The U.S. DOT Volpe Center Advances UAS Integration in the National Airspace System
Wake turbulence is the rotational movement of air created behind any aircraft under lift generation. The U.S. DOT Volpe Center’s measurements and analyses of wake turbulence on behalf of FAA have been instrumental in enabling FAA to make positive operational changes in the National Airspace System (NAS). The result has been improved wake turbulence safety, reduced airport delays, shortened final approaches to airports, and reduced fuel burn and greenhouse gas emissions.
FAA’s long-term goal is to fully integrate unmanned aircraft systems (UAS) into the NAS. From a wake turbulence perspective, the full scope of UAS integration of any sizes has been a continuing topic of discussion and research between FAA and various stakeholders. However, the question of how to integrate larger UAS into the NAS when they require air traffic control (ATC) services presented immediate and near-term challenges to air traffic controllers, because no UAS had yet received a categorical wake turbulence designation.
For example, the General Atomics Q9 is a large, fixed-wing UAS that Department of Defense (DOD) currently operates in airspace with mixed civilian and military traffic. FAA guidance recommends that until an aircraft receives a categorical wake turbulence designation, it receives a NOWGT (no weight) designation, which requires controllers to place 10 nautical miles (NM) between the Q9 and another aircraft—both in front and behind the aircraft being assessed. In 2021, the FAA NextGen Wake Turbulence Research and Development Program (WTRDP) asked the U.S. DOT Volpe Center to evaluate the wake turbulence separation requirement for the Q9 as a specific integration scenario.
Mission, performance, aerodynamic design requirements, and operating procedures of any UAS can be significantly different from those of commercial aircraft. For this reason, the U.S. DOT Volpe Center first needed to examine the details of the current wake turbulence separation assessment in order to determine its extendibility and necessary revisions with respect to the Q9. Ultimately, U.S. DOT Volpe Center’s analysis included wake parameters such as speed profile, weight, wingspan, roll resistance, and roll performance of the Q9. The assessment employed a dynamic wake encounter model developed by the WTRDP along with other data-driven tools; in combination, these could provide conservative estimates of the wake strength evolution of the Q9. Additionally, the U.S. DOT Volpe Center team used aircraft performance characteristics provided by the airframe manufacturer and operational experience input from Q9 subject matter experts to develop the recommended wake separation.
The U.S. DOT Volpe Center’s analysis demonstrated the wake separation minima for all variants of the Q9 can be safely reduced below 10 NM. Moreover, the Q9 could be safely designated to one of the FAA’s wake turbulence categorical schemes already in use. The WTRDP and FAA Chief Scientist and Technical Advisor for Wake Turbulence, upon reviewing the analysis from the U.S. DOT Volpe Center as well as factoring in other perspectives, made a joint recommendation for the Q9 wake separation minima to FAA's Air Traffic Organization in late 2021. The minima were published and operationalized in FAA JO 7360.1G, issued April 8, 2022.
The Q9 is the first UAS in the NAS to receive a categorical wake turbulence designation. The designation is specific to the Q9 and cannot be generalized to any other UAS aircraft. However, the U.S. DOT Volpe Center’s work has yielded both a valuable case study and initiated what may eventually become a repeatable process for developing recommended minima, both for other DOD large, fixed-wing UAS operating in the NAS, and for future aircraft entrants (including other UAS designs as well as Urban Air Mobility)—a significant step toward achieving FAA’s long-term goal of integrating UAS into the NAS. This achievement has implications for both safety and economic competitiveness.