Differences training instructor

[Say again s l o w l y]

Differences training is specific to Variable Pitch Propellors, Turbo/Supercharging, Retractable Undercarriage, Tail wheel and Pressurisation. Soon it will encompass single power lever controls and glass cockpits. (ie a TAA- Technically Advanced Aeroplane)

So, most PPL's will not require any differences training with an FI if they are simply moving between one SEP (Single Engine Piston) type of aircraft. A club or syndicate may enforce a checkout on a specific TYPE, but the class rating means that this isn't a legal responsibility. (Though a very sensible one.)

You may however be thinking about a CRI (Class Rating Instructor) rather than an FI.

As is usual in questions like these, the answer lies in LASORS which I'll reproduce here.

CRI(SPA) PRIVILEGES
The holder of a CRI(SPA) rating can instruct licence holders for the issue of a type or class rating for single-pilot aeroplanes. The holder may instruct on single pilot single-engine or multi-engine aeroplanes, subject to being appropriately qualified, as per JAR-FCL 1.310(a).

H3.2 CRI(SPA) COURSE FOR SINGLE-ENGINE CLASS RATING
An applicant for the issue of a CRI(SPA) rating for single-engine aeroplanes shall have:
a) A valid SEP (Land), SET (Land), TMG Class Rating or a Single-Pilot Single-Engine Type Rating.
b) Completed at least 300 hours flight time as pilot of aeroplanes.
c) Completed at least 30 hours as PIC on the applicable type or class of aeroplane prior to commencing the course.
d) Completed an approved course at an approved FTO or TRTO of at least three hours flight instruction on the aeroplane or a flight simulator given by an instructor approved for this purpose
(see Appendix 2 to JAR-FCL 1.380)
;and
e) Passed the relevant elements of a FI Skill Test in a single pilot, single engine aeroplane with a suitably qualified FIE(A)