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The new flaw, traced to how data is being processed by the flight computer, affected pilots' abilities to quickly and easily follow the required recovery procedures for the runaway stabilizer, sources told ABC News, who said is not related to reported problems with an anti-stall system, called MCAS, but connected to a broader anti-stall system called "speed trim."
The FAA released an updated statement on the 737 MAX Wednesday evening stating that they "recently found a potential risk that Boeing must mitigate" as they evaluate updates to the Boeing 737 MAX software.
Boeing said in a SEC filing on Wednesday that they are working on the required software to address the FAA's request, and that they will not offer the 737 MAX for certification by the FAA until they "have satisfied all requirements for certification of the MAX and its safe return to service."
The 737 Max jets have been grounded since March following two fatal plane crashes in five months that killed a total of 346 people.
FAA officials said the agency will lift the aircraft's prohibition order "when they deem it safe to do so."