Can a passenger realistically replace suddenly incapacitated pilots?
I guess this is a secret dream of many of you here: You sit on a scheduled flight, and hear in the PA system: "Both our pilots just passed out. Any volunteers to take over their duties?"

Won't happen.

Think of what would happen next: Panic! The volunteers won't even make it to the cockpit. All cabin crew are focussed to keep the cabin calm, so this announcement will never be made.

Instead, one of the cabin crew will take over. That is part of their duties, actually, and some airlines even give them basic training so they have a fair chance when they are in contact with someone on the ground who can talk them through all steps.



Edit: Thanks to some commenters for pointing out that the question also includes GA traffic. Yes, I did not cover this. Generally, I think a passenger should have an easier time to take over, when he/she is sitting already in the right seat. He/she could watch the pilot before and has a full set of controls available. GA planes move much more slowly, so things happen at a slower pace and much less kinetic energy is involved if the touchdown is not perfect. Much depends on the constitution of the new pilot: Some people will simply panic and not be able to follow even the best remote help. But if they keep their cool, a good landing (meaning one you can walk away from) should be possible.

I had been in the right seat of a Lancair once and the pilot turned the plane over to me on final. The Lancair was so sensitive in pitch that I porpoised the aircraft - I first had to get adjusted. So even a pilot in a new type might not be able to land it immediately when he gets to fly it first time on short final!