In the UK, (and, I presume the rest of EASA-land), for any Commercial Air Transport flight, the ‘second pilot’ in:
• A helicopter certificated for single pilot operations
• And, if configured for 9 or fewer passenger seats, and not additionally required by the AOC to operate with two pilots
• (and in this case the Operations Manual must also specify procedures for 2-pilot ops)
….has no status, and may not be part of the operating crew unless:
• a CPL (H) or higher qualification, and
• Type rated, and
• Current OPC on type/class, and
• Current Line Check on type, unless:
o undergoing line training (if and where permitted) on a revenue Commercial Air Transport flight, in which case Pu/t or P2, or
o If he is undergoing a Line Check as if the Commander, which, if successful, may be logged as PiCu/s.
The UK CAA produced a circular (FODCOM, probably now in CAP 789, ‘Guidance to Operators’) a few years back reminding AOC operators that a so-called ‘Pilot’s Assistant’ (ie not meeting the above requirements) must not in any way be involved in the operation of the aircraft on a CAT flight. I guess opening and closing the pax doors on the ground is allowable though!
In general, at any time only the Commander of an aircraft may log PiC time; a second pilot may be able to log PiCu/s or P1 for the time he/she actually is in charge of the flying controls, otherwise P2 (but only if it is a 2-pilot certificated aircraft) or Pu/t (Dual) if undergoing instruction.
A ‘safety pilot’ for insurance or lookout purposes cannot log flight time. He/she is just an extra well-qualified pair of eyes. (However, for safety reasons the how and why of any intervention by the SP should be pre-briefed by the Commander!)
Perhaps others can add to this or correct any of my errors in the above, doodled in a hotel room….