This has been discussed many times before. There are a number of definitions of 'flight time' each of which is tailored towards its use in the applicable regulation. The three main uses are:
1. to establish the hours for continuing airworthiness - which is wheels up to wheels down;
2. to establish the hours for qualification in licensing terms - which is usually regarded as stick time; and
3. to establish the basis for responsibility and flight and duty times - which is as defined in ICAO Annex 6.
With respect to the 3, States look to ICAO to set the definition and that, reproduced from ICAO Annex 6 Part III is as follows:
Flight time — helicopters. The total time from the moment a helicopter’s rotor blades start turning until the moment the helicopter finally comes to rest at the end of the flight, and the rotor blades are stopped.
Note 1.— The State may provide guidance in those cases where the definition of flight time does not describe or permit normal practices. Examples are: crew change without stopping the rotors; and rotors running engine wash procedure following a flight. In any case, the time when rotors are running between sectors of a flight is included within the calculation of flight time.
Note 2.— This definition is intended only for the purpose of flight and duty time regulations.
States, in their basic regulations, should differentiate the three uses for the very reasons that have been discussed in this thread (which repeats a discussion from other threads in PPRune).
Jim