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(a) Subject to the provisions of subparagraph (c) of this paragraph, the holder of a pilot’s licence or a flight engineer’s licence shall not be entitled to exercise the privileges of an aircraft rating contained in the licence on a flight unless the licence bears a valid certificate of test or a valid certificate of experience, which certificate shall in either case be appropriate to the functions he is to perform on that flight in accordance with Part C of the said Schedule and shall otherwise comply with that Part:Provided that the holder of a Private Pilot’s Licence (Balloons and Airships) or a Commercial Pilot’s Licence (Balloons) shall be entitled to exercise the privileges of an aircraft rating contained in the licence on a flight when the licence does not bear such a certificate. (b) (c) In any case where the Private Pilot’s Licence is in such a form that it is not possible to include certificates therein, the holder of such a licence shall not be entitled to exercise the privileges of an aircraft rating contained in the licence on a flight unless the certificate of test or certificate of experience required by subparagraph (a) of this paragraph is included in the personal flying log book required to be kept by him under Article 22 of this Order.