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by error503 1291 days ago
The software on the MAX worked as designed/specified; unlike Therac-25 there was no bug in the critical path, it was a series of design and oversight failures pushed by business and cost cutting interests, and the actual accidents were triggered (though I wouldn't consider it causal) by hardware failure in one of the AoA sensors. There was a bug regarding displaying an AoA disagree warning to the pilots, which despite being known wasn't fixed by Boeing, but this wouldn't have actually changed anything about the plane's behaviour.

To the credit of systems engineers, I can't think of a recent high profile fatal accident that could be reasonably blamed primarily on software, but that's not so much because software is infallible, but because systems are designed to fail safe.
1 comments
ionwake 1291 days ago
Correct me if I’m wrong but wasn’t there an overflow in the software if the aircraft climbed too steeply ?

I’m sure I read that

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error503 1290 days ago
I don't think so? Maybe a bug of this form was found, but I'm sure nothing like that was involved in the crashes. The flight control software performed as it was designed to, it wasn't software that sent the trim wheels spinning, but a bad AOA sensor and a lack of proper safety analysis, training and procedures.

There was a bug that caused the AOA DISAGREE alert on the EICAS not to be displayed, because at some point someone misunderstood the requirement that the AOA indicator should be hidden if they didn't pay for the upgrade, but this was just an indication and wouldn't have affected control at all (though likely would have hinted the pilots to a more appropriate cause of action).

One could also consider the lack of cross-checking between the two flight computers and associated AOA sensors to be a bug, but that was how the system was intentionally designed, because AOA wasn't considered a flight-critical measurement in the system's safety assessment, so they didn't consider this required. A holistic safety analysis was never really done inclusive of MCAS though, and this requirement probably just followed on from 737NG and wasn't really considered (at least thoughtfully...) in MCAS' design.

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ilyt 1290 days ago
The main bug was:

* each flight computer used its own angle of attack sensor with no way to detect failure. Craft had 2 sensors, but they were not used together (apparently that was extra paid option...) * pilots didn't know how the system worked or when it is active, so they were fighting against it

don't remember anything about overflow

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