AB33Zab,
I must admit I know nothing about the AP/FD functioning and am just trying to understand. The FCU vertical control area has an altitude window and a VS / FPA window. The pushbutton toggles the latter window between V/S and FPA. My understanding is that the FD guides toward the selected altitude or the selected VS/FPA, depending on the active AP/FD vertical mode, which in cruise (I think) would normally be "ALT CRZ", unless "V/S or FPA" was specifically selected. Am I wrong?
Last edited by HazelNuts39; 4th Jan 2012 at 09:42.