Then it would seem that the answer to
bmbahia's question is, in part, type-specific. Certainly, to the best of my recollection, Boeing system philosophy will lead to the AP immediately adjusting vertically to regain the selected altitude if there is a subscale change on the engaged AP/ADC, hence my reply earlier in this thread.
However if the A-320/330 design is such that once ALT GREEN is annunciated the AP will not react to a subscale change, whether it be large or small or from 1013 to QNH, then the answer to the original question must be that TCAS cannot be affected by momentarily setting QNH, regardless of which AP is engaged.
I do have one more question -
bmbahia says in his original post that
I know that I can make the altimeter alert sing
What then does this alert denote, if the AP is not registering a vertical deviation from the correct flight path?
JD
