14 C.F.R. § 38.19 – Change criteria.

(a) For an airplane that has been shown to comply with § 38.17, any subsequent version of that airplane must demonstrate compliance with § 38.17 if the subsequent version incorporates a modification that either increases:
1. (1) The maximum takeoff mass; or
2. (2) The fuel efficiency metric value by a percentage that is more than the following calculated thresholds.
  1. (i) For airplanes with a MTOM greater than or equal to 5,700 kg, the threshold decreases linearly from 1.35 percent for an airplane with a MTOM of 5,700 kg to 0.75 percent for an airplane with a MTOM of 60,000 kg.
  2. (ii) For airplanes with a MTOM greater than or equal to 60,000 kg, the threshold decreases linearly from 0.75 percent for an airplane with a MTOM of 60,000 kg to 0.70 percent for airplanes with a MTOM of 600,000 kg.
  3. (iii) For airplanes with a MTOM greater than or equal to 600,000 kg, the threshold is 0.70 percent.
(b) For an airplane that has been shown to comply with § 38.17, and for any subsequent version of that airplane that incorporates modifications that do not increase the MTOM or the fuel efficiency metric value in excess of the levels shown in paragraph (a) of this section, the fuel efficiency metric value of the modified airplane may be reported to be the same as the value prior to modification.
(c) For an airplane that meets the criteria of § 38.1(a)(4) or (5), on or after January 1, 2023, and before January 1, 2028, the airplane must demonstrate compliance with § 38.17 if it incorporates any modification that increases the fuel efficiency metric value of the airplane prior to modification by more than 1.5 percent.