That whole question was never really completely explained by the FAA. But here a few more of my cents:
So if a co-pilot ever wishes to log PIC time, the accompanying captain must be an FAA CFI?
Nope. The captain is not acting as a CFI in normal circumstances. The co-pilot will only be able to get SIC time while in a airplane or operation that requires two pilots.
61.51
(f) Logging second-in-command flight time. A person may log second-in-command time only for that flight time during which that person:
(1) Is qualified in accordance with the second-in-command requirements of §61.55 of this part, and occupies a crewmember station in an aircraft that requires more than one pilot by the aircraft's type certificate; or
(2) Holds the appropriate category, class, and instrument rating (if an instrument rating is required for the flight) for the aircraft being flown, and more than one pilot is required under the type certification of the aircraft or the regulations under which the flight is being conducted.
Acctually what I said in the last post is bs.
Look
here for more information about logging PIC time in FAA land.
And acctually yes, pilot flying or non flying is not the same as PIC responsibilities. As you will see, even a safety pilot may log PIC time, if he is taking the responsibility.
And now read this and you will be completely confused:
61.55 Second-in-command qualifications.
I would say ... and I am absolutely not sure here and it does not bother me, because I don't fly this kind of ships ... if you are the copilot (providing operation and/or aircraft requires nada nada nada ...) you get SIC time, because you are not responsible for the safe conduct of the flight. That's the captain. If you are the captain you get PIC time ... period ... and now don't ask me about the relief pilot. We don't do plank questions here but generaly in a more-than-one-pilot-required environement the sole-manipulator-of-the-controls rule does not apply.
You may say, that this is not logic, because a CFI can always log PIC time and the student (as long as he/she is already a rated pilot) may also log PIC time. But that's two different things and another story.