Operability enhancement is defined as the extension of operating range for which fully developed rotating stall is avoided. It is proved that the rate-saturated control law is the optimal control that maximizes the operability enhancement. For the controlled system with the rate-saturated controller, analytic formulas are derived for the operability enhancement as a function of noise level. actuator saturation limits, and the shape of the compressor characteristic. which is the major nonlinearity in the model. The shape of the compressor characteristic, especially the unstable part. is critical to the rate required for robust operability near the peak for the closed-loop system, Experiments are carried out on a single-stage low-speed axial compressor using different levels of steady air injections to generate compressor characteristics with different shapes, The rate requirement predicted by the theoretical formulas agrees qualitatively with experiments and simulations of a higher order model (37 states).