I have a closely related question:
How much is known about the processes by which pitot tubes ice up at altitude? I have read a number of other reports which suggest that the culprit is not liquid water (or at least do so to me since pitot tube icing is the only icing mentioned, and in at least one case, ice crystals and graupel were reported at FLs 180-310, while icing if pitot tubes occurred at FLs 390 and 410).
Is it possible that the ice crystals deposit and fuse inside the tubes, thus obstructing them? Of am I missing something major?