What are the differences between fail operational and fail passive autolands? The only difference I notice is an added button above the center CDU.

It's a bit like the difference between wearing a belt, versus wearing a belt and braces. A fail-passive system is usually a dual channel autopilot system on most aircraft, which means that if one of those systems goes awry, although the aircraft will still be able to fly an approach automatically, it will require the crew to disengage the autopilot at some point before touchdown and carry out the actual flare and touchdown themselves, since a fully-automated landing requires two autopilots to be operational from a safety and legality point of view. A fail-operational system has more built-in redundancy (i.e. more channels), whereby if one system goes inoperable, there will still be enough systems available to carry out an automatic landing in spite of the malfunction. Thus a fail-operational system literally means what its name says, i.e. there can be a failure in a system but that failure will not render the system's full capabilities inoperable. There is actually a little bit more to it than that if you want to get really technical, but that's the gist of it. Since you can't actually break regulations in FS as you can in real life, and there is little danger of killing yourself or anyone else when flying a pretend aeroplane on a PC, it's not something that makes a huge difference in FS, although if you like following all the real-world procedures (which you probably do if you've bought a PMDG airliner), it can add to the fun and games if you simulate failures. Al

Thanks. Do they both have the same engagement requirements?