14 CFR 36.9 - Acoustical change: Propeller-driven small airplanes and propeller-driven commuter category airplanes.
For propeller-driven small airplanes in the primary, normal, utility, acrobatic, transport, and restricted categories and for propeller-driven, commuter category airplanes for which an acoustical change approval is applied for under § 21.93(b) of this chapter after January 1, 1975, the following apply:
(b) If the airplane was not type certificated under this part prior to a change in type design, it may not exceed the higher of the two following values:
(1) The noise limit specified in § 36.501 of this part, or
(2) The noise level created prior to the change in type design, measured and corrected as prescribed in § 36.501 of this part.