It doesn't happen. It hasn't happened in nearly 40 years of 747 operations up and down African through the ITCZ, across the Indian Ocean through monsoons and hell, and through storms in Asia. BA has been operating up to 114 widebody aircraft on worldwide routes. I've done my share flying them. The rule is below -40, you can forget icing. I've not known of a single case to contradict it- had it happened, we would have changed our procedures. My procedures are as outlined, and as far as I know, BAs are as well.
The proof being thrown at me that it does happen seems to consist of airey-fairey theoretical papers and unsubstantiated accident theories, in particular AF447 which appears to have had substandard systems, but nobody really knows. When the advice and official procedures are changed, I will gladly go along with the new procedures. But until then, some of you may as well save flapping your lips together, because you are spouting unproven theories that I do not accept or believe. Is that clear enough for some of you? I tried to say it nicely, but that's how it comes!