Evaluating “Critical-Engine-Inoperative” Conditions for Distributed Hybrid-Electric Aircraft

To ensure flight safety, a set of flight certification rules are defined at critical-engine-inoperative (CEI) conditions for conventional fuel-powered tube-and-wing aircraft. However, the definition of CEI is unclear for distributed hybrid-electric propulsion aircraft, where a single propulsor can be driven by multiple power sources in parallel and/or propulsive power can be unevenly distributed among power sources. These innovative designs necessitate a revisit of CEI conditions for the demonstration of certification compliance. This paper proposes a potential solution for determining critical propulsion failure states and demonstrating the equivalent safety level for distributed hybrid-electric aircraft through reliability analysis and flight dynamic simulations. As an example, NASA’s PEGASUS aircraft is selected as the use case. The failure rates of propulsive failure states computed using a Bayesian modeling approach indicate that PEGASUS’s overall reliability is improved compared to its conventional counterpart despite its more complex propulsion architecture. The flight dynamic simulations show that PEGASUS requires at least 45% of maximum power at sea level for takeoff and maintaining steady level flight. To satisfy the controllability constraints specified in CEI rules, the yawing moment induced by asymmetric loss of thrust may not exceed 1.7 times that induced by the critical engine failure of the ATR 42-500.