The design of near-optimal observer-based controllers for discrete two
-time-scale systems with inaccessible states is considered. The contro
ller is established by the design of the controllers of slow and fast
subsystems via the quasi-steady-state method. The stability bound of t
he singular perturbation parameter and the value of the minimum perfor
mance index for the actual closed-loop system are determined. A digita
l flight control system of a twin-engine aircraft is used to demonstra
te the desired scheme.