14 CFR § 125.111 - General.
(a) Except as provided in paragraph (b) of this section, no certificate holder may use an airplane powered by airplane engines rated at more than 600 horsepower each for maximum continuous operation unless that airplane meets the requirements of §§ 125.113 through 125.181.
(b) If the Administrator determines that, for a particular model of airplane used in cargo service, literal compliance with any requirement under paragraph (a) of this section would be extremely difficult and that compliance would not contribute materially to the objective sought, the Administrator may require compliance with only those requirements that are necessary to accomplish the basic objectives of this part.
(c) This section does not apply to any airplane certificated under -
(1) Part 4b of the Civil Air Regulations in effect after October 31, 1946;
(2) Part 25 of this chapter; or
(3) Special Civil Air Regulation 422, 422A, or 422B.