Land 3(fail operational) is available with 3 electrical power sources. No distinction is made for which three generators are required. As well, the MEL for one generator inoperative makes no mention of landing restrictions. Therefore there must be another generator that powers the Sync Bus if #4 is unavailable. Which one I don't know. And seeing as the electrical synoptic will not display when Land 3 is active, it won't help by looking at it in the sim if you want to try.
That being said, generator #4 normally doesn't power any autopilots which is why it is the only one powering the sync bus for Land 3. Therefore if one of the other three gererators is lost(meaning one of the generators powering an autopilot when busses are isolated is lost), then I suppose the #4 BTB remains closed as usual along with a second BTB, being the one for the failed generator's bus. Never thought about that before.
Last edited by JammedStab; 10th Apr 2015 at 15:04.