Originally Posted by CaptainMongo
Absent a failure indication (or inappropriate FMGC programming) the aircraft will level off at FCU selected altitude
Not necessarily. Some time ago I had an A320 bust an altitude during a descent. AP on, everything in managed, nothing abnormal going on. I (PM for the leg) called thousand to level off only to later see the aircraft passing thru 17000', with all the proper mode switching mind you! We stopped it at 250' below the cleared altitude, even the altitude alerter went off. I've never fully trusted the A320 automation, but I do it even less ever since.
Originally Posted by
AerocatS2A
On some older autopilot systems, changing the vertical mode during capture will disarm the alt and if not caught you end up barreling through the cleared level. Knowing that the capture mode is active is critical in these types.
I was about to say this too. In heading mode there's no mode reversion if you switch the target value near "capture". However, switching the altitude target in Alt*/Cap will revert the pitch mode, hence it's quite useful to know when capture is occurring so you don't revert. The A320 is now a 30yr old design so I guess it qualifies as an old aircraft now, but this same logic was true for the DHC-8 (another old aircraft by now)
As a side note, I had noticed the "target over speed" preference in Alt* as I've seen the aircraft get below the speed in certain circumstances during capture whereas it didn't during the rest of the climb. Nice to see my suspicion was correct