I can't think of any regulations that prohibit the pilot in command being in the back seat, UNLESS the POH (which has legal standing of-course) requires the PiC to be in a particular seat.
But I'm reminded, as I often am, of an old boss of mine at Boscombe Down who gave me the advice "Before making any decision in aviation, always run through your mind the phrase 'at the subsequent board of inquiry' ".
In that case, I think that anybody who thinks that an FI in the back, with no qualified pilot in the front is acting professionally as PiC, is daft. But the case suggested above of an FE in the back checking an FI in the front who is instructing a student sat next to them: the FE being Captain. Well it's unusual, but no more problematic than the Captain of an airliner taking a rest break whilst two qualified pilots remain in the cockpit mid Atlantic.
Another instance might be a 2-seat tandem with no dual controls, current and qualified pilot at the controls, FI or CRI conducting a biennial from the other seat, logging PiC. What's wrong with that?
The quoted original case (four pilots in a PA28) sounds distinctly dodgy, as it sounds very much like four mates going flying together, and the one who happens to be an instructor trying to log everything. You are only, as an instructor, PiC in two circumstances. Either you're flying the thing yourself, or it is formally briefed, etc. as an instructional sortie. Simply being on board with an instructors rating does not make you able to log it as Captain. I've had a couple of instances where (as a CRI but really just going flying with a friend) they *afterwards* asked if they could count it as their biennal - to which I have always said no (but sweetened it by offering to do the biennial, properly briefed, for free in their own aircraft whenever they like, but strangely nobody has ever taken me up on that).
G
Last edited by Genghis the Engineer; 24th Jun 2020 at 08:03.