to having our work done but, of course, in all cases the regulators decide when we're complete and whether they need any more information from us. We still expect to get that airplane certified and be able to make deliveries in the 2022 timeframe,” he adds.
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