"Severe icing indicates that the rate of accumulation is so fast that ice protection systems fail to remove the accumulation of ice (1cm in less than 5 minutes). The crew need to exit this condition immediately.
Severe icing is usually a product of clear or mixed icing encounter. Severe icing occurs most frequently in areas where the air has high levels of liquid water or there are very large droplets.
Immediate pilot action is required. Performance and handling may be seriously affected after only a few minutes exposure. Activate the ice protection system and work to exit the conditions immediately."
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"The AIM 7-20 provides the following definition for the icing severity index: “Severe”. The rate of accumulation is such that deicing/anti-icing equipment fails to reduce or control the hazard. Immediate flight diversion is necessary.”
Copy from ATR '
Be prepared for icing' @ post #85
https://www.dropbox.com/scl/fi/ucyq9...bq58sahy5&dl=0
Even after 20 yrs of the manufacturing industry being aware and promoting the need for a definition of sever icing, the regulators have yet to catch up.
EASA 2024 Annual Safety Recommendations Review ~ page 40
"The Norwegian Safety Investigation Authority recommends that EASA clarifies the inadequate definitions as well as the existing inconsistency relating to icing problems and ensure that the results are harmonised with other international authorities."
"The European Union Aviation Safety Agency (EASA) acknowledges that severe icing is not defined within the scope of … Nevertheless, the existing provisions in … clearly state that procedures related to flight in icing conditions are dependent on aircraft flight manual (AFM) limitations defined in the AFM and applied certification standards. This is consistent with the Certification Specifications (CS) for large airplanes (CS-25) on which it is deducible that ‘severe icing conditions’ correspond to conditions for which the airplane is not certified. The procedures to be established by the operator should notably take account of the design, the equipment, and relevant AFM limitations,
… EASA has therefore concluded that introducing the definition of severe icing into the regulations may not necessarily lead to clearer operational guidance on avoidance of flight in severe icing conditions, as same levels of icing may have a different effect on different aircraft. Therefore, it would not be appropriate to amend the existing rules, … Ultimately, EASA considers this might rather be a topic for implementation at the level of the International Civil Aviation Organization (ICAO), with no immediate action on existing European Union rules.
https://www.easa.europa.eu/en/downloads/140203/en
i.e. No action, place responsibility on operators.