A neutron hitting a cell on a microprocessor can change the cell’s electrical charge, flipping its binary state from 0 to 1 or from 1 to 0. The result is that although the software code is right and the inputs to the computer are correct, the output is corrupted by this one wrong bit.
So for example, a value of 1 on a single bit might indicate that the jet’s wing flaps are up, while a 0 would mean they are down. A value of 1 on a different bit might tell the computer that the MAX’s problematic flight-control system called MCAS is engaged, while a 0 would indicate it is not.
This isn’t as alarming as it may sound . . .
. . . for these simulations, the five bits flipped were chosen in light of the two deadly crashes to create the worst possible combinations of failures to test if the pilots could cope.
I'm only an electronics amateur, but for such important information, wouldn't entire packets of data be needed to convey say, flap position or things of equivalent importance? How else would you do a checksum?
In one scenario, the bits chosen first told the computer that MCAS was engaged when it wasn’t. This had the effect of disabling the cut-off switches inside the pilot-control column, which normally stop any uncommanded movement of the horizontal tail if the pilot pulls in the opposite direction. MCAS cannot work with those cut-off switches active and so the computer, fooled into thinking MCAS was operating, disabled them.
I'm at a loss. It has to be said that in the early days of these threads, there were clear statements that one of the column switches had been removed, but then a skilled poster showed the circuitry which seemed to say it was just circuit logic that had been altered. However, I don't recall a time when MCAS was disabled by pulling back, it being said that it would mean pulling would negate the very function needed at that time. Of course, we'd have to be completely sure what 'uncommanded' means. And so it goes on.