Freezing Rain (and Freezing Drizzle) have the "freezing factor" (coldness) in the transported liquid.
If the OAT is at or above 0° C and the rain freezes than we talk about "rain on cold soaked wing", which is a seprate category in the hold-over-time tables and gives the shortest protection at all.
Looking into incident statistics, "Rain on cold soaked wings" is the most critical one, as a lot of ground and flight crews completely ignore the fact of a cold wing. Otherwise, how you can explain that we still have engine FOD's by clear ice after identifying the problem 25 years ago.
Finnair did a few years ago a study on the wing temperature durign the take-off run and take-off. The result had been that in the lift-off face the wing temperature drops dramatically. This fatcor must be taken additonally into consideration.
Looking on the existing test data, the accident and incident statistics as well as from my own experience:
- No hold-over-time for Type I Fluids at all, except in "Active Frost"
- Deletion of the 3° buffer for Type I Fluids in a Two-Step Procedure
- Starting the project to install temperature probes in the wings to get more precise information about the wing temperature