My (limited) experience of flying in icing conditions has shown me that ice accretion is incipient rather than sudden. I doubt that the pitots 'blocked' due ice but may well have become restricted. A blockage would be easy to spot, a restriction not so.
Various AB/AF docs posted on this forum have shown concerns with blockage of pitot drains and have queried the blocking agent; is corrosion/erosion a suspect, perhaps aided by the scouring effect of ice crystals?
The AD systems would not be concerned about different readings from the pitots (due position etc.) as long as the differences were small and relatively static (forgive the pun), the system may then permit a slow build up of ice cristals in the pitot tubes.
A restriction of the drain holes would cause an increase of pressure in the pitot tube. The AP would see this as a potential overspeed and slow the aircraft nearer to the stall.
Could this have been the start of the upset?
Editted:Spelling
Last edited by EGMA; 30th Jun 2009 at 03:59.