Peter wrote:
I think I understand the reasoning behind Va, the max maneuvering
speed, being that the wing will stall (and thus dispose of the
loading) before it breaks. This is why Va falls as the weight falls,
because at any given IAS a higher weight takes the aircraft closer to
stall already.
So, how is it possible to have aircraft destruction due to weather,
e.g. flying into a strong updraught in a CB, if flying below Va?
A DOWNdraught would do it more easily because most aircraft designs
have a lower design limit for negative G.

Find a copy of "FLYING THE BEECH BONANZA", by John C Eckalbar.
There is a section in the book expressly dealing with this topic.