Noise from ETAs - Experience from the Industry

(Updated September 2022)


During the CAEP12 cycle, several companies in Europe, the U.S. and China accomplished first flights of full-scale prototypes of Emerging Technology Aircraft (ETA). It is recognized by regulators and industry that these emerging aircraft pose a noise certification challenge because the existing reference conditions and testing procedures in Annex 16 Volume I are inadequate for ETA designs and flight operations. ETA designs range from small remotely piloted vehicles with limited cargo capacity, to larger designs with significant cargo capacity, and finally to more advanced designs transporting passengers that may be operated by an onboard pilot. Most ETA designs incorporate vertical takeoff and landing (VTOL) capability. Although significant noise is expected to occur during VTOL operations for ETA vehicles, vertiport planning and location by infrastructure developers as well as automated flight profiles designed by industry are being implemented to minimize public exposure during these conditions.


For the more advanced category of ETA that are designed to carry passengers, manufacturers have recognized the need to design products substantially more quiet than current conventional helicopters considering that community noise concerns can be a barrier to successful entry for this industry. Most ETA designs are differentiated from conventional helicopter and tiltrotor designs by the use of distributed propulsion (multiple prop/rotors instead of one or two main prop/rotors) and integrated flight controls. Instead of commanding changes in aircraft state (thrust and control surface position) as with most conventional aircraft, ETA pilots command changes in aircraft velocity vector while onboard computers automate these commands to changes in control surface position, engine speed, propeller or rotor pitch, tilt angle and other parameters. Distributed propulsion enables lower tip speeds and thereby lower noise than conventional tiltrotors and is used for stability and attitude control as well as forward thrust. Integrated flight controls permit the aircraft to respond automatically to hazards and failure conditions, greatly improving flight safety.


Various gaps have been identified when attempting to define ETA noise certification procedures using existing standards prescribed by Annex 16. Consider that conventional helicopter and tiltrotor procedures require fixed rotor speed, which is not viable when rotor speed is adjusted continuously to accomplish aircraft control for ETA. Also consider that conventional aircraft pilots generally target a desired airspeed when transporting cargo and passengers, whereas ETA flight controls are being designed with automated, more sophisticated techniques. Considering the emerging technology enablers and noise certification gaps discussed above, it is proposed to extend use of the Variable Noise Reduction System (VNRS) as described in the Environmental Technical Manual (SGAR 2018 revision Doc 9501) 3.7.1 for which specific guidance has been generally accepted by certificating authorities for the takeoff flight test procedure. Use of VNRS should be extended to applicable flyover, takeoff, and/or approach flight test procedures when automated and integrated ETA flight controls are employed by manufacturers. Application of VNRS will require highly automated flight control procedures to be developed by industry for normal operating procedures in and out of communities affected by ETA operations.


The U.S. has proposed the first noise certification basis of an ETA, using a simplified version of the light helicopter procedures similar to those in Annex 16, Volume I, Appendix 4. An additional requirement was added for the applicant to provide detailed acoustic test data over a range of operating conditions to inform further FAA research. It is suggested that the CAEP13 cycle should include regular updates from the international authorities certificating ETA on the procedures they have found effective in responding to certification requests for these vehicles. Particular attention should be paid to the identification of flight conditions, research and consideration of existing and alternate noise certification metrics, and associated threshold noise level (perhaps a future ETA noise limit) with the highest potential for negative reactions to community noise. As ETA noise certification procedures mature and develop during CAEP13, it may be necessary to categorize UAS differently than aircraft used to transport passengers.


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