Backup controller for large transport aircraft with insufficient natural stability

For today's commercial airliners, modern fly-by-wire systems provide excellent handling qualities and disturbance rejection capability, which significantly reduce pilot workload during manual flight, thus potentially increasing flight safety. However, in case of severe system failures or unexpected dynamics, many systems immediately degrade to direct mode, where the expected functions and protections degrade or even disappear. This paper assesses whether, with only pitch rate measurable, an emergency longitudinal controller built for a large transport aircraft featuring unstable pitch-up nonlinearities can provide acceptable handling qualities. A requirement-driven, sequential synthesis controller design process is developed, integrating quantitative feedback theory and L₁ piecewise constant output-feedback control theory. The quantitative feedback technique is used to generate a fixed-gain controller to avoid airspeed or dynamic pressure based gain scheduling and provide level 1 handling qualities without existence of the pitch-up nonlinearity. L₁ augmentations are further introduced to eliminate the performance degradation caused by the pitch-up nonlinearity. The fixed-gain controller and the L₁ controllers perform satisfactorily regarding performance and robust metrics within the entire envelope.