[bumby]

There's levels to safety-criticality so these aren't binary decisions. Often they are based on severity/probability/detectability measures.

For example, airbags on your car may be mandatory but automatic emergency braking not so much.

[marksweston]

I agree, but was responding to a post that was heavily criticising Lion Air for not paying for the option and implicitly laying the blame for the crash at their door. Either the option was safety critical, in which case Boeing had no business making it optional, or it wasn’t, in which case blaming Lion Air is unreasonable.

Given that thanks to Boeing, the Lion Air crew didn’t even know the MCAS subsystem existed, and that MCAS would continue to rely on the single faulty sensor however many extra redundancies were installed, the decision not to buy seems kind of irrelevant.

[bumby]

> Either the option was safety critical, in which case Boeing had no business making it optional

This is the part I was trying to clarify. I don't know Boeing's internal processes, but there certainly could be options that are safety-critical but still optional because they are ranked low enough on the severity/probability/detectability scale.

I agree that it's wrong to blame Lion Air as the consumer. The fault seems to be in Boeing misapplying the risk categorization.