This work presents a simplified FDI strategy tailored to the quadcopter’s nonlinear dynamics. The approach employs a nonlinear observer to reconstruct system states and generate residuals by comparing observer-based estimates with measured angular rates. Fault-sensitive decision statistics are computed over finite sample windows and evaluated against adaptive thresholds to detect and isolate loss-of-effectiveness (LOE) actuator faults. Simulation studies demonstrate that the proposed method achieves accurate and timely fault detection and correct isolation over a range of fault severities, while maintaining low computational overhead. These results indicate that the observer-based FDI scheme offers a practical solution for onboard implementation in resource-constrained quadcopter platforms.