FCL.940.FI specifies the renewal requirements for a Flight Instructor certificate. If you are required to complete both refresher training and assessment of competence then you
should complete the refresher training before the AoC in accordance with FCL.1030(ii):
FCL.1030 When conducting skill tests, proficiency checks and assessments of competence, examiners shall:
(i) ensure that communication with the applicant can be established without language barriers
(ii) verify that the applicant complies with all the qualification, training and experience requirements in Part FCL for the issue, revalidation or renewal of the licence, rating or certificate for which the skill test, proficiency check or assessment of competence is taken
(iii) make the applicant aware of the consequences of providing incomplete, inaccurate or false information related to their training and flight experience
However, UK CAA are happy to accept the refresher training (seminar) and AoC in any order. That said, under
AMC1 FCL.940.FI paragraph (b), for renewal where your FI has expired for a longer period, you may be assessed as requiring ground and flight training to meet the required standard prior to AoC. Speak to an ATO providing instructor training to see what they recommend.
EASA takes FCL.1030 more literally, where your refresher training must be completed prior to AoC.
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