<p>This work proposes a disturbance observer (DOB)-based fault-tolerant control (FTC) approach for a quadcopter subject to total rotor failure. The main goal is to develop a robust control strategy capable of maintaining partial flight control and stabilizing the quadcopter even after a complete failure of one rotor. This is a major challenge, as losing one rotor breaks the thrust symmetry, leading to instability. The proposed approach joins two key components: a primary axis controller that enables the quadrotor to regulate both its spatial position and inclination, and a DOB designed to estimate and compensate for disturbances caused by rotor failures and cross-coupling effects. Leveraging nested control loops and quadratic programming optimization, the control signals are dynamically allocated in response to real-time angular acceleration feedback, thereby reducing reliance on an accurate plant model. The method is shown in high-fidelity MATLAB/Simulink simulations to stabilize the quadcopter post-failure, with comparative analysis indicating high performance over a used estimator in a prior investigation.</p>