It's obvious enough. They have a total-air-temp displayed on the FMC and a TAT above 10°C OR not in visible moisture OR SAT* below -40°C is a good enough proxy for "not in icing conditions".
The problem isn't knowing whether or not you're in possible icing conditions if you think to ask the question, but rather reliably remembering to evaluate it every single time you enter and exit possible icing conditions.
* SAT - static air temp (air temp before the ram rise).
That's not obvious. Something with critical damage potential shouldn't be hidden in a reading in an obscure display page where a value has to be evaluated by several rules.
It should be a caution light or something at least.
I'm not advocating that Boeing is proposing a sensible path here; I think they're not.
But ATPs don't have any trouble evaluating "am I in potential icing conditions?" as they've been doing it for one to many thousands of hours previously.
I understand. It's just that most accidents have not just one cause but a whole chain of them. Something small happens that causes the pilots to be distracted trying to fix it, meanwhile ignoring the elephant in the room or forgetting a 'routine check'. The flight deck isn't always a relaxed place and this is when these things can get out of hand so easily.
This kind of mistake has been made so often and lives have been paid that I find it crazy that it's still being proposed.
based on the article, it doesn't sound like this has anything specifically to do with temperature - the issue is more about whether there is moisture in the air.
If it were just a temperature thing, you'd think it could be automated, but I don't think that's really the main thing they're dealing with here.
In theory this means switching the system off when you exit the clouds.
This is usually pretty noticeable, but maybe less so at night, since you might be breaking out into a pocket or a clear layer between other layers, without visibly seeing much of anything outside.
I would hesitate to comment on the feasibility of this beyond what the interviewed persons have said, precisely because they're not clarifying (to the readers in any case) what the actual thresholds here are. And the interviewees don't seem convinced that this is a reasonable/safe requirement.
The problem isn't knowing whether or not you're in possible icing conditions if you think to ask the question, but rather reliably remembering to evaluate it every single time you enter and exit possible icing conditions.
* SAT - static air temp (air temp before the ram rise).