Integrated FADEC Architecture for Turboshaft Engines: How the Fuel Controller Shapes Protection-Layer Behaviour

Turboshaft engines must maintain nearly constant power-turbine speed under rapid rotor-load variations while respecting limits on spool speed, turbine temperature, and compressor pressure. Performance regulation and engine protection are often designed and assessed separately, leaving their interaction insufficiently explored. This paper presents an integrated Full Authority Digital Engine Control (FADEC) architecture for a GE T700-class turboshaft engine, developed using a nonlinear T-MATS model. The architecture combines a sliding mode fuel-flow controller, two-dimensional variable guide vane scheduling based on corrected gas-generator speed and Mach number, actuator and sensor dynamics with transport delays, and an industrial Min–Max protection layer with anti-windup compensation. A gain-scheduled PI controller designed using the same loop-shaping specification serves as the baseline, allowing the comparison to isolate the effects of control structure rather than loop bandwidth. The vane schedule improved compressor surge margin in three of the four mission segments. During scheduled operating-point transitions, the two controllers exhibited comparable performance, as the shared feedforward schedule dominated the fuel command. Under ±20% and ±40% load disturbances, the sliding mode controller reduced the peak power-turbine speed deviation at every event, with an average reduction of 4.1%, while requiring 40% fewer protection-limiter activations. This difference is substantially larger than the advantage indicated by conventional transient metrics, which do not capture the residual baseline oscillations when they remain within the settling band. Automatic code generation produced 301 lines of code and 192 bytes of static RAM, corresponding to 39% less code than the baseline with identical memory requirements. Software-in-the-loop verification further confirmed the functional equivalence of the generated implementation.