Section 38.13 - Specific air range(a) For each airplane subject to this part, the SAR of an airplane must be determined by either:(1) Direct flight test measurements; or(2) Using a performance model that is:(i) Validated by actual SAR flight test data; and(ii) Approved by the FAA before any SAR calculations are submitted.(b) For the airplane model, establish a 1/SAR value at each of the following reference airplane masses: (1) High gross mass: 92 percent MTOM.(2) Low gross mass: (0.45 * MTOM) + (0.63 * (MTOM^0.924)).(3) Mid gross mass: simple arithmetic average of high gross mass and low gross mass.(c) To obtain (1/SAR)avg as required to determine the fuel efficiency metric value described in § 38.11 , calculate the average of the three 1/SAR values described in paragraph (b) of this section. Do not include auxiliary power units in any 1/SAR calculation.(d) All determinations made under this section must be made in accordance with the procedures applicable to SAR as described in appendix A to this part.