Hi,
In 2007, EASA has described the risk of blocking probes pitot "at least hazardous" I remember the definition, according to EASA, from "hazardous"
CS-25 BOOK 2 page 2-F-7
(4) Hazardous: Failure Conditions, Which would Reduce the capability of the airplane or the ability of the crew to cope with adverse operating, conditions to the extent that there would be:
(i) A large reduction in safety margins or functional capabilities
(ii) Physical distress or excessive workload such that the flight crew can not be relied upon to perform their tasks accurately or completely
(iii) Serious or fatal injury to a relatively small number of the occupants other than the flight crew
Les dossiers noirs du transport aérien