@SN3:
Never heard about a TAS-limitation. Only either IAS (Vmo) or M (Mmo).
I did! (It was a question) - Part 1:
I shouldn't have said 'TAS', as it's not quite that: I'm given to understand Vne as marked on the ASI is predicated on standard temp & pressure, and should be factored for altitude - i.e. the IAS will underread at alt due to density, but VNE is a constant *airstream* speed. Or to put it really simply, cruising along at 20,000ft you could hit flutter while the ASI was well short of the redline.
Part 2:
I gather big tin generally doesn't have Vne, but Vmo - am I correct in thinking that varies / is varied with altitude. If so, how does that work - presumably it's driven by factors different to Vne/flutter (not just airstream speed).