Admiral, better watch out.
A controller can tell us more, but I know that depending on software installed, the comtrollers can see SELECTED alt, hdg and speed.
This is certainly true for the London TMA and parts in Germany.
Once, flying out of Luton, we were cleared to to climb to FL120.
FL120 was selected and read back, however the PnF had doubts and subsequently asked the controller to confirm 'you want us to climb to 12- or 130?'
Response from the controller: The level you've got SELECTED is correct, climb FL120.
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Arrowhead, what exactly do you mean by 'setting an unattainable IAS can confuse the FMGS'. I am far from being an expert on the subject, but I have never heared of this.
So what kind of confusion are we talking about???