HFD,
The reference to training captains was in response to someone saying that instruction was not allowed unless there was an instructor on board.
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Mad jock,
But you have missed the vital point that the CAA has to approve the logging of Co-pilot time.
The CAA has to approve applications for flights during which there is no rule which requires a co-pilot. If there is already provision in the licensing requirements / ANO for two pilots then the minimum crew for that flight is 2.
Example - simulated instrument flight - 2 crew minimum.
Commercial flight in Senneca to destination with ILS and RVR of 600m but no autopilot - 2 crew minimum.
Do the same flight with a PIC that meets certain experience requirements and it can be single crew.
Flight in same aircraft to aerodrome short distance away VFR - single crew.
Some operations require 2 crew some don't. Companies apply to the CAA to operate 2 crew all the time rather than chop and change from day to day.
If the insurance requirments require what is in effect a line check of new group members then it has to be a 2 crew operation. There is no other way to do it. Passengers are not crewmembers.