but why are Airbus making an easy job difficult by imposing these restrictions? Why not just let the pilot use the best mode for the job; or allow airlines to design their own SOP's within a/c design capabilities?
Our A330 FCOM simply states the following:
"MDA/DA shall not be set in the FCU ALT window."
No explanation is provided in the FCOM or FCTM and there doesn't appear to be any latitude when it comes to circling approaches. I don't know the reasoning behind the ban; as vilas suggested, perhaps it's because Airbus doesn't want to have different FCU altitude setting procedures for different types of approach. Or perhaps it's because they think the majority of non-precision approaches are flown as CDFAs and there are other ways to deal with the automation in the event of a circling approach.
In any case, levelling off at MDA for circling isn't difficult in an Airbus; it's just a matter of pressing the ALT button at the appropriate time. Just make sure you don't forget to do it and allow sufficient time for the aircraft to level off so you don't bust the MDA. That said, I believe the Boeing procedure provides an extra measure of safety that guards against distractions at the time you should be pressing the ALT button. In the B777, the MDA can be set to within 10ft on the MCP.
Anyway, this discussion is somewhat irrelevant at my outfit - we are no longer permitted to fly circling approaches!