<p>This paper proposes a novel fixed-time sliding mode control (FTSMC) approach for accurate trajectory tracking of an underactuated three degrees-of-freedom (3-DOF) laboratory helicopter. A fixed-time extended state observer (FESO) is designed to estimate disturbance and unmeasurable states to augment the robustness of the system against uncertainties and disturbances. In order to force the tracking error dynamics to a predefined finite-time convergence, an FTSMC is proposed. A smooth continuous control law is developed that mitigates the chattering phenomena associated with the existence of fast-switching terms, which is facilitated by a novel advanced saturation function that allows it to switch smoothly and continuously. The Lyapunov stability criterion is then employed to articulate a proof demonstrating the fixed-time closed-loop stability of the system, thereby guaranteeing that the tracking error converges in a predefined time to a bounded neighborhood around zero despite the presence of disturbances, unmodeled dynamics, and selected initial states. The effectiveness and efficiency of the proposed controller scheme are validated via a rigorous stability analysis and extensive experimental evaluations, which include comparative analysis and additional tests over a variety of reference signals. The practical results clearly demonstrate that the suggested control scheme yields the required trajectory-tracking performance.</p>