Ok, I've been looking and can't find an answer to this question.

Can a single-pilot leave his station for any reason?

You can imagine how this came up. I recently learned of a situation where a 208 pilot turned on the auto pilot and walked to the back of the plane to use the "pee tube." They're part 91 and he did have a company employee on board but the only difference that makes is somebody was there to tell on him.

My opinion is that although this is certainly questionable judgement I can't seem to find a regulation that addresses this specific scenario.

91.105 (a) During takeoff and landing, and while en route, each required flight crewmember shall-
(1) Be at the crewmember station unless the absence is necessary to perform duties in connection with the operation of the aircraft or in connection with physiological needs...



Is that it? It's legal to walk to the back for a pee break? This just doesn't seem like a complete answer to me. Somebody please tell me I'm missing an advisory circular or something.