Originally Posted by oberwolfach

Certainly, I get that the UK decision is not binding elsewhere. One difficulty is that there really is not a very good chain of binding precedent in most issues related to what are and are not "extraordinary circumstances". The UK ruling is just the easiest to find without having to dig into sheaves of historical documents, and it's hard to systematically collect information on past decisions. From what I've seen, a medical diversion may be an "extraordinary circumstance" on the flight that was diverted, depending on the details and who's adjudicating it. It's not an "extraordinary circumstance" for the subsequent flight unless the airline took "all reasonable measures" to avoid the delay.

On sending another plane, my thought is that it is reasonable to expect a business operates with spare capacity at a hub like OSL, and when the airline was informed the plane from OSL to LYR was diverting back, it could have sent a different plane with a different crew without passengers to LYR so the delay on LYR to OSL would end up being only 1-2 hours rather than 4. I don't see why this could not have happened before the diverted plane returned to OSL, unless the airline didn't have a spare plane and crew at one of its hubs.