During tests intended to check for malfunctions of the 737 Max’s redesigned flight control software, the pilots still managed to lose control of a simulated aircraft during ground exercises. The test which caused them to lose control involved flipping bits in the memory of one of the Boeing’s two flight control computers.
Flipping bits is intended to simulate a rare but not impossible situation where cosmic rays striking the memory of electronic components in flight can reset bits from 0 to 1. With 737s routinely flying up to 37,000ft, the possibility of this happening increases with altitude.
Testing focused on flipping five bits, said to control some of the most crucial parameters: positioning of flight controls and activation state of flight control systems, such as the infamous MCAS anti-stall system.
Astonishingly, until the 737 Max crashes, the aircraft was flying with no redundancy at all for the flight control computers. If the active one failed or suffered inversion of critical bits in memory, there was no standby unit ready to cut in and continue.
Read the full article at: https://www.theregister.co.uk/2019/08/02/737_max_cosmic_bit_flipping_test/