The basic premise is as follows:
The extension if IR(H) privileges from one MEH type to another is a matter only of the transfer of existing instrument flying skills to the new type and, therefore, may be conducted by an appropriately qualified TRI. When extending privileges from SEH to MEH, however, new instrument flying skills are required and only a suitably qualified FI(H) or an IRI(H) may deliver the training.
The privileges of the IR(A) clearly cannot be extended to a helicopter type; a different rating is required. Consequently, it is necessary to complete an initial IR(H) course, although the flight training may be reduced to not less than 10 hours. The course may be completed on either a SEH or a MEH and the instruction must be given by either a suitably qualified FI(H) or an IRI(H).