EASA Aircrew Regulation Annex II on pages 163 et seq.
http://eur-lex.europa.eu/LexUriServ/...01:0193:EN:PDF
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FCL.905 FI privileges state:
FCL.905.FI FI — Privileges and conditions
The privileges of an FI are to conduct flight instruction for the issue, revalidation or renewal of:
(f) a towing or aerobatic rating, provided that such privileges are held and the FI has demonstrated the ability to instruct for that rating to an FI qualified in accordance with (i) below;
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(i) an FI, IRI, CRI, STI or MI certificate provided that the FI has:
(1) completed at least:
(i) in the case of an FI(S), at least 50 hours or 150 launches of flight instruction on sailplanes;
(ii) in the case of an FI(B), at least 50 hours or 50 take-offs of flight instruction on balloons;
(iii) in all other cases, 500 hours of flight instruction in the appropriate aircraft category;
(2) passed an assessment of competence in accordance with FCL.935 in the appropriate aircraft category to demon* strate to a Flight Instructor Examiner (FIE) the ability to instruct for the FI certificate;
So, in order to instruct for the towing or aerobatic rating one must demonstrate proficiency to a FIC Instructor (as described at paragraph (i)).
To become a FIC Instructor, the assessment of competence described at FCL.935 is a FI test in the role of an instructor-instructor.
HTH