14 CFR § 38.11 - Fuel efficiency metric.

For each airplane subject to this part, or to determine whether a modification makes an airplane subject to this part under the change criteria of § 38.19, a fuel efficiency metric value must be calculated, using the following equation, rounded to three decimal places:

Where:

The SAR is determined in accordance with § 38.13, and the RGF is determined in accordance with § 38.15. The fuel efficiency metric value is expressed in units of kilograms of fuel consumed per kilometer.