Originally Posted by
Notanatp
What is the basis for your statement that flipping five bits "is the standard procedure for this category of tests"?
Originally Posted by Seattle Times
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This was standard testing that’s typically done in certifying an airplane, but this time it was deliberately set up to produce specific effects similar to what happened on the Lion Air and Ethiopian flights.
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During the tests, 33 different scenarios were artificially induced by deliberately flipping five bits on the microprocessor, an error rate determined appropriate by prior analysis. For all five bits, each 1 became a 0 and each 0 became a 1. This is considered a single fault, on the assumption that some cause, whether cosmic rays or something else, might flip all five bits at once.
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.
(my emphasis)
About the bit selection ; 1 bit for MCAS status "engaged" (disabling the control column switches), 1 bit for nose down pitch trim command, and 3 bits for undetailed "
complications" (for instance: airspeed disagree warning).
The later description of how the scenario unrolled clearly shows that it's the first 2 bits plus a 3 seconds startling delay that are critical for dooming the plane in 1 occurrence over 3.