I have my issues with the FAA/EASA defined HIRF "threat" (there is one particularly high frequency where the threat is defined as, IIRC, 18,000 volts/meter. It's so high that we have issues generating the field strength in a lab environment (and if encountered in the real world would probably mean you'll never have any more children, at least not 'normal' children
).
But that being said, the FAA/EASA consider HIRF to be a 'common mode threat' - i.e. redundancy doesn't help because the whole airplane is going to be exposed to that threat. In other words, if one display blanks due to HIRF, ALL displays will blank - and I can't dispute their reasoning. Further, it's assumed that the
external HIRF threat is greater closer to the ground (since that's where the high power radar and similar HIRF generators are located). Again, hard to dispute their reasoning.
So as far as the FAA/EASA is concerned, the threat is that all flight deck displays will malfunction at the same time, at the worst possible time during final approach.
Given that recent experience has demonstrated that some flight crews have a hard time landing a 777 on a 2 mile long runway on a perfect summer day with everything working correctly, it's not exactly unreasonable to assume that some flight crews might have a bad day if all the displays blank while doing a CAT III landing under extremely challenging conditions
