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Run Time Assurance for Electric Vertical Takeoff and Landing AircraftNASA is conducting research to demonstrate and evaluate the application of Run Time Assurance (RTA) as a means to assure safety in Electric Vertical Takeoff and Landing (eVTOL) aircraft with highly automated or autonomous flight capability supervised by a single onboard pilot. The work described in this report demonstrates an application of RTA and examines the implications for design and analysis of aircraft functions and systems; aircraft safety hazards; safety assurance; development assurance; and pilot tasks and performance. This research effort also seeks to assess the efficacy of the combined application of traditional Functional Hazard Analysis (FHA) and the more modern System Theoretic Process Analysis (STPA) techniques to perform hazard analyses on aircraft with complex automated and autonomous systems and an onboard pilot. During the research effort we developed architectural designs of two alternate eVTOL aircraft, generally following the process characterized in the SAE standards ARP4754 and ARP4761. The design has focused on the control architectures of these aircraft, which are identical except that one incorporates RTA techniques to reduce the criticality of some key software components. Artifacts of this process include a taxonomy of aircraft-level functions, aircraft-level architecture diagrams, aircraft-level functional hazard assessments (AFHA), function allocations onto aircraft systems and subsystems, functional block diagrams for a select set of control-related functions, and system-level functional hazard assessments (SFHA) for those functions. This project has highlighted the notion that DAL D is something of a sweet spot for low-confidence controllers in an RTA-based design. Among the many activities described in DO-178C, the activities related to requirement verifiability, algorithmic accuracy, and test coverage can be the most challenging for the kinds of advanced control techniques that may be desirable in novel UAM designs, such as adaptive control, machine-learning, artificial intelligence, numerical search, and Monte Carlo based algorithms. Moreover, the standard requires that development teams demonstrate that errors leading to unacceptable failure conditions have been removed from the software. The RTA architecture, which cordons off the low-confidence function, makes it much easier to show this for these kinds of algorithms. With regard to the use of STPA and FHA as complementary hazard analysis techniques, our research effort led us to the conclusion that STPA should be used to derive requirements for hardware and software systems and/or components. Also, STPA is a natural complement to other processes in ARP4754A involving design studies and iteration.
Document ID
Acquisition Source
Langley Research Center
Document Type
Contractor Report (CR)
Michael DeVore
(Barron Associates (United States) Charlottesville, Virginia, United States)
Jared Cooper
(Barron Associates (United States) Charlottesville, Virginia, United States)
Andy Wallington
(Electron International II Phoenix, Arizona, United States)
Robert Crouse
(Electron International II Phoenix, Arizona, United States)
Gust Tsikalas
(Electron International II Phoenix, Arizona, United States)
Komal Verma
(Electron International II Phoenix, Arizona, United States)
Cody H Fleming
(Iowa State University Ames, Iowa, United States)
Greg Carr
(Architecture Technology Corporation (United States) Eden Prairie, Minnesota, United States)
Newton Kirby ORCID
(Architecture Technology Corporation (United States) Eden Prairie, Minnesota, United States)
Date Acquired
January 14, 2022
Publication Date
March 1, 2022
Subject Category
Air Transportation And Safety
Report/Patent Number
Funding Number(s)
WBS: 340428.
TASK: 80LARC20F0089
Distribution Limits
Public Use Permitted.
Technical Review
Single Expert
Run-time assurance
Electric vertical takeoff and landing aircraft
System Theoretic Process Analysis (STPA)
Urban air mobility
Functional hazard analysis
Safety assessment
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