Decoupled flight control for propulsion-controlled four-engine transport aircraft

Abstract

Total control-surface failure is a catastrophic fault in large transport aircraft that eliminates conventional aerodynamic maneuverability. Propulsion-Controlled Aircraft (PCA) technology, which achieves emergency flight control solely through engine thrust, is the critical technical solution to guarantee flight safety in such failure scenarios. However, existing PCA studies predominantly focus on twin- and tri-engine aircraft, lacking a systematic method for decoupling four thrust inputs in four-engine large transport aircraft; multi-thrust coupling has become the core bottleneck constraining the development of four-engine PCA technology. To address this issue, this paper establishes a six-degree-of-freedom (6-DOF) flight dynamics model for a four-engine transport aircraft under total control-surface failure, re-derives the forces and moments generated by engine thrust, and performs small-disturbance linearization. A linear input transformation based on the control matrix is designed to achieve decoupling control between the longitudinal and lateral-directional channels, and a cascaded attitude control system combining inner-loop stability augmentation and outer-loop PID control is constructed. Aiming at the insufficient full-envelope adaptability of PID control, a two-level nonlinear dynamic inversion (NDI) attitude controller and a dedicated incremental thrust control allocation strategy for four engines are proposed. Simulation results demonstrate that the decoupling control method effectively suppresses roll-yaw coupling, and all control channels achieve fast, low-oscillation tracking with zero steady-state error. The NDI controller realizes global feedback linearization without gain scheduling and maintains stable control performance across the full flight envelope. This research provides an effective technical scheme for emergency propulsion control of four-engine large transport aircraft in the event of total control-surface failure, enhancing their flight safety.